{-# LANGUAGE CPP #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE MultiWayIf #-}

module GHC.Tc.Solver.Canonical(
     canonicalize,
     unifyWanted,
     makeSuperClasses,
     StopOrContinue(..), stopWith, continueWith, andWhenContinue,
     solveCallStack    -- For GHC.Tc.Solver
  ) where

import GHC.Prelude

import GHC.Tc.Types.Constraint
import GHC.Core.Predicate
import GHC.Tc.Types.Origin
import GHC.Tc.Utils.Unify
import GHC.Tc.Utils.TcType
import GHC.Core.Type
import GHC.Tc.Solver.Rewrite
import GHC.Tc.Solver.Monad
import GHC.Tc.Solver.InertSet
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.EvTerm
import GHC.Core.Class
import GHC.Core.DataCon ( dataConName )
import GHC.Core.TyCon
import GHC.Core.Multiplicity
import GHC.Core.TyCo.Rep   -- cleverly decomposes types, good for completeness checking
import GHC.Core.Coercion
import GHC.Core.Coercion.Axiom
import GHC.Core.Reduction
import GHC.Core
import GHC.Types.Id( mkTemplateLocals )
import GHC.Core.FamInstEnv ( FamInstEnvs )
import GHC.Tc.Instance.Family ( tcTopNormaliseNewTypeTF_maybe )
import GHC.Types.Var
import GHC.Types.Var.Env( mkInScopeSet )
import GHC.Types.Var.Set( delVarSetList, anyVarSet )
import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Utils.Panic.Plain
import GHC.Builtin.Types ( anyTypeOfKind )
import GHC.Types.Name.Set
import GHC.Types.Name.Reader
import GHC.Hs.Type( HsIPName(..) )
import GHC.Types.Unique  ( hasKey )
import GHC.Builtin.Names ( coercibleTyConKey )

import GHC.Data.Pair
import GHC.Utils.Misc
import GHC.Data.Bag
import GHC.Utils.Monad
import Control.Monad
import Data.Maybe ( isJust, isNothing )
import Data.List  ( zip4 )
import GHC.Types.Basic

import qualified Data.Semigroup as S
import Data.Bifunctor ( bimap )
import Data.Foldable ( traverse_ )

{-
************************************************************************
*                                                                      *
*                      The Canonicaliser                               *
*                                                                      *
************************************************************************

Note [Canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~

Canonicalization converts a simple constraint to a canonical form. It is
unary (i.e. treats individual constraints one at a time).

Constraints originating from user-written code come into being as
CNonCanonicals. We know nothing about these constraints. So, first:

     Classify CNonCanoncal constraints, depending on whether they
     are equalities, class predicates, or other.

Then proceed depending on the shape of the constraint. Generally speaking,
each constraint gets rewritten and then decomposed into one of several forms
(see type Ct in GHC.Tc.Types).

When an already-canonicalized constraint gets kicked out of the inert set,
it must be recanonicalized. But we know a bit about its shape from the
last time through, so we can skip the classification step.

-}

-- Top-level canonicalization
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

canonicalize :: Ct -> TcS (StopOrContinue Ct)
canonicalize :: Ct -> TcS (StopOrContinue Ct)
canonicalize (CNonCanonical { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev })
  = {-# SCC "canNC" #-}
    CtEvidence -> TcS (StopOrContinue Ct)
canNC CtEvidence
ev

canonicalize (CQuantCan (QCI { qci_ev :: QCInst -> CtEvidence
qci_ev = CtEvidence
ev, qci_pend_sc :: QCInst -> Bool
qci_pend_sc = Bool
pend_sc }))
  = CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev Bool
pend_sc

canonicalize (CIrredCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev })
  = CtEvidence -> TcS (StopOrContinue Ct)
canNC CtEvidence
ev
    -- Instead of rewriting the evidence before classifying, it's possible we
    -- can make progress without the rewrite. Try this first.
    -- For insolubles (all of which are equalities), do /not/ rewrite the arguments
    -- In #14350 doing so led entire-unnecessary and ridiculously large
    -- type function expansion.  Instead, canEqNC just applies
    -- the substitution to the predicate, and may do decomposition;
    --    e.g. a ~ [a], where [G] a ~ [Int], can decompose

canonicalize (CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class  = Class
cls
                       , cc_tyargs :: Ct -> [Type]
cc_tyargs = [Type]
xis, cc_pend_sc :: Ct -> Bool
cc_pend_sc = Bool
pend_sc
                       , cc_fundeps :: Ct -> Bool
cc_fundeps = Bool
fds })
  = {-# SCC "canClass" #-}
    CtEvidence
-> Class -> [Type] -> Bool -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Type]
xis Bool
pend_sc Bool
fds

canonicalize (CEqCan { cc_ev :: Ct -> CtEvidence
cc_ev     = CtEvidence
ev
                     , cc_lhs :: Ct -> CanEqLHS
cc_lhs    = CanEqLHS
lhs
                     , cc_rhs :: Ct -> Type
cc_rhs    = Type
rhs
                     , cc_eq_rel :: Ct -> EqRel
cc_eq_rel = EqRel
eq_rel })
  = {-# SCC "canEqLeafTyVarEq" #-}
    CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
ev EqRel
eq_rel (CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs) Type
rhs

canNC :: CtEvidence -> TcS (StopOrContinue Ct)
canNC :: CtEvidence -> TcS (StopOrContinue Ct)
canNC CtEvidence
ev =
  case Type -> Pred
classifyPredType Type
pred of
      ClassPred Class
cls [Type]
tys     -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:cls" (forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr [Type]
tys)
                                  CtEvidence -> Class -> [Type] -> TcS (StopOrContinue Ct)
canClassNC CtEvidence
ev Class
cls [Type]
tys
      EqPred EqRel
eq_rel Type
ty1 Type
ty2 -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:eq" (forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
ty2)
                                  CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC    CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
      IrredPred {}          -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:irred" (forall a. Outputable a => a -> SDoc
ppr Type
pred)
                                  CtEvidence -> TcS (StopOrContinue Ct)
canIrred CtEvidence
ev
      ForAllPred [TcTyVar]
tvs [Type]
th Type
p   -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:forall" (forall a. Outputable a => a -> SDoc
ppr Type
pred)
                                  CtEvidence
-> [TcTyVar] -> [Type] -> Type -> TcS (StopOrContinue Ct)
canForAllNC CtEvidence
ev [TcTyVar]
tvs [Type]
th Type
p

  where
    pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev

{-
************************************************************************
*                                                                      *
*                      Class Canonicalization
*                                                                      *
************************************************************************
-}

canClassNC :: CtEvidence -> Class -> [Type] -> TcS (StopOrContinue Ct)
-- "NC" means "non-canonical"; that is, we have got here
-- from a NonCanonical constraint, not from a CDictCan
-- Precondition: EvVar is class evidence
canClassNC :: CtEvidence -> Class -> [Type] -> TcS (StopOrContinue Ct)
canClassNC CtEvidence
ev Class
cls [Type]
tys
  | CtEvidence -> Bool
isGiven CtEvidence
ev  -- See Note [Eagerly expand given superclasses]
  = do { [Ct]
sc_cts <- CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [] [] Class
cls [Type]
tys
       ; [Ct] -> TcS ()
emitWork [Ct]
sc_cts
       ; CtEvidence
-> Class -> [Type] -> Bool -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Type]
tys Bool
False Bool
fds }

  | CtWanted { ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } <- CtEvidence
ev
  , Just FastString
ip_name <- Class -> [Type] -> Maybe FastString
isCallStackPred Class
cls [Type]
tys
  , CtOrigin -> Bool
isPushCallStackOrigin CtOrigin
orig
  -- If we're given a CallStack constraint that arose from a function
  -- call, we need to push the current call-site onto the stack instead
  -- of solving it directly from a given.
  -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
  -- and Note [Solving CallStack constraints] in GHC.Tc.Solver.Types
  = do { -- First we emit a new constraint that will capture the
         -- given CallStack.
       ; let new_loc :: CtLoc
new_loc = CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin CtLoc
loc (HsIPName -> CtOrigin
IPOccOrigin (FastString -> HsIPName
HsIPName FastString
ip_name))
                            -- We change the origin to IPOccOrigin so
                            -- this rule does not fire again.
                            -- See Note [Overview of implicit CallStacks]
                            -- in GHC.Tc.Types.Evidence

       ; CtEvidence
new_ev <- CtLoc -> RewriterSet -> Type -> TcS CtEvidence
newWantedEvVarNC CtLoc
new_loc RewriterSet
rewriters Type
pred

         -- Then we solve the wanted by pushing the call-site
         -- onto the newly emitted CallStack
       ; let ev_cs :: EvCallStack
ev_cs = FastString -> RealSrcSpan -> EvExpr -> EvCallStack
EvCsPushCall (CtOrigin -> FastString
callStackOriginFS CtOrigin
orig)
                                  (CtLoc -> RealSrcSpan
ctLocSpan CtLoc
loc) (HasDebugCallStack => CtEvidence -> EvExpr
ctEvExpr CtEvidence
new_ev)
       ; CtEvidence -> EvCallStack -> TcS ()
solveCallStack CtEvidence
ev EvCallStack
ev_cs

       ; CtEvidence
-> Class -> [Type] -> Bool -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
new_ev Class
cls [Type]
tys
                  Bool
False -- No superclasses
                  Bool
False -- No top level instances for fundeps
       }

  | Bool
otherwise
  = CtEvidence
-> Class -> [Type] -> Bool -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Type]
tys (Class -> Bool
has_scs Class
cls) Bool
fds

  where
    has_scs :: Class -> Bool
has_scs Class
cls = Bool -> Bool
not (forall (t :: * -> *) a. Foldable t => t a -> Bool
null (Class -> [Type]
classSCTheta Class
cls))
    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    orig :: CtOrigin
orig = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc
    pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev
    fds :: Bool
fds  = Class -> Bool
classHasFds Class
cls

solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
-- Also called from GHC.Tc.Solver when defaulting call stacks
solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
solveCallStack CtEvidence
ev EvCallStack
ev_cs = do
  -- We're given ev_cs :: CallStack, but the evidence term should be a
  -- dictionary, so we have to coerce ev_cs to a dictionary for
  -- `IP ip CallStack`. See Note [Overview of implicit CallStacks]
  EvExpr
cs_tm <- forall (m :: * -> *).
(MonadThings m, HasModule m, HasDynFlags m) =>
EvCallStack -> m EvExpr
evCallStack EvCallStack
ev_cs
  let ev_tm :: EvTerm
ev_tm = EvExpr -> TcCoercion -> EvTerm
mkEvCast EvExpr
cs_tm (Type -> TcCoercion
wrapIP (CtEvidence -> Type
ctEvPred CtEvidence
ev))
  CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev EvTerm
ev_tm

canClass :: CtEvidence
         -> Class -> [Type]
         -> Bool            -- True <=> un-explored superclasses
         -> Bool            -- True <=> unexploited fundep(s)
         -> TcS (StopOrContinue Ct)
-- Precondition: EvVar is class evidence

canClass :: CtEvidence
-> Class -> [Type] -> Bool -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Type]
tys Bool
pend_sc Bool
fds
  = -- all classes do *nominal* matching
    forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (CtEvidence -> Role
ctEvRole CtEvidence
ev forall a. Eq a => a -> a -> Bool
== Role
Nominal) (forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [Type]
tys) forall a b. (a -> b) -> a -> b
$
    do { (redns :: Reductions
redns@(Reductions [TcCoercion]
_ [Type]
xis), RewriterSet
rewriters) <- CtEvidence -> TyCon -> [Type] -> TcS (Reductions, RewriterSet)
rewriteArgsNom CtEvidence
ev TyCon
cls_tc [Type]
tys
       ; let redn :: Reduction
redn@(Reduction TcCoercion
_ Type
xi) = Class -> Reductions -> Reduction
mkClassPredRedn Class
cls Reductions
redns
             mk_ct :: CtEvidence -> Ct
mk_ct CtEvidence
new_ev = CDictCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
new_ev
                                     , cc_tyargs :: [Type]
cc_tyargs = [Type]
xis
                                     , cc_class :: Class
cc_class = Class
cls
                                     , cc_pend_sc :: Bool
cc_pend_sc = Bool
pend_sc
                                     , cc_fundeps :: Bool
cc_fundeps = Bool
fds }
       ; StopOrContinue CtEvidence
mb <- RewriterSet
-> CtEvidence -> Reduction -> TcS (StopOrContinue CtEvidence)
rewriteEvidence RewriterSet
rewriters CtEvidence
ev Reduction
redn
       ; String -> SDoc -> TcS ()
traceTcS String
"canClass" ([SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
                                   , forall a. Outputable a => a -> SDoc
ppr Type
xi, forall a. Outputable a => a -> SDoc
ppr StopOrContinue CtEvidence
mb ])
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap CtEvidence -> Ct
mk_ct StopOrContinue CtEvidence
mb) }
  where
    cls_tc :: TyCon
cls_tc = Class -> TyCon
classTyCon Class
cls

{- Note [The superclass story]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We need to add superclass constraints for two reasons:

* For givens [G], they give us a route to proof.  E.g.
    f :: Ord a => a -> Bool
    f x = x == x
  We get a Wanted (Eq a), which can only be solved from the superclass
  of the Given (Ord a).

* For wanteds [W], they may give useful
  functional dependencies.  E.g.
     class C a b | a -> b where ...
     class C a b => D a b where ...
  Now a [W] constraint (D Int beta) has (C Int beta) as a superclass
  and that might tell us about beta, via C's fundeps.  We can get this
  by generating a [W] (C Int beta) constraint. We won't use the evidence,
  but it may lead to unification.

See Note [Why adding superclasses can help].

For these reasons we want to generate superclass constraints for both
Givens and Wanteds. But:

* (Minor) they are often not needed, so generating them aggressively
  is a waste of time.

* (Major) if we want recursive superclasses, there would be an infinite
  number of them.  Here is a real-life example (#10318);

     class (Frac (Frac a) ~ Frac a,
            Fractional (Frac a),
            IntegralDomain (Frac a))
         => IntegralDomain a where
      type Frac a :: *

  Notice that IntegralDomain has an associated type Frac, and one
  of IntegralDomain's superclasses is another IntegralDomain constraint.

So here's the plan:

1. Eagerly generate superclasses for given (but not wanted)
   constraints; see Note [Eagerly expand given superclasses].
   This is done using mkStrictSuperClasses in canClassNC, when
   we take a non-canonical Given constraint and cannonicalise it.

   However stop if you encounter the same class twice.  That is,
   mkStrictSuperClasses expands eagerly, but has a conservative
   termination condition: see Note [Expanding superclasses] in GHC.Tc.Utils.TcType.

2. Solve the wanteds as usual, but do no further expansion of
   superclasses for canonical CDictCans in solveSimpleGivens or
   solveSimpleWanteds; Note [Danger of adding superclasses during solving]

   However, /do/ continue to eagerly expand superclasses for new /given/
   /non-canonical/ constraints (canClassNC does this).  As #12175
   showed, a type-family application can expand to a class constraint,
   and we want to see its superclasses for just the same reason as
   Note [Eagerly expand given superclasses].

3. If we have any remaining unsolved wanteds
        (see Note [When superclasses help] in GHC.Tc.Types.Constraint)
   try harder: take both the Givens and Wanteds, and expand
   superclasses again.  See the calls to expandSuperClasses in
   GHC.Tc.Solver.simpl_loop and solveWanteds.

   This may succeed in generating (a finite number of) extra Givens,
   and extra Wanteds. Both may help the proof.

3a An important wrinkle: only expand Givens from the current level.
   Two reasons:
      - We only want to expand it once, and that is best done at
        the level it is bound, rather than repeatedly at the leaves
        of the implication tree
      - We may be inside a type where we can't create term-level
        evidence anyway, so we can't superclass-expand, say,
        (a ~ b) to get (a ~# b).  This happened in #15290.

4. Go round to (2) again.  This loop (2,3,4) is implemented
   in GHC.Tc.Solver.simpl_loop.

The cc_pend_sc flag in a CDictCan records whether the superclasses of
this constraint have been expanded.  Specifically, in Step 3 we only
expand superclasses for constraints with cc_pend_sc set to true (i.e.
isPendingScDict holds).

Why do we do this?  Two reasons:

* To avoid repeated work, by repeatedly expanding the superclasses of
  same constraint,

* To terminate the above loop, at least in the -XNoRecursiveSuperClasses
  case.  If there are recursive superclasses we could, in principle,
  expand forever, always encountering new constraints.

When we take a CNonCanonical or CIrredCan, but end up classifying it
as a CDictCan, we set the cc_pend_sc flag to False.

Note [Superclass loops]
~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
  class C a => D a
  class D a => C a

Then, when we expand superclasses, we'll get back to the self-same
predicate, so we have reached a fixpoint in expansion and there is no
point in fruitlessly expanding further.  This case just falls out from
our strategy.  Consider
  f :: C a => a -> Bool
  f x = x==x
Then canClassNC gets the [G] d1: C a constraint, and eager emits superclasses
G] d2: D a, [G] d3: C a (psc).  (The "psc" means it has its sc_pend flag set.)
When processing d3 we find a match with d1 in the inert set, and we always
keep the inert item (d1) if possible: see Note [Replacement vs keeping] in
GHC.Tc.Solver.Interact.  So d3 dies a quick, happy death.

Note [Eagerly expand given superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In step (1) of Note [The superclass story], why do we eagerly expand
Given superclasses by one layer?  (By "one layer" we mean expand transitively
until you meet the same class again -- the conservative criterion embodied
in expandSuperClasses.  So a "layer" might be a whole stack of superclasses.)
We do this eagerly for Givens mainly because of some very obscure
cases like this:

   instance Bad a => Eq (T a)

   f :: (Ord (T a)) => blah
   f x = ....needs Eq (T a), Ord (T a)....

Here if we can't satisfy (Eq (T a)) from the givens we'll use the
instance declaration; but then we are stuck with (Bad a).  Sigh.
This is really a case of non-confluent proofs, but to stop our users
complaining we expand one layer in advance.

Note [Instance and Given overlap] in GHC.Tc.Solver.Interact.

We also want to do this if we have

   f :: F (T a) => blah

where
   type instance F (T a) = Ord (T a)

So we may need to do a little work on the givens to expose the
class that has the superclasses.  That's why the superclass
expansion for Givens happens in canClassNC.

This same scenario happens with quantified constraints, whose superclasses
are also eagerly expanded. Test case: typecheck/should_compile/T16502b
These are handled in canForAllNC, analogously to canClassNC.

Note [Why adding superclasses can help]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Examples of how adding superclasses can help:

    --- Example 1
        class C a b | a -> b
    Suppose we want to solve
         [G] C a b
         [W] C a beta
    Then adding [W] beta~b will let us solve it.

    -- Example 2 (similar but using a type-equality superclass)
        class (F a ~ b) => C a b
    And try to sllve:
         [G] C a b
         [W] C a beta
    Follow the superclass rules to add
         [G] F a ~ b
         [W] F a ~ beta
    Now we get [W] beta ~ b, and can solve that.

    -- Example (tcfail138)
      class L a b | a -> b
      class (G a, L a b) => C a b

      instance C a b' => G (Maybe a)
      instance C a b  => C (Maybe a) a
      instance L (Maybe a) a

    When solving the superclasses of the (C (Maybe a) a) instance, we get
      [G] C a b, and hance by superclasses, [G] G a, [G] L a b
      [W] G (Maybe a)
    Use the instance decl to get
      [W] C a beta
    Generate its superclass
      [W] L a beta.  Now using fundeps, combine with [G] L a b to get
      [W] beta ~ b
    which is what we want.

Note [Danger of adding superclasses during solving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here's a serious, but now out-dated example, from #4497:

   class Num (RealOf t) => Normed t
   type family RealOf x

Assume the generated wanted constraint is:
   [W] RealOf e ~ e
   [W] Normed e

If we were to be adding the superclasses during simplification we'd get:
   [W] RealOf e ~ e
   [W] Normed e
   [W] RealOf e ~ fuv
   [W] Num fuv
==>
   e := fuv, Num fuv, Normed fuv, RealOf fuv ~ fuv

While looks exactly like our original constraint. If we add the
superclass of (Normed fuv) again we'd loop.  By adding superclasses
definitely only once, during canonicalisation, this situation can't
happen.

Note [Nested quantified constraint superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (typecheck/should_compile/T17202)

  class C1 a
  class (forall c. C1 c) => C2 a
  class (forall b. (b ~ F a) => C2 a) => C3 a

Elsewhere in the code, we get a [G] g1 :: C3 a. We expand its superclass
to get [G] g2 :: (forall b. (b ~ F a) => C2 a). This constraint has a
superclass, as well. But we now must be careful: we cannot just add
(forall c. C1 c) as a Given, because we need to remember g2's context.
That new constraint is Given only when forall b. (b ~ F a) is true.

It's tempting to make the new Given be (forall b. (b ~ F a) => forall c. C1 c),
but that's problematic, because it's nested, and ForAllPred is not capable
of representing a nested quantified constraint. (We could change ForAllPred
to allow this, but the solution in this Note is much more local and simpler.)

So, we swizzle it around to get (forall b c. (b ~ F a) => C1 c).

More generally, if we are expanding the superclasses of
  g0 :: forall tvs. theta => cls tys
and find a superclass constraint
  forall sc_tvs. sc_theta => sc_inner_pred
we must have a selector
  sel_id :: forall cls_tvs. cls cls_tvs -> forall sc_tvs. sc_theta => sc_inner_pred
and thus build
  g_sc :: forall tvs sc_tvs. theta => sc_theta => sc_inner_pred
  g_sc = /\ tvs. /\ sc_tvs. \ theta_ids. \ sc_theta_ids.
         sel_id tys (g0 tvs theta_ids) sc_tvs sc_theta_ids

Actually, we cheat a bit by eta-reducing: note that sc_theta_ids are both the
last bound variables and the last arguments. This avoids the need to produce
the sc_theta_ids at all. So our final construction is

  g_sc = /\ tvs. /\ sc_tvs. \ theta_ids.
         sel_id tys (g0 tvs theta_ids) sc_tvs

  -}

makeSuperClasses :: [Ct] -> TcS [Ct]
-- Returns strict superclasses, transitively, see Note [The superclass story]
-- See Note [The superclass story]
-- The loop-breaking here follows Note [Expanding superclasses] in GHC.Tc.Utils.TcType
-- Specifically, for an incoming (C t) constraint, we return all of (C t)'s
--    superclasses, up to /and including/ the first repetition of C
--
-- Example:  class D a => C a
--           class C [a] => D a
-- makeSuperClasses (C x) will return (D x, C [x])
--
-- NB: the incoming constraints have had their cc_pend_sc flag already
--     flipped to False, by isPendingScDict, so we are /obliged/ to at
--     least produce the immediate superclasses
makeSuperClasses :: [Ct] -> TcS [Ct]
makeSuperClasses [Ct]
cts = forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
concatMapM Ct -> TcS [Ct]
go [Ct]
cts
  where
    go :: Ct -> TcS [Ct]
go (CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class = Class
cls, cc_tyargs :: Ct -> [Type]
cc_tyargs = [Type]
tys })
      = CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [] [] Class
cls [Type]
tys
    go (CQuantCan (QCI { qci_pred :: QCInst -> Type
qci_pred = Type
pred, qci_ev :: QCInst -> CtEvidence
qci_ev = CtEvidence
ev }))
      = forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (Type -> Bool
isClassPred Type
pred) (forall a. Outputable a => a -> SDoc
ppr Type
pred) forall a b. (a -> b) -> a -> b
$  -- The cts should all have
                                                   -- class pred heads
        CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
      where
        ([TcTyVar]
tvs, [Type]
theta, Class
cls, [Type]
tys) = Type -> ([TcTyVar], [Type], Class, [Type])
tcSplitDFunTy (CtEvidence -> Type
ctEvPred CtEvidence
ev)
    go Ct
ct = forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"makeSuperClasses" (forall a. Outputable a => a -> SDoc
ppr Ct
ct)

mkStrictSuperClasses
    :: CtEvidence
    -> [TyVar] -> ThetaType  -- These two args are non-empty only when taking
                             -- superclasses of a /quantified/ constraint
    -> Class -> [Type] -> TcS [Ct]
-- Return constraints for the strict superclasses of
--   ev :: forall as. theta => cls tys
mkStrictSuperClasses :: CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
  = NameSet
-> CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mk_strict_superclasses (Name -> NameSet
unitNameSet (Class -> Name
className Class
cls))
                           CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys

mk_strict_superclasses :: NameSet -> CtEvidence
                       -> [TyVar] -> ThetaType
                       -> Class -> [Type] -> TcS [Ct]
-- Always return the immediate superclasses of (cls tys);
-- and expand their superclasses, provided none of them are in rec_clss
-- nor are repeated
mk_strict_superclasses :: NameSet
-> CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mk_strict_superclasses NameSet
rec_clss (CtGiven { ctev_evar :: CtEvidence -> TcTyVar
ctev_evar = TcTyVar
evar, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc })
                       [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
  = forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
concatMapM TcTyVar -> TcS [Ct]
do_one_given forall a b. (a -> b) -> a -> b
$
    Class -> [TcTyVar]
classSCSelIds Class
cls
  where
    dict_ids :: [TcTyVar]
dict_ids  = [Type] -> [TcTyVar]
mkTemplateLocals [Type]
theta
    size :: TypeSize
size      = [Type] -> TypeSize
sizeTypes [Type]
tys

    do_one_given :: TcTyVar -> TcS [Ct]
do_one_given TcTyVar
sel_id
      | HasDebugCallStack => Type -> Bool
isUnliftedType Type
sc_pred
         -- NB: class superclasses are never representation-polymorphic,
         -- so isUnliftedType is OK here.
      , Bool -> Bool
not (forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs Bool -> Bool -> Bool
&& forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Type]
theta)
      = -- See Note [Equality superclasses in quantified constraints]
        forall (m :: * -> *) a. Monad m => a -> m a
return []
      | Bool
otherwise
      = do { CtEvidence
given_ev <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
sc_loc forall a b. (a -> b) -> a -> b
$
                         TcTyVar -> Type -> (Type, EvTerm)
mk_given_desc TcTyVar
sel_id Type
sc_pred
           ; NameSet -> CtEvidence -> [TcTyVar] -> [Type] -> Type -> TcS [Ct]
mk_superclasses NameSet
rec_clss CtEvidence
given_ev [TcTyVar]
tvs [Type]
theta Type
sc_pred }
      where
        sc_pred :: Type
sc_pred = TcTyVar -> [Type] -> Type
classMethodInstTy TcTyVar
sel_id [Type]
tys

      -- See Note [Nested quantified constraint superclasses]
    mk_given_desc :: Id -> PredType -> (PredType, EvTerm)
    mk_given_desc :: TcTyVar -> Type -> (Type, EvTerm)
mk_given_desc TcTyVar
sel_id Type
sc_pred
      = (Type
swizzled_pred, EvTerm
swizzled_evterm)
      where
        ([TcTyVar]
sc_tvs, Type
sc_rho)          = Type -> ([TcTyVar], Type)
splitForAllTyCoVars Type
sc_pred
        ([Scaled Type]
sc_theta, Type
sc_inner_pred) = Type -> ([Scaled Type], Type)
splitFunTys Type
sc_rho

        all_tvs :: [TcTyVar]
all_tvs       = [TcTyVar]
tvs forall a. [a] -> [a] -> [a]
`chkAppend` [TcTyVar]
sc_tvs
        all_theta :: [Type]
all_theta     = [Type]
theta forall a. [a] -> [a] -> [a]
`chkAppend` (forall a b. (a -> b) -> [a] -> [b]
map forall a. Scaled a -> a
scaledThing [Scaled Type]
sc_theta)
        swizzled_pred :: Type
swizzled_pred = [TcTyVar] -> [Type] -> Type -> Type
mkInfSigmaTy [TcTyVar]
all_tvs [Type]
all_theta Type
sc_inner_pred

        -- evar :: forall tvs. theta => cls tys
        -- sel_id :: forall cls_tvs. cls cls_tvs
        --                        -> forall sc_tvs. sc_theta => sc_inner_pred
        -- swizzled_evterm :: forall tvs sc_tvs. theta => sc_theta => sc_inner_pred
        swizzled_evterm :: EvTerm
swizzled_evterm = EvExpr -> EvTerm
EvExpr forall a b. (a -> b) -> a -> b
$
          forall b. [b] -> Expr b -> Expr b
mkLams [TcTyVar]
all_tvs forall a b. (a -> b) -> a -> b
$
          forall b. [b] -> Expr b -> Expr b
mkLams [TcTyVar]
dict_ids forall a b. (a -> b) -> a -> b
$
          forall b. TcTyVar -> Expr b
Var TcTyVar
sel_id
            forall b. Expr b -> [Type] -> Expr b
`mkTyApps` [Type]
tys
            forall b. Expr b -> Expr b -> Expr b
`App` (TcTyVar -> EvExpr
evId TcTyVar
evar forall b. Expr b -> [TcTyVar] -> Expr b
`mkVarApps` ([TcTyVar]
tvs forall a. [a] -> [a] -> [a]
++ [TcTyVar]
dict_ids))
            forall b. Expr b -> [TcTyVar] -> Expr b
`mkVarApps` [TcTyVar]
sc_tvs

    sc_loc :: CtLoc
sc_loc
       | Class -> Bool
isCTupleClass Class
cls
       = CtLoc
loc   -- For tuple predicates, just take them apart, without
               -- adding their (large) size into the chain.  When we
               -- get down to a base predicate, we'll include its size.
               -- #10335

       |  Class -> Bool
isEqPredClass Class
cls
       Bool -> Bool -> Bool
|| Class
cls forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
coercibleTyConKey
       = CtLoc
loc   -- The only superclasses of ~, ~~, and Coercible are primitive
               -- equalities, and they don't use the InstSCOrigin mechanism
               -- detailed in Note [Solving superclass constraints] in
               -- GHC.Tc.TyCl.Instance. Skip for a tiny performance win.

         -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
         -- for explantation of InstSCOrigin and Note [Replacement vs keeping] in
         -- GHC.Tc.Solver.Interact for why we need OtherSCOrigin and depths
       | Bool
otherwise
       = CtLoc
loc { ctl_origin :: CtOrigin
ctl_origin = CtOrigin
new_orig }
       where
         new_orig :: CtOrigin
new_orig = case CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc of
            -- these cases are when we have something that's already a superclass constraint
           InstSCOrigin  Int
sc_depth TypeSize
n  -> Int -> TypeSize -> CtOrigin
InstSCOrigin  (Int
sc_depth forall a. Num a => a -> a -> a
+ Int
1) (TypeSize
n forall a. Ord a => a -> a -> a
`max` TypeSize
size)
           OtherSCOrigin Int
sc_depth SkolemInfoAnon
si -> Int -> SkolemInfoAnon -> CtOrigin
OtherSCOrigin (Int
sc_depth forall a. Num a => a -> a -> a
+ Int
1) SkolemInfoAnon
si

            -- these cases do not already have a superclass constraint: depth starts at 1
           GivenOrigin SkolemInfoAnon
InstSkol      -> Int -> TypeSize -> CtOrigin
InstSCOrigin  Int
1 TypeSize
size
           GivenOrigin SkolemInfoAnon
other_skol    -> Int -> SkolemInfoAnon -> CtOrigin
OtherSCOrigin Int
1 SkolemInfoAnon
other_skol

           CtOrigin
other_orig                -> forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"Given constraint without given origin" forall a b. (a -> b) -> a -> b
$
                                        forall a. Outputable a => a -> SDoc
ppr TcTyVar
evar SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr CtOrigin
other_orig

mk_strict_superclasses NameSet
rec_clss CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
  | forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all Type -> Bool
noFreeVarsOfType [Type]
tys
  = forall (m :: * -> *) a. Monad m => a -> m a
return [] -- Wanteds with no variables yield no superclass constraints.
              -- See Note [Improvement from Ground Wanteds]

  | Bool
otherwise -- Wanted case, just add Wanted superclasses
              -- that can lead to improvement.
  = forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs Bool -> Bool -> Bool
&& forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Type]
theta) (forall a. Outputable a => a -> SDoc
ppr [TcTyVar]
tvs SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [Type]
theta) forall a b. (a -> b) -> a -> b
$
    forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
concatMapM Type -> TcS [Ct]
do_one (Class -> [Type] -> [Type]
immSuperClasses Class
cls [Type]
tys)
  where
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
`updateCtLocOrigin` Type -> CtOrigin -> CtOrigin
WantedSuperclassOrigin (CtEvidence -> Type
ctEvPred CtEvidence
ev)

    do_one :: Type -> TcS [Ct]
do_one Type
sc_pred
      = do { String -> SDoc -> TcS ()
traceTcS String
"mk_strict_superclasses Wanted" (forall a. Outputable a => a -> SDoc
ppr (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys) SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
sc_pred)
           ; CtEvidence
sc_ev <- CtLoc -> RewriterSet -> Type -> TcS CtEvidence
newWantedNC CtLoc
loc (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
ev) Type
sc_pred
           ; NameSet -> CtEvidence -> [TcTyVar] -> [Type] -> Type -> TcS [Ct]
mk_superclasses NameSet
rec_clss CtEvidence
sc_ev [] [] Type
sc_pred }

{- Note [Improvement from Ground Wanteds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose class C b a => D a b
and consider
  [W] D Int Bool
Is there any point in emitting [W] C Bool Int?  No!  The only point of
emitting superclass constraints for W constraints is to get
improvement, extra unifications that result from functional
dependencies.  See Note [Why adding superclasses can help] above.

But no variables means no improvement; case closed.
-}

mk_superclasses :: NameSet -> CtEvidence
                -> [TyVar] -> ThetaType -> PredType -> TcS [Ct]
-- Return this constraint, plus its superclasses, if any
mk_superclasses :: NameSet -> CtEvidence -> [TcTyVar] -> [Type] -> Type -> TcS [Ct]
mk_superclasses NameSet
rec_clss CtEvidence
ev [TcTyVar]
tvs [Type]
theta Type
pred
  | ClassPred Class
cls [Type]
tys <- Type -> Pred
classifyPredType Type
pred
  = NameSet
-> CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mk_superclasses_of NameSet
rec_clss CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys

  | Bool
otherwise   -- Superclass is not a class predicate
  = forall (m :: * -> *) a. Monad m => a -> m a
return [CtEvidence -> Ct
mkNonCanonical CtEvidence
ev]

mk_superclasses_of :: NameSet -> CtEvidence
                   -> [TyVar] -> ThetaType -> Class -> [Type]
                   -> TcS [Ct]
-- Always return this class constraint,
-- and expand its superclasses
mk_superclasses_of :: NameSet
-> CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mk_superclasses_of NameSet
rec_clss CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
  | Bool
loop_found = do { String -> SDoc -> TcS ()
traceTcS String
"mk_superclasses_of: loop" (forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr [Type]
tys)
                    ; forall (m :: * -> *) a. Monad m => a -> m a
return [Ct
this_ct] }  -- cc_pend_sc of this_ct = True
  | Bool
otherwise  = do { String -> SDoc -> TcS ()
traceTcS String
"mk_superclasses_of" ([SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr [Type]
tys
                                                          , forall a. Outputable a => a -> SDoc
ppr (Class -> Bool
isCTupleClass Class
cls)
                                                          , forall a. Outputable a => a -> SDoc
ppr NameSet
rec_clss
                                                          ])
                    ; [Ct]
sc_cts <- NameSet
-> CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mk_strict_superclasses NameSet
rec_clss' CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
                    ; forall (m :: * -> *) a. Monad m => a -> m a
return (Ct
this_ct forall a. a -> [a] -> [a]
: [Ct]
sc_cts) }
                                   -- cc_pend_sc of this_ct = False
  where
    cls_nm :: Name
cls_nm     = Class -> Name
className Class
cls
    loop_found :: Bool
loop_found = Bool -> Bool
not (Class -> Bool
isCTupleClass Class
cls) Bool -> Bool -> Bool
&& Name
cls_nm Name -> NameSet -> Bool
`elemNameSet` NameSet
rec_clss
                 -- Tuples never contribute to recursion, and can be nested
    rec_clss' :: NameSet
rec_clss'  = NameSet
rec_clss NameSet -> Name -> NameSet
`extendNameSet` Name
cls_nm

    this_ct :: Ct
this_ct | forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs, forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Type]
theta
            = CDictCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Class
cc_class = Class
cls, cc_tyargs :: [Type]
cc_tyargs = [Type]
tys
                       , cc_pend_sc :: Bool
cc_pend_sc = Bool
loop_found, cc_fundeps :: Bool
cc_fundeps = Class -> Bool
classHasFds Class
cls }
                 -- NB: If there is a loop, we cut off, so we have not
                 --     added the superclasses, hence cc_pend_sc = True
            | Bool
otherwise
            = QCInst -> Ct
CQuantCan (QCI { qci_tvs :: [TcTyVar]
qci_tvs = [TcTyVar]
tvs, qci_pred :: Type
qci_pred = Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys
                             , qci_ev :: CtEvidence
qci_ev = CtEvidence
ev
                             , qci_pend_sc :: Bool
qci_pend_sc = Bool
loop_found })


{- Note [Equality superclasses in quantified constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#15359, #15593, #15625)
  f :: (forall a. theta => a ~ b) => stuff

It's a bit odd to have a local, quantified constraint for `(a~b)`,
but some people want such a thing (see the tickets). And for
Coercible it is definitely useful
  f :: forall m. (forall p q. Coercible p q => Coercible (m p) (m q)))
                 => stuff

Moreover it's not hard to arrange; we just need to look up /equality/
constraints in the quantified-constraint environment, which we do in
GHC.Tc.Solver.Interact.doTopReactOther.

There is a wrinkle though, in the case where 'theta' is empty, so
we have
  f :: (forall a. a~b) => stuff

Now, potentially, the superclass machinery kicks in, in
makeSuperClasses, giving us a a second quantified constraint
       (forall a. a ~# b)
BUT this is an unboxed value!  And nothing has prepared us for
dictionary "functions" that are unboxed.  Actually it does just
about work, but the simplifier ends up with stuff like
   case (/\a. eq_sel d) of df -> ...(df @Int)...
and fails to simplify that any further.  And it doesn't satisfy
isPredTy any more.

So for now we simply decline to take superclasses in the quantified
case.  Instead we have a special case in GHC.Tc.Solver.Interact.doTopReactOther,
which looks for primitive equalities specially in the quantified
constraints.

See also Note [Evidence for quantified constraints] in GHC.Core.Predicate.


************************************************************************
*                                                                      *
*                      Irreducibles canonicalization
*                                                                      *
************************************************************************
-}

canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
-- Precondition: ty not a tuple and no other evidence form
canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
canIrred CtEvidence
ev
  = do { let pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev
       ; String -> SDoc -> TcS ()
traceTcS String
"can_pred" (String -> SDoc
text String
"IrredPred = " SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
pred)
       ; (Reduction
redn, RewriterSet
rewriters) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
pred
       ; RewriterSet
-> CtEvidence -> Reduction -> TcS (StopOrContinue CtEvidence)
rewriteEvidence RewriterSet
rewriters CtEvidence
ev Reduction
redn forall a b.
TcS (StopOrContinue a)
-> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
`andWhenContinue` \ CtEvidence
new_ev ->

    do { -- Re-classify, in case rewriting has improved its shape
         -- Code is like the canNC, except
         -- that the IrredPred branch stops work
       ; case Type -> Pred
classifyPredType (CtEvidence -> Type
ctEvPred CtEvidence
new_ev) of
           ClassPred Class
cls [Type]
tys     -> CtEvidence -> Class -> [Type] -> TcS (StopOrContinue Ct)
canClassNC CtEvidence
new_ev Class
cls [Type]
tys
           EqPred EqRel
eq_rel Type
ty1 Type
ty2 -> -- IrredPreds have kind Constraint, so
                                    -- cannot become EqPreds
                                    forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"canIrred: EqPred"
                                      (forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
ty2)
           ForAllPred [TcTyVar]
tvs [Type]
th Type
p   -> -- this is highly suspect; Quick Look
                                    -- should never leave a meta-var filled
                                    -- in with a polytype. This is #18987.
                                    do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:forall" (forall a. Outputable a => a -> SDoc
ppr Type
pred)
                                       CtEvidence
-> [TcTyVar] -> [Type] -> Type -> TcS (StopOrContinue Ct)
canForAllNC CtEvidence
ev [TcTyVar]
tvs [Type]
th Type
p
           IrredPred {}          -> forall a. a -> TcS (StopOrContinue a)
continueWith forall a b. (a -> b) -> a -> b
$
                                    CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
IrredShapeReason CtEvidence
new_ev } }

{- *********************************************************************
*                                                                      *
*                      Quantified predicates
*                                                                      *
********************************************************************* -}

{- Note [Quantified constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The -XQuantifiedConstraints extension allows type-class contexts like this:

  data Rose f x = Rose x (f (Rose f x))

  instance (Eq a, forall b. Eq b => Eq (f b))
        => Eq (Rose f a)  where
    (Rose x1 rs1) == (Rose x2 rs2) = x1==x2 && rs1 == rs2

Note the (forall b. Eq b => Eq (f b)) in the instance contexts.
This quantified constraint is needed to solve the
 [W] (Eq (f (Rose f x)))
constraint which arises form the (==) definition.

The wiki page is
  https://gitlab.haskell.org/ghc/ghc/wikis/quantified-constraints
which in turn contains a link to the GHC Proposal where the change
is specified, and a Haskell Symposium paper about it.

We implement two main extensions to the design in the paper:

 1. We allow a variable in the instance head, e.g.
      f :: forall m a. (forall b. m b) => D (m a)
    Notice the 'm' in the head of the quantified constraint, not
    a class.

 2. We support superclasses to quantified constraints.
    For example (contrived):
      f :: (Ord b, forall b. Ord b => Ord (m b)) => m a -> m a -> Bool
      f x y = x==y
    Here we need (Eq (m a)); but the quantified constraint deals only
    with Ord.  But we can make it work by using its superclass.

Here are the moving parts
  * Language extension {-# LANGUAGE QuantifiedConstraints #-}
    and add it to ghc-boot-th:GHC.LanguageExtensions.Type.Extension

  * A new form of evidence, EvDFun, that is used to discharge
    such wanted constraints

  * checkValidType gets some changes to accept forall-constraints
    only in the right places.

  * Predicate.Pred gets a new constructor ForAllPred, and
    and classifyPredType analyses a PredType to decompose
    the new forall-constraints

  * GHC.Tc.Solver.Monad.InertCans gets an extra field, inert_insts,
    which holds all the Given forall-constraints.  In effect,
    such Given constraints are like local instance decls.

  * When trying to solve a class constraint, via
    GHC.Tc.Solver.Interact.matchInstEnv, use the InstEnv from inert_insts
    so that we include the local Given forall-constraints
    in the lookup.  (See GHC.Tc.Solver.Monad.getInstEnvs.)

  * GHC.Tc.Solver.Canonical.canForAll deals with solving a
    forall-constraint.  See
       Note [Solving a Wanted forall-constraint]

  * We augment the kick-out code to kick out an inert
    forall constraint if it can be rewritten by a new
    type equality; see GHC.Tc.Solver.Monad.kick_out_rewritable

Note that a quantified constraint is never /inferred/
(by GHC.Tc.Solver.simplifyInfer).  A function can only have a
quantified constraint in its type if it is given an explicit
type signature.

-}

canForAllNC :: CtEvidence -> [TyVar] -> TcThetaType -> TcPredType
            -> TcS (StopOrContinue Ct)
canForAllNC :: CtEvidence
-> [TcTyVar] -> [Type] -> Type -> TcS (StopOrContinue Ct)
canForAllNC CtEvidence
ev [TcTyVar]
tvs [Type]
theta Type
pred
  | CtEvidence -> Bool
isGiven CtEvidence
ev  -- See Note [Eagerly expand given superclasses]
  , Just (Class
cls, [Type]
tys) <- Maybe (Class, [Type])
cls_pred_tys_maybe
  = do { [Ct]
sc_cts <- CtEvidence -> [TcTyVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [TcTyVar]
tvs [Type]
theta Class
cls [Type]
tys
       ; [Ct] -> TcS ()
emitWork [Ct]
sc_cts
       ; CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev Bool
False }

  | Bool
otherwise
  = CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev (forall a. Maybe a -> Bool
isJust Maybe (Class, [Type])
cls_pred_tys_maybe)

  where
    cls_pred_tys_maybe :: Maybe (Class, [Type])
cls_pred_tys_maybe = Type -> Maybe (Class, [Type])
getClassPredTys_maybe Type
pred

canForAll :: CtEvidence -> Bool -> TcS (StopOrContinue Ct)
-- We have a constraint (forall as. blah => C tys)
canForAll :: CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev Bool
pend_sc
  = do { -- First rewrite it to apply the current substitution
         let pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev
       ; (Reduction
redn, RewriterSet
rewriters) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
pred
       ; RewriterSet
-> CtEvidence -> Reduction -> TcS (StopOrContinue CtEvidence)
rewriteEvidence RewriterSet
rewriters CtEvidence
ev Reduction
redn forall a b.
TcS (StopOrContinue a)
-> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
`andWhenContinue` \ CtEvidence
new_ev ->

    do { -- Now decompose into its pieces and solve it
         -- (It takes a lot less code to rewrite before decomposing.)
       ; case Type -> Pred
classifyPredType (CtEvidence -> Type
ctEvPred CtEvidence
new_ev) of
           ForAllPred [TcTyVar]
tvs [Type]
theta Type
pred
              -> CtEvidence
-> [TcTyVar] -> [Type] -> Type -> Bool -> TcS (StopOrContinue Ct)
solveForAll CtEvidence
new_ev [TcTyVar]
tvs [Type]
theta Type
pred Bool
pend_sc
           Pred
_  -> forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"canForAll" (forall a. Outputable a => a -> SDoc
ppr CtEvidence
new_ev)
    } }

solveForAll :: CtEvidence -> [TyVar] -> TcThetaType -> PredType -> Bool
            -> TcS (StopOrContinue Ct)
solveForAll :: CtEvidence
-> [TcTyVar] -> [Type] -> Type -> Bool -> TcS (StopOrContinue Ct)
solveForAll ev :: CtEvidence
ev@(CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest, ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc })
            [TcTyVar]
tvs [Type]
theta Type
pred Bool
_pend_sc
  = -- See Note [Solving a Wanted forall-constraint]
    forall a. TcLclEnv -> TcS a -> TcS a
setLclEnv (CtLoc -> TcLclEnv
ctLocEnv CtLoc
loc) forall a b. (a -> b) -> a -> b
$
    -- This setLclEnv is important: the emitImplicationTcS uses that
    -- TcLclEnv for the implication, and that in turn sets the location
    -- for the Givens when solving the constraint (#21006)
    do { SkolemInfo
skol_info <- forall (m :: * -> *). MonadIO m => SkolemInfoAnon -> m SkolemInfo
mkSkolemInfo SkolemInfoAnon
QuantCtxtSkol
       ; let empty_subst :: Subst
empty_subst = InScopeSet -> Subst
mkEmptySubst forall a b. (a -> b) -> a -> b
$ VarSet -> InScopeSet
mkInScopeSet forall a b. (a -> b) -> a -> b
$
                           [Type] -> VarSet
tyCoVarsOfTypes (Type
predforall a. a -> [a] -> [a]
:[Type]
theta) VarSet -> [TcTyVar] -> VarSet
`delVarSetList` [TcTyVar]
tvs
       ; (Subst
subst, [TcTyVar]
skol_tvs) <- SkolemInfo -> Subst -> [TcTyVar] -> TcS (Subst, [TcTyVar])
tcInstSkolTyVarsX SkolemInfo
skol_info Subst
empty_subst [TcTyVar]
tvs
       ; [TcTyVar]
given_ev_vars <- forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM Type -> TcS TcTyVar
newEvVar (HasDebugCallStack => Subst -> [Type] -> [Type]
substTheta Subst
subst [Type]
theta)

       ; (TcLevel
lvl, (TcTyVar
w_id, Bag Ct
wanteds))
             <- forall a. SDoc -> TcS a -> TcS (TcLevel, a)
pushLevelNoWorkList (forall a. Outputable a => a -> SDoc
ppr SkolemInfo
skol_info) forall a b. (a -> b) -> a -> b
$
                do { CtEvidence
wanted_ev <- CtLoc -> RewriterSet -> Type -> TcS CtEvidence
newWantedEvVarNC CtLoc
loc RewriterSet
rewriters forall a b. (a -> b) -> a -> b
$
                                  HasDebugCallStack => Subst -> Type -> Type
substTy Subst
subst Type
pred
                   ; forall (m :: * -> *) a. Monad m => a -> m a
return ( CtEvidence -> TcTyVar
ctEvEvId CtEvidence
wanted_ev
                            , forall a. a -> Bag a
unitBag (CtEvidence -> Ct
mkNonCanonical CtEvidence
wanted_ev)) }

      ; TcEvBinds
ev_binds <- TcLevel
-> SkolemInfoAnon
-> [TcTyVar]
-> [TcTyVar]
-> Bag Ct
-> TcS TcEvBinds
emitImplicationTcS TcLevel
lvl (SkolemInfo -> SkolemInfoAnon
getSkolemInfo SkolemInfo
skol_info) [TcTyVar]
skol_tvs
                                       [TcTyVar]
given_ev_vars Bag Ct
wanteds

      ; TcEvDest -> EvTerm -> TcS ()
setWantedEvTerm TcEvDest
dest forall a b. (a -> b) -> a -> b
$
        EvFun { et_tvs :: [TcTyVar]
et_tvs = [TcTyVar]
skol_tvs, et_given :: [TcTyVar]
et_given = [TcTyVar]
given_ev_vars
              , et_binds :: TcEvBinds
et_binds = TcEvBinds
ev_binds, et_body :: TcTyVar
et_body = TcTyVar
w_id }

      ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Wanted forall-constraint" }

 -- See Note [Solving a Given forall-constraint]
solveForAll ev :: CtEvidence
ev@(CtGiven {}) [TcTyVar]
tvs [Type]
_theta Type
pred Bool
pend_sc
  = do { QCInst -> TcS ()
addInertForAll QCInst
qci
       ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Given forall-constraint" }
  where
    qci :: QCInst
qci = QCI { qci_ev :: CtEvidence
qci_ev = CtEvidence
ev, qci_tvs :: [TcTyVar]
qci_tvs = [TcTyVar]
tvs
              , qci_pred :: Type
qci_pred = Type
pred, qci_pend_sc :: Bool
qci_pend_sc = Bool
pend_sc }

{- Note [Solving a Wanted forall-constraint]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Solving a wanted forall (quantified) constraint
  [W] df :: forall ab. (Eq a, Ord b) => C x a b
is delightfully easy.   Just build an implication constraint
    forall ab. (g1::Eq a, g2::Ord b) => [W] d :: C x a
and discharge df thus:
    df = /\ab. \g1 g2. let <binds> in d
where <binds> is filled in by solving the implication constraint.
All the machinery is to hand; there is little to do.

Note [Solving a Given forall-constraint]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For a Given constraint
  [G] df :: forall ab. (Eq a, Ord b) => C x a b
we just add it to TcS's local InstEnv of known instances,
via addInertForall.  Then, if we look up (C x Int Bool), say,
we'll find a match in the InstEnv.

************************************************************************
*                                                                      *
*        Equalities
*                                                                      *
************************************************************************

Note [Canonicalising equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to canonicalise an equality, we look at the structure of the
two types at hand, looking for similarities. A difficulty is that the
types may look dissimilar before rewriting but similar after rewriting.
However, we don't just want to jump in and rewrite right away, because
this might be wasted effort. So, after looking for similarities and failing,
we rewrite and then try again. Of course, we don't want to loop, so we
track whether or not we've already rewritten.

It is conceivable to do a better job at tracking whether or not a type
is rewritten, but this is left as future work. (Mar '15)

Note [Decomposing FunTy]
~~~~~~~~~~~~~~~~~~~~~~~~
can_eq_nc' may attempt to decompose a FunTy that is un-zonked.  This
means that we may very well have a FunTy containing a type of some
unknown kind. For instance, we may have,

    FunTy (a :: k) Int

Where k is a unification variable. So the calls to getRuntimeRep_maybe may
fail (returning Nothing).  In that case we'll fall through, zonk, and try again.
Zonking should fill the variable k, meaning that decomposition will succeed the
second time around.

Also note that we require the AnonArgFlag to match.  This will stop
us decomposing
   (Int -> Bool)  ~  (Show a => blah)
It's as if we treat (->) and (=>) as different type constructors.
-}

canEqNC :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
  = do { Either (Pair Type) Type
result <- Type -> Type -> TcS (Either (Pair Type) Type)
zonk_eq_types Type
ty1 Type
ty2
       ; case Either (Pair Type) Type
result of
           Left (Pair Type
ty1' Type
ty2') -> Bool
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
False CtEvidence
ev EqRel
eq_rel Type
ty1' Type
ty1 Type
ty2' Type
ty2
           Right Type
ty              -> CtEvidence -> EqRel -> Type -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
ty }

can_eq_nc
   :: Bool            -- True => both types are rewritten
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue Ct)
can_eq_nc :: Bool
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
rewritten CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr Bool
rewritten, forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel, forall a. Outputable a => a -> SDoc
ppr Type
ty1, forall a. Outputable a => a -> SDoc
ppr Type
ps_ty1, forall a. Outputable a => a -> SDoc
ppr Type
ty2, forall a. Outputable a => a -> SDoc
ppr Type
ps_ty2 ]
       ; GlobalRdrEnv
rdr_env <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
       ; (FamInstEnv, FamInstEnv)
fam_insts <- TcS (FamInstEnv, FamInstEnv)
getFamInstEnvs
       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
fam_insts CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2 }

can_eq_nc'
   :: Bool           -- True => both input types are rewritten
   -> GlobalRdrEnv   -- needed to see which newtypes are in scope
   -> FamInstEnvs    -- needed to unwrap data instances
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue Ct)

-- See Note [Comparing nullary type synonyms] in GHC.Core.Type.
can_eq_nc' :: Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
_flat GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: Type
ty1@(TyConApp TyCon
tc1 []) Type
_ps_ty1 (TyConApp TyCon
tc2 []) Type
_ps_ty2
  | TyCon
tc1 forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  = CtEvidence -> EqRel -> Type -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
ty1

-- Expand synonyms first; see Note [Type synonyms and canonicalization]
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  | Just Type
ty1' <- Type -> Maybe Type
tcView Type
ty1 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
ty1' Type
ps_ty1 Type
ty2  Type
ps_ty2
  | Just Type
ty2' <- Type -> Maybe Type
tcView Type
ty2 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
ty1  Type
ps_ty1 Type
ty2' Type
ps_ty2

-- need to check for reflexivity in the ReprEq case.
-- See Note [Eager reflexivity check]
-- Check only when rewritten because the zonk_eq_types check in canEqNC takes
-- care of the non-rewritten case.
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
ReprEq Type
ty1 Type
_ Type
ty2 Type
_
  | Type
ty1 HasDebugCallStack => Type -> Type -> Bool
`tcEqType` Type
ty2
  = CtEvidence -> EqRel -> Type -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
ReprEq Type
ty1

-- When working with ReprEq, unwrap newtypes.
-- See Note [Unwrap newtypes first]
-- This must be above the TyVarTy case, in order to guarantee (TyEq:N)
can_eq_nc' Bool
_rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), Type)
stuff1 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv
-> Type
-> Maybe ((Bag GlobalRdrElt, TcCoercion), Type)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env Type
ty1
  = CtEvidence
-> SwapFlag
-> Type
-> ((Bag GlobalRdrElt, TcCoercion), Type)
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_newtype_nc CtEvidence
ev SwapFlag
NotSwapped Type
ty1 ((Bag GlobalRdrElt, TcCoercion), Type)
stuff1 Type
ty2 Type
ps_ty2

  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), Type)
stuff2 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv
-> Type
-> Maybe ((Bag GlobalRdrElt, TcCoercion), Type)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env Type
ty2
  = CtEvidence
-> SwapFlag
-> Type
-> ((Bag GlobalRdrElt, TcCoercion), Type)
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_newtype_nc CtEvidence
ev SwapFlag
IsSwapped Type
ty2 ((Bag GlobalRdrElt, TcCoercion), Type)
stuff2 Type
ty1 Type
ps_ty1

-- Then, get rid of casts
can_eq_nc' Bool
rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel (CastTy Type
ty1 TcCoercion
co1) Type
_ Type
ty2 Type
ps_ty2
  | forall a. Maybe a -> Bool
isNothing (Type -> Maybe CanEqLHS
canEqLHS_maybe Type
ty2)  -- See (3) in Note [Equalities with incompatible kinds]
  = Bool
-> CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> TcCoercion
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCast Bool
rewritten CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped Type
ty1 TcCoercion
co1 Type
ty2 Type
ps_ty2
can_eq_nc' Bool
rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ps_ty1 (CastTy Type
ty2 TcCoercion
co2) Type
_
  | forall a. Maybe a -> Bool
isNothing (Type -> Maybe CanEqLHS
canEqLHS_maybe Type
ty1)  -- See (3) in Note [Equalities with incompatible kinds]
  = Bool
-> CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> TcCoercion
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCast Bool
rewritten CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped Type
ty2 TcCoercion
co2 Type
ty1 Type
ps_ty1

----------------------
-- Otherwise try to decompose
----------------------

-- Literals
can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: Type
ty1@(LitTy TyLit
l1) Type
_ (LitTy TyLit
l2) Type
_
 | TyLit
l1 forall a. Eq a => a -> a -> Bool
== TyLit
l2
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev (TcCoercion -> EvTerm
evCoercion forall a b. (a -> b) -> a -> b
$ Role -> Type -> TcCoercion
mkReflCo (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
ty1)
       ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Equal LitTy" }

-- Decompose FunTy: (s -> t) and (c => t)
-- NB: don't decompose (Int -> blah) ~ (Show a => blah)
can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           (FunTy { ft_mult :: Type -> Type
ft_mult = Type
am1, ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
af1, ft_arg :: Type -> Type
ft_arg = Type
ty1a, ft_res :: Type -> Type
ft_res = Type
ty1b }) Type
_ps_ty1
           (FunTy { ft_mult :: Type -> Type
ft_mult = Type
am2, ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
af2, ft_arg :: Type -> Type
ft_arg = Type
ty2a, ft_res :: Type -> Type
ft_res = Type
ty2b }) Type
_ps_ty2
  | AnonArgFlag
af1 forall a. Eq a => a -> a -> Bool
== AnonArgFlag
af2   -- Don't decompose (Int -> blah) ~ (Show a => blah)
  , Just Type
ty1a_rep <- HasDebugCallStack => Type -> Maybe Type
getRuntimeRep_maybe Type
ty1a  -- getRutimeRep_maybe:
  , Just Type
ty1b_rep <- HasDebugCallStack => Type -> Maybe Type
getRuntimeRep_maybe Type
ty1b  -- see Note [Decomposing FunTy]
  , Just Type
ty2a_rep <- HasDebugCallStack => Type -> Maybe Type
getRuntimeRep_maybe Type
ty2a
  , Just Type
ty2b_rep <- HasDebugCallStack => Type -> Maybe Type
getRuntimeRep_maybe Type
ty2b
  = CtEvidence
-> EqRel -> TyCon -> [Type] -> [Type] -> TcS (StopOrContinue Ct)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
funTyCon
                              [Type
am1, Type
ty1a_rep, Type
ty1b_rep, Type
ty1a, Type
ty1b]
                              [Type
am2, Type
ty2a_rep, Type
ty2b_rep, Type
ty2a, Type
ty2b]

-- Decompose type constructor applications
-- NB: we have expanded type synonyms already
can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Type
ty1 Type
_ Type
ty2 Type
_
  | Just (TyCon
tc1, [Type]
tys1) <- HasCallStack => Type -> Maybe (TyCon, [Type])
tcSplitTyConApp_maybe Type
ty1
  , Just (TyCon
tc2, [Type]
tys2) <- HasCallStack => Type -> Maybe (TyCon, [Type])
tcSplitTyConApp_maybe Type
ty2
   -- we want to catch e.g. Maybe Int ~ (Int -> Int) here for better
   -- error messages rather than decomposing into AppTys;
   -- hence no direct match on TyConApp
  , Bool -> Bool
not (TyCon -> Bool
isTypeFamilyTyCon TyCon
tc1)
  , Bool -> Bool
not (TyCon -> Bool
isTypeFamilyTyCon TyCon
tc2)
  = CtEvidence
-> EqRel
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> TcS (StopOrContinue Ct)
canTyConApp CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Type]
tys1 TyCon
tc2 [Type]
tys2

can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           s1 :: Type
s1@(ForAllTy (Bndr TcTyVar
_ ArgFlag
vis1) Type
_) Type
_
           s2 :: Type
s2@(ForAllTy (Bndr TcTyVar
_ ArgFlag
vis2) Type
_) Type
_
  | ArgFlag
vis1 ArgFlag -> ArgFlag -> Bool
`sameVis` ArgFlag
vis2 -- Note [ForAllTy and typechecker equality]
  = CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel Type
s1 Type
s2

-- See Note [Canonicalising type applications] about why we require rewritten types
-- Use tcSplitAppTy, not matching on AppTy, to catch oversaturated type families
-- NB: Only decompose AppTy for nominal equality. See Note [Decomposing equality]
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
NomEq Type
ty1 Type
_ Type
ty2 Type
_
  | Just (Type
t1, Type
s1) <- Type -> Maybe (Type, Type)
tcSplitAppTy_maybe Type
ty1
  , Just (Type
t2, Type
s2) <- Type -> Maybe (Type, Type)
tcSplitAppTy_maybe Type
ty2
  = CtEvidence
-> Type -> Type -> Type -> Type -> TcS (StopOrContinue Ct)
can_eq_app CtEvidence
ev Type
t1 Type
s1 Type
t2 Type
s2

-------------------
-- Can't decompose.
-------------------

-- No similarity in type structure detected. Rewrite and try again.
can_eq_nc' Bool
False GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
_ Type
ps_ty1 Type
_ Type
ps_ty2
  = do { (redn1 :: Reduction
redn1@(Reduction TcCoercion
_ Type
xi1), RewriterSet
rewriters1) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ps_ty1
       ; (redn2 :: Reduction
redn2@(Reduction TcCoercion
_ Type
xi2), RewriterSet
rewriters2) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ps_ty2
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence (RewriterSet
rewriters1 forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters2) CtEvidence
ev SwapFlag
NotSwapped Reduction
redn1 Reduction
redn2
       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
True GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
new_ev EqRel
eq_rel Type
xi1 Type
xi1 Type
xi2 Type
xi2 }

----------------------------
-- Look for a canonical LHS. See Note [Canonical LHS].
-- Only rewritten types end up below here.
----------------------------

-- NB: pattern match on True: we want only rewritten types sent to canEqLHS
-- This means we've rewritten any variables and reduced any type family redexes
-- See also Note [No top-level newtypes on RHS of representational equalities]
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  | Just CanEqLHS
can_eq_lhs1 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
ty1
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped CanEqLHS
can_eq_lhs1 Type
ps_ty1 Type
ty2 Type
ps_ty2

  | Just CanEqLHS
can_eq_lhs2 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
ty2
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
can_eq_lhs2 Type
ps_ty2 Type
ty1 Type
ps_ty1

     -- If the type is TyConApp tc1 args1, then args1 really can't be less
     -- than tyConArity tc1. It could be *more* than tyConArity, but then we
     -- should have handled the case as an AppTy. That case only fires if
     -- _both_ sides of the equality are AppTy-like... but if one side is
     -- AppTy-like and the other isn't (and it also isn't a variable or
     -- saturated type family application, both of which are handled by
     -- can_eq_nc'), we're in a failure mode and can just fall through.

----------------------------
-- Fall-through. Give up.
----------------------------

-- We've rewritten and the types don't match. Give up.
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Type
_ Type
ps_ty1 Type
_ Type
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc' catch-all case" (forall a. Outputable a => a -> SDoc
ppr Type
ps_ty1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
ps_ty2)
       ; case EqRel
eq_rel of -- See Note [Unsolved equalities]
            EqRel
ReprEq -> forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
ReprEqReason CtEvidence
ev)
            EqRel
NomEq  -> forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
ShapeMismatchReason CtEvidence
ev) }
          -- No need to call canEqFailure/canEqHardFailure because they
          -- rewrite, and the types involved here are already rewritten

{- Note [Unsolved equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have an unsolved equality like
  (a b ~R# Int)
that is not necessarily insoluble!  Maybe 'a' will turn out to be a newtype.
So we want to make it a potentially-soluble Irred not an insoluble one.
Missing this point is what caused #15431

Note [ForAllTy and typechecker equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Should GHC type-check the following program (adapted from #15740)?

  {-# LANGUAGE PolyKinds, ... #-}
  data D a
  type family F :: forall k. k -> Type
  type instance F = D

Due to the way F is declared, any instance of F must have a right-hand side
whose kind is equal to `forall k. k -> Type`. The kind of D is
`forall {k}. k -> Type`, which is very close, but technically uses distinct
Core:

  -----------------------------------------------------------
  | Source Haskell    | Core                                |
  -----------------------------------------------------------
  | forall  k.  <...> | ForAllTy (Bndr k Specified) (<...>) |
  | forall {k}. <...> | ForAllTy (Bndr k Inferred)  (<...>) |
  -----------------------------------------------------------

We could deem these kinds to be unequal, but that would imply rejecting
programs like the one above. Whether a kind variable binder ends up being
specified or inferred can be somewhat subtle, however, especially for kinds
that aren't explicitly written out in the source code (like in D above).
For now, we decide to not make the specified/inferred status of an invisible
type variable binder affect GHC's notion of typechecker equality
(see Note [Typechecker equality vs definitional equality] in
GHC.Tc.Utils.TcType). That is, we have the following:

  --------------------------------------------------
  | Type 1            | Type 2            | Equal? |
  --------------------|-----------------------------
  | forall k. <...>   | forall k. <...>   | Yes    |
  |                   | forall {k}. <...> | Yes    |
  |                   | forall k -> <...> | No     |
  --------------------------------------------------
  | forall {k}. <...> | forall k. <...>   | Yes    |
  |                   | forall {k}. <...> | Yes    |
  |                   | forall k -> <...> | No     |
  --------------------------------------------------
  | forall k -> <...> | forall k. <...>   | No     |
  |                   | forall {k}. <...> | No     |
  |                   | forall k -> <...> | Yes    |
  --------------------------------------------------

We implement this nuance by using the GHC.Types.Var.sameVis function in
GHC.Tc.Solver.Canonical.canEqNC and GHC.Tc.Utils.TcType.tcEqType, which
respect typechecker equality. sameVis puts both forms of invisible type
variable binders into the same equivalence class.

Note that we do /not/ use sameVis in GHC.Core.Type.eqType, which implements
/definitional/ equality, a slighty more coarse-grained notion of equality
(see Note [Non-trivial definitional equality] in GHC.Core.TyCo.Rep) that does
not consider the ArgFlag of ForAllTys at all. That is, eqType would equate all
of forall k. <...>, forall {k}. <...>, and forall k -> <...>.
-}

---------------------------------
can_eq_nc_forall :: CtEvidence -> EqRel
                 -> Type -> Type    -- LHS and RHS
                 -> TcS (StopOrContinue Ct)
-- (forall as. phi1) ~ (forall bs. phi2)
-- Check for length match of as, bs
-- Then build an implication constraint: forall as. phi1 ~ phi2[as/bs]
-- But remember also to unify the kinds of as and bs
--  (this is the 'go' loop), and actually substitute phi2[as |> cos / bs]
-- Remember also that we might have forall z (a:z). blah
--  so we must proceed one binder at a time (#13879)

can_eq_nc_forall :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel Type
s1 Type
s2
 | CtWanted { ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc, ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
orig_dest, ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } <- CtEvidence
ev
 = do { let free_tvs :: VarSet
free_tvs       = [Type] -> VarSet
tyCoVarsOfTypes [Type
s1,Type
s2]
            ([TyVarBinder]
bndrs1, Type
phi1) = Type -> ([TyVarBinder], Type)
tcSplitForAllTyVarBinders Type
s1
            ([TyVarBinder]
bndrs2, Type
phi2) = Type -> ([TyVarBinder], Type)
tcSplitForAllTyVarBinders Type
s2
      ; if Bool -> Bool
not (forall a b. [a] -> [b] -> Bool
equalLength [TyVarBinder]
bndrs1 [TyVarBinder]
bndrs2)
        then do { String -> SDoc -> TcS ()
traceTcS String
"Forall failure" forall a b. (a -> b) -> a -> b
$
                     [SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr Type
s1, forall a. Outputable a => a -> SDoc
ppr Type
s2, forall a. Outputable a => a -> SDoc
ppr [TyVarBinder]
bndrs1, forall a. Outputable a => a -> SDoc
ppr [TyVarBinder]
bndrs2
                          , forall a. Outputable a => a -> SDoc
ppr (forall a b. (a -> b) -> [a] -> [b]
map forall tv argf. VarBndr tv argf -> argf
binderArgFlag [TyVarBinder]
bndrs1)
                          , forall a. Outputable a => a -> SDoc
ppr (forall a b. (a -> b) -> [a] -> [b]
map forall tv argf. VarBndr tv argf -> argf
binderArgFlag [TyVarBinder]
bndrs2) ]
                ; CtEvidence -> Type -> Type -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Type
s1 Type
s2 }
        else
   do { String -> SDoc -> TcS ()
traceTcS String
"Creating implication for polytype equality" forall a b. (a -> b) -> a -> b
$ forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
      ; let empty_subst1 :: Subst
empty_subst1 = InScopeSet -> Subst
mkEmptySubst forall a b. (a -> b) -> a -> b
$ VarSet -> InScopeSet
mkInScopeSet VarSet
free_tvs
      ; SkolemInfo
skol_info <- forall (m :: * -> *). MonadIO m => SkolemInfoAnon -> m SkolemInfo
mkSkolemInfo (Type -> SkolemInfoAnon
UnifyForAllSkol Type
phi1)
      ; (Subst
subst1, [TcTyVar]
skol_tvs) <- SkolemInfo -> Subst -> [TcTyVar] -> TcS (Subst, [TcTyVar])
tcInstSkolTyVarsX SkolemInfo
skol_info Subst
empty_subst1 forall a b. (a -> b) -> a -> b
$
                              forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [TyVarBinder]
bndrs1

      ; let phi1' :: Type
phi1' = HasDebugCallStack => Subst -> Type -> Type
substTy Subst
subst1 Type
phi1

            -- Unify the kinds, extend the substitution
            go :: [TcTyVar] -> Subst -> [TyVarBinder]
               -> TcS (TcCoercion, Cts)
            go :: [TcTyVar] -> Subst -> [TyVarBinder] -> TcS (TcCoercion, Bag Ct)
go (TcTyVar
skol_tv:[TcTyVar]
skol_tvs) Subst
subst (TyVarBinder
bndr2:[TyVarBinder]
bndrs2)
              = do { let tv2 :: TcTyVar
tv2 = forall tv argf. VarBndr tv argf -> tv
binderVar TyVarBinder
bndr2
                   ; (TcCoercion
kind_co, Bag Ct
wanteds1) <- CtLoc
-> RewriterSet -> Role -> Type -> Type -> TcS (TcCoercion, Bag Ct)
unify CtLoc
loc RewriterSet
rewriters Role
Nominal (TcTyVar -> Type
tyVarKind TcTyVar
skol_tv)
                                                  (HasDebugCallStack => Subst -> Type -> Type
substTy Subst
subst (TcTyVar -> Type
tyVarKind TcTyVar
tv2))
                   ; let subst' :: Subst
subst' = Subst -> TcTyVar -> Type -> Subst
extendTvSubstAndInScope Subst
subst TcTyVar
tv2
                                       (Type -> TcCoercion -> Type
mkCastTy (TcTyVar -> Type
mkTyVarTy TcTyVar
skol_tv) TcCoercion
kind_co)
                         -- skol_tv is already in the in-scope set, but the
                         -- free vars of kind_co are not; hence "...AndInScope"
                   ; (TcCoercion
co, Bag Ct
wanteds2) <- [TcTyVar] -> Subst -> [TyVarBinder] -> TcS (TcCoercion, Bag Ct)
go [TcTyVar]
skol_tvs Subst
subst' [TyVarBinder]
bndrs2
                   ; forall (m :: * -> *) a. Monad m => a -> m a
return ( TcTyVar -> TcCoercion -> TcCoercion -> TcCoercion
mkTcForAllCo TcTyVar
skol_tv TcCoercion
kind_co TcCoercion
co
                            , Bag Ct
wanteds1 forall a. Bag a -> Bag a -> Bag a
`unionBags` Bag Ct
wanteds2 ) }

            -- Done: unify phi1 ~ phi2
            go [] Subst
subst [TyVarBinder]
bndrs2
              = forall a. HasCallStack => Bool -> a -> a
assert (forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVarBinder]
bndrs2) forall a b. (a -> b) -> a -> b
$
                CtLoc
-> RewriterSet -> Role -> Type -> Type -> TcS (TcCoercion, Bag Ct)
unify CtLoc
loc RewriterSet
rewriters (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
phi1' (Subst -> Type -> Type
substTyUnchecked Subst
subst Type
phi2)

            go [TcTyVar]
_ Subst
_ [TyVarBinder]
_ = forall a. String -> a
panic String
"cna_eq_nc_forall"  -- case (s:ss) []

            empty_subst2 :: Subst
empty_subst2 = InScopeSet -> Subst
mkEmptySubst (Subst -> InScopeSet
getSubstInScope Subst
subst1)

      ; (TcLevel
lvl, (TcCoercion
all_co, Bag Ct
wanteds)) <- forall a. SDoc -> TcS a -> TcS (TcLevel, a)
pushLevelNoWorkList (forall a. Outputable a => a -> SDoc
ppr SkolemInfo
skol_info) forall a b. (a -> b) -> a -> b
$
                                    [TcTyVar] -> Subst -> [TyVarBinder] -> TcS (TcCoercion, Bag Ct)
go [TcTyVar]
skol_tvs Subst
empty_subst2 [TyVarBinder]
bndrs2
      ; TcLevel -> SkolemInfoAnon -> [TcTyVar] -> Bag Ct -> TcS ()
emitTvImplicationTcS TcLevel
lvl (SkolemInfo -> SkolemInfoAnon
getSkolemInfo SkolemInfo
skol_info) [TcTyVar]
skol_tvs Bag Ct
wanteds

      ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
orig_dest TcCoercion
all_co
      ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Deferred polytype equality" } }

 | Bool
otherwise
 = do { String -> SDoc -> TcS ()
traceTcS String
"Omitting decomposition of given polytype equality" forall a b. (a -> b) -> a -> b
$
        Type -> Type -> SDoc
pprEq Type
s1 Type
s2    -- See Note [Do not decompose Given polytype equalities]
      ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Discard given polytype equality" }

 where
    unify :: CtLoc -> RewriterSet -> Role -> TcType -> TcType -> TcS (TcCoercion, Cts)
    -- This version returns the wanted constraint rather
    -- than putting it in the work list
    unify :: CtLoc
-> RewriterSet -> Role -> Type -> Type -> TcS (TcCoercion, Bag Ct)
unify CtLoc
loc RewriterSet
rewriters Role
role Type
ty1 Type
ty2
      | Type
ty1 HasDebugCallStack => Type -> Type -> Bool
`tcEqType` Type
ty2
      = forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Type -> TcCoercion
mkTcReflCo Role
role Type
ty1, forall a. Bag a
emptyBag)
      | Bool
otherwise
      = do { (CtEvidence
wanted, TcCoercion
co) <- CtLoc
-> RewriterSet
-> Role
-> Type
-> Type
-> TcS (CtEvidence, TcCoercion)
newWantedEq CtLoc
loc RewriterSet
rewriters Role
role Type
ty1 Type
ty2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion
co, forall a. a -> Bag a
unitBag (CtEvidence -> Ct
mkNonCanonical CtEvidence
wanted)) }

---------------------------------
-- | Compare types for equality, while zonking as necessary. Gives up
-- as soon as it finds that two types are not equal.
-- This is quite handy when some unification has made two
-- types in an inert Wanted to be equal. We can discover the equality without
-- rewriting, which is sometimes very expensive (in the case of type functions).
-- In particular, this function makes a ~20% improvement in test case
-- perf/compiler/T5030.
--
-- Returns either the (partially zonked) types in the case of
-- inequality, or the one type in the case of equality. canEqReflexive is
-- a good next step in the 'Right' case. Returning 'Left' is always safe.
--
-- NB: This does *not* look through type synonyms. In fact, it treats type
-- synonyms as rigid constructors. In the future, it might be convenient
-- to look at only those arguments of type synonyms that actually appear
-- in the synonym RHS. But we're not there yet.
zonk_eq_types :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
zonk_eq_types :: Type -> Type -> TcS (Either (Pair Type) Type)
zonk_eq_types = Type -> Type -> TcS (Either (Pair Type) Type)
go
  where
    go :: Type -> Type -> TcS (Either (Pair Type) Type)
go (TyVarTy TcTyVar
tv1) (TyVarTy TcTyVar
tv2) = TcTyVar -> TcTyVar -> TcS (Either (Pair Type) Type)
tyvar_tyvar TcTyVar
tv1 TcTyVar
tv2
    go (TyVarTy TcTyVar
tv1) Type
ty2           = SwapFlag -> TcTyVar -> Type -> TcS (Either (Pair Type) Type)
tyvar SwapFlag
NotSwapped TcTyVar
tv1 Type
ty2
    go Type
ty1 (TyVarTy TcTyVar
tv2)           = SwapFlag -> TcTyVar -> Type -> TcS (Either (Pair Type) Type)
tyvar SwapFlag
IsSwapped  TcTyVar
tv2 Type
ty1

    -- We handle FunTys explicitly here despite the fact that they could also be
    -- treated as an application. Why? Well, for one it's cheaper to just look
    -- at two types (the argument and result types) than four (the argument,
    -- result, and their RuntimeReps). Also, we haven't completely zonked yet,
    -- so we may run into an unzonked type variable while trying to compute the
    -- RuntimeReps of the argument and result types. This can be observed in
    -- testcase tc269.
    go Type
ty1 Type
ty2
      | Just (Scaled Type
w1 Type
arg1, Type
res1) <- Maybe (Scaled Type, Type)
split1
      , Just (Scaled Type
w2 Type
arg2, Type
res2) <- Maybe (Scaled Type, Type)
split2
      , Type -> Type -> Bool
eqType Type
w1 Type
w2
      = do { Either (Pair Type) Type
res_a <- Type -> Type -> TcS (Either (Pair Type) Type)
go Type
arg1 Type
arg2
           ; Either (Pair Type) Type
res_b <- Type -> Type -> TcS (Either (Pair Type) Type)
go Type
res1 Type
res2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (Type -> Type -> Type -> Type
mkVisFunTy Type
w1) Either (Pair Type) Type
res_b Either (Pair Type) Type
res_a
           }
      | forall a. Maybe a -> Bool
isJust Maybe (Scaled Type, Type)
split1 Bool -> Bool -> Bool
|| forall a. Maybe a -> Bool
isJust Maybe (Scaled Type, Type)
split2
      = forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Type
ty1 Type
ty2
      where
        split1 :: Maybe (Scaled Type, Type)
split1 = Type -> Maybe (Scaled Type, Type)
tcSplitFunTy_maybe Type
ty1
        split2 :: Maybe (Scaled Type, Type)
split2 = Type -> Maybe (Scaled Type, Type)
tcSplitFunTy_maybe Type
ty2

    go Type
ty1 Type
ty2
      | Just (TyCon
tc1, [Type]
tys1) <- HasDebugCallStack => Type -> Maybe (TyCon, [Type])
tcRepSplitTyConApp_maybe Type
ty1
      , Just (TyCon
tc2, [Type]
tys2) <- HasDebugCallStack => Type -> Maybe (TyCon, [Type])
tcRepSplitTyConApp_maybe Type
ty2
      = if TyCon
tc1 forall a. Eq a => a -> a -> Bool
== TyCon
tc2 Bool -> Bool -> Bool
&& [Type]
tys1 forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
tys2
          -- Crucial to check for equal-length args, because
          -- we cannot assume that the two args to 'go' have
          -- the same kind.  E.g go (Proxy *      (Maybe Int))
          --                        (Proxy (*->*) Maybe)
          -- We'll call (go (Maybe Int) Maybe)
          -- See #13083
        then TyCon -> [Type] -> [Type] -> TcS (Either (Pair Type) Type)
tycon TyCon
tc1 [Type]
tys1 [Type]
tys2
        else forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Type
ty1 Type
ty2

    go Type
ty1 Type
ty2
      | Just (Type
ty1a, Type
ty1b) <- Type -> Maybe (Type, Type)
tcRepSplitAppTy_maybe Type
ty1
      , Just (Type
ty2a, Type
ty2b) <- Type -> Maybe (Type, Type)
tcRepSplitAppTy_maybe Type
ty2
      = do { Either (Pair Type) Type
res_a <- Type -> Type -> TcS (Either (Pair Type) Type)
go Type
ty1a Type
ty2a
           ; Either (Pair Type) Type
res_b <- Type -> Type -> TcS (Either (Pair Type) Type)
go Type
ty1b Type
ty2b
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev Type -> Type -> Type
mkAppTy Either (Pair Type) Type
res_b Either (Pair Type) Type
res_a }

    go ty1 :: Type
ty1@(LitTy TyLit
lit1) (LitTy TyLit
lit2)
      | TyLit
lit1 forall a. Eq a => a -> a -> Bool
== TyLit
lit2
      = forall (m :: * -> *) a. Monad m => a -> m a
return (forall a b. b -> Either a b
Right Type
ty1)

    go Type
ty1 Type
ty2 = forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Type
ty1 Type
ty2
      -- We don't handle more complex forms here

    bale_out :: a -> a -> m (Either (Pair a) b)
bale_out a
ty1 a
ty2 = forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ forall a b. a -> Either a b
Left (forall a. a -> a -> Pair a
Pair a
ty1 a
ty2)

    tyvar :: SwapFlag -> TcTyVar -> TcType
          -> TcS (Either (Pair TcType) TcType)
      -- Try to do as little as possible, as anything we do here is redundant
      -- with rewriting. In particular, no need to zonk kinds. That's why
      -- we don't use the already-defined zonking functions
    tyvar :: SwapFlag -> TcTyVar -> Type -> TcS (Either (Pair Type) Type)
tyvar SwapFlag
swapped TcTyVar
tv Type
ty
      = case TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv of
          MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
            -> do { MetaDetails
cts <- forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
                  ; case MetaDetails
cts of
                      MetaDetails
Flexi        -> TcS (Either (Pair Type) Type)
give_up
                      Indirect Type
ty' -> do { forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TcTyVar
tv Type
ty'
                                         ; forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped Type -> Type -> TcS (Either (Pair Type) Type)
go Type
ty' Type
ty } }
          TcTyVarDetails
_ -> TcS (Either (Pair Type) Type)
give_up
      where
        give_up :: TcS (Either (Pair Type) Type)
give_up = forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ forall a b. a -> Either a b
Left forall a b. (a -> b) -> a -> b
$ forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped forall a. a -> a -> Pair a
Pair (TcTyVar -> Type
mkTyVarTy TcTyVar
tv) Type
ty

    tyvar_tyvar :: TcTyVar -> TcTyVar -> TcS (Either (Pair Type) Type)
tyvar_tyvar TcTyVar
tv1 TcTyVar
tv2
      | TcTyVar
tv1 forall a. Eq a => a -> a -> Bool
== TcTyVar
tv2 = forall (m :: * -> *) a. Monad m => a -> m a
return (forall a b. b -> Either a b
Right (TcTyVar -> Type
mkTyVarTy TcTyVar
tv1))
      | Bool
otherwise  = do { (Type
ty1', Bool
progress1) <- TcTyVar -> TcS (Type, Bool)
quick_zonk TcTyVar
tv1
                        ; (Type
ty2', Bool
progress2) <- TcTyVar -> TcS (Type, Bool)
quick_zonk TcTyVar
tv2
                        ; if Bool
progress1 Bool -> Bool -> Bool
|| Bool
progress2
                          then Type -> Type -> TcS (Either (Pair Type) Type)
go Type
ty1' Type
ty2'
                          else forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ forall a b. a -> Either a b
Left (forall a. a -> a -> Pair a
Pair (TcTyVar -> Type
TyVarTy TcTyVar
tv1) (TcTyVar -> Type
TyVarTy TcTyVar
tv2)) }

    trace_indirect :: a -> a -> TcS ()
trace_indirect a
tv a
ty
       = String -> SDoc -> TcS ()
traceTcS String
"Following filled tyvar (zonk_eq_types)"
                  (forall a. Outputable a => a -> SDoc
ppr a
tv SDoc -> SDoc -> SDoc
<+> SDoc
equals SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr a
ty)

    quick_zonk :: TcTyVar -> TcS (Type, Bool)
quick_zonk TcTyVar
tv = case TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv of
      MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
        -> do { MetaDetails
cts <- forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
              ; case MetaDetails
cts of
                  MetaDetails
Flexi        -> forall (m :: * -> *) a. Monad m => a -> m a
return (TcTyVar -> Type
TyVarTy TcTyVar
tv, Bool
False)
                  Indirect Type
ty' -> do { forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TcTyVar
tv Type
ty'
                                     ; forall (m :: * -> *) a. Monad m => a -> m a
return (Type
ty', Bool
True) } }
      TcTyVarDetails
_ -> forall (m :: * -> *) a. Monad m => a -> m a
return (TcTyVar -> Type
TyVarTy TcTyVar
tv, Bool
False)

      -- This happens for type families, too. But recall that failure
      -- here just means to try harder, so it's OK if the type function
      -- isn't injective.
    tycon :: TyCon -> [TcType] -> [TcType]
          -> TcS (Either (Pair TcType) TcType)
    tycon :: TyCon -> [Type] -> [Type] -> TcS (Either (Pair Type) Type)
tycon TyCon
tc [Type]
tys1 [Type]
tys2
      = do { [Either (Pair Type) Type]
results <- forall (m :: * -> *) a b c.
Applicative m =>
(a -> b -> m c) -> [a] -> [b] -> m [c]
zipWithM Type -> Type -> TcS (Either (Pair Type) Type)
go [Type]
tys1 [Type]
tys2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ case [Either (Pair Type) Type] -> Either (Pair [Type]) [Type]
combine_results [Either (Pair Type) Type]
results of
               Left Pair [Type]
tys  -> forall a b. a -> Either a b
Left (TyCon -> [Type] -> Type
mkTyConApp TyCon
tc forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair [Type]
tys)
               Right [Type]
tys -> forall a b. b -> Either a b
Right (TyCon -> [Type] -> Type
mkTyConApp TyCon
tc [Type]
tys) }

    combine_results :: [Either (Pair TcType) TcType]
                    -> Either (Pair [TcType]) [TcType]
    combine_results :: [Either (Pair Type) Type] -> Either (Pair [Type]) [Type]
combine_results = forall (p :: * -> * -> *) a b c d.
Bifunctor p =>
(a -> b) -> (c -> d) -> p a c -> p b d
bimap (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap forall a. [a] -> [a]
reverse) forall a. [a] -> [a]
reverse forall b c a. (b -> c) -> (a -> b) -> a -> c
.
                      forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl' (forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (:)) (forall a b. b -> Either a b
Right [])

      -- combine (in reverse) a new result onto an already-combined result
    combine_rev :: (a -> b -> c)
                -> Either (Pair b) b
                -> Either (Pair a) a
                -> Either (Pair c) c
    combine_rev :: forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev a -> b -> c
f (Left Pair b
list) (Left Pair a
elt) = forall a b. a -> Either a b
Left (a -> b -> c
f forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair a
elt     forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> Pair b
list)
    combine_rev a -> b -> c
f (Left Pair b
list) (Right a
ty) = forall a b. a -> Either a b
Left (a -> b -> c
f forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> forall (f :: * -> *) a. Applicative f => a -> f a
pure a
ty forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> Pair b
list)
    combine_rev a -> b -> c
f (Right b
tys) (Left Pair a
elt) = forall a b. a -> Either a b
Left (a -> b -> c
f forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair a
elt     forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> forall (f :: * -> *) a. Applicative f => a -> f a
pure b
tys)
    combine_rev a -> b -> c
f (Right b
tys) (Right a
ty) = forall a b. b -> Either a b
Right (a -> b -> c
f a
ty b
tys)

{- See Note [Unwrap newtypes first]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  newtype N m a = MkN (m a)
Then N will get a conservative, Nominal role for its second parameter 'a',
because it appears as an argument to the unknown 'm'. Now consider
  [W] N Maybe a  ~R#  N Maybe b

If we decompose, we'll get
  [W] a ~N# b

But if instead we unwrap we'll get
  [W] Maybe a ~R# Maybe b
which in turn gives us
  [W] a ~R# b
which is easier to satisfy.

Bottom line: unwrap newtypes before decomposing them!
c.f. #9123 comment:52,53 for a compelling example.

Note [Newtypes can blow the stack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)
  newtype Y = MkY (Int -> Y)

and now wish to prove

  [W] X ~R Y

This Wanted will loop, expanding out the newtypes ever deeper looking
for a solid match or a solid discrepancy. Indeed, there is something
appropriate to this looping, because X and Y *do* have the same representation,
in the limit -- they're both (Fix ((->) Int)). However, no finitely-sized
coercion will ever witness it. This loop won't actually cause GHC to hang,
though, because we check our depth when unwrapping newtypes.

Note [Eager reflexivity check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)

and

  [W] X ~R X

Naively, we would start unwrapping X and end up in a loop. Instead,
we do this eager reflexivity check. This is necessary only for representational
equality because the rewriter technology deals with the similar case
(recursive type families) for nominal equality.

Note that this check does not catch all cases, but it will catch the cases
we're most worried about, types like X above that are actually inhabited.

Here's another place where this reflexivity check is key:
Consider trying to prove (f a) ~R (f a). The AppTys in there can't
be decomposed, because representational equality isn't congruent with respect
to AppTy. So, when canonicalising the equality above, we get stuck and
would normally produce a CIrredCan. However, we really do want to
be able to solve (f a) ~R (f a). So, in the representational case only,
we do a reflexivity check.

(This would be sound in the nominal case, but unnecessary, and I [Richard
E.] am worried that it would slow down the common case.)
-}

------------------------
-- | We're able to unwrap a newtype. Update the bits accordingly.
can_eq_newtype_nc :: CtEvidence           -- ^ :: ty1 ~ ty2
                  -> SwapFlag
                  -> TcType                                    -- ^ ty1
                  -> ((Bag GlobalRdrElt, TcCoercion), TcType)  -- ^ :: ty1 ~ ty1'
                  -> TcType               -- ^ ty2
                  -> TcType               -- ^ ty2, with type synonyms
                  -> TcS (StopOrContinue Ct)
can_eq_newtype_nc :: CtEvidence
-> SwapFlag
-> Type
-> ((Bag GlobalRdrElt, TcCoercion), Type)
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_newtype_nc CtEvidence
ev SwapFlag
swapped Type
ty1 ((Bag GlobalRdrElt
gres, TcCoercion
co1), Type
ty1') Type
ty2 Type
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_newtype_nc" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped, forall a. Outputable a => a -> SDoc
ppr TcCoercion
co1, forall a. Outputable a => a -> SDoc
ppr Bag GlobalRdrElt
gres, forall a. Outputable a => a -> SDoc
ppr Type
ty1', forall a. Outputable a => a -> SDoc
ppr Type
ty2 ]

         -- check for blowing our stack:
         -- See Note [Newtypes can blow the stack]
       ; CtLoc -> Type -> TcS ()
checkReductionDepth (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev) Type
ty1

         -- Next, we record uses of newtype constructors, since coercing
         -- through newtypes is tantamount to using their constructors.
       ; [GlobalRdrElt] -> TcS ()
addUsedGREs [GlobalRdrElt]
gre_list
         -- If a newtype constructor was imported, don't warn about not
         -- importing it...
       ; forall (t :: * -> *) (f :: * -> *) a b.
(Foldable t, Applicative f) =>
(a -> f b) -> t a -> f ()
traverse_ Name -> TcS ()
keepAlive forall a b. (a -> b) -> a -> b
$ forall a b. (a -> b) -> [a] -> [b]
map GlobalRdrElt -> Name
greMangledName [GlobalRdrElt]
gre_list
         -- ...and similarly, if a newtype constructor was defined in the same
         -- module, don't warn about it being unused.
         -- See Note [Tracking unused binding and imports] in GHC.Tc.Utils.

       ; let redn1 :: Reduction
redn1 = TcCoercion -> Type -> Reduction
mkReduction TcCoercion
co1 Type
ty1'

       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                     Reduction
redn1
                     (Role -> Type -> Reduction
mkReflRedn Role
Representational Type
ps_ty2)
       ; Bool
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
False CtEvidence
new_ev EqRel
ReprEq Type
ty1' Type
ty1' Type
ty2 Type
ps_ty2 }
  where
    gre_list :: [GlobalRdrElt]
gre_list = forall a. Bag a -> [a]
bagToList Bag GlobalRdrElt
gres

---------
-- ^ Decompose a type application.
-- All input types must be rewritten. See Note [Canonicalising type applications]
-- Nominal equality only!
can_eq_app :: CtEvidence       -- :: s1 t1 ~N s2 t2
           -> Xi -> Xi         -- s1 t1
           -> Xi -> Xi         -- s2 t2
           -> TcS (StopOrContinue Ct)

-- AppTys only decompose for nominal equality, so this case just leads
-- to an irreducible constraint; see typecheck/should_compile/T10494
-- See Note [Decomposing AppTy at representational role]
can_eq_app :: CtEvidence
-> Type -> Type -> Type -> Type -> TcS (StopOrContinue Ct)
can_eq_app CtEvidence
ev Type
s1 Type
t1 Type
s2 Type
t2
  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest, ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } <- CtEvidence
ev
  = do { TcCoercion
co_s <- RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
Nominal Type
s1 Type
s2
       ; let arg_loc :: CtLoc
arg_loc
               | Type -> Bool
isNextArgVisible Type
s1 = CtLoc
loc
               | Bool
otherwise           = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
updateCtLocOrigin CtLoc
loc CtOrigin -> CtOrigin
toInvisibleOrigin
       ; TcCoercion
co_t <- RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
arg_loc Role
Nominal Type
t1 Type
t2
       ; let co :: TcCoercion
co = TcCoercion -> TcCoercion -> TcCoercion
mkAppCo TcCoercion
co_s TcCoercion
co_t
       ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co
       ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed [W] AppTy" }

    -- If there is a ForAll/(->) mismatch, the use of the Left coercion
    -- below is ill-typed, potentially leading to a panic in splitTyConApp
    -- Test case: typecheck/should_run/Typeable1
    -- We could also include this mismatch check above (for W and D), but it's slow
    -- and we'll get a better error message not doing it
  | Type
s1k Type -> Type -> Bool
`mismatches` Type
s2k
  = CtEvidence -> Type -> Type -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev (Type
s1 Type -> Type -> Type
`mkAppTy` Type
t1) (Type
s2 Type -> Type -> Type
`mkAppTy` Type
t2)

  | CtGiven { ctev_evar :: CtEvidence -> TcTyVar
ctev_evar = TcTyVar
evar } <- CtEvidence
ev
  = do { let co :: TcCoercion
co   = TcTyVar -> TcCoercion
mkTcCoVarCo TcTyVar
evar
             co_s :: TcCoercion
co_s = LeftOrRight -> TcCoercion -> TcCoercion
mkTcLRCo LeftOrRight
CLeft  TcCoercion
co
             co_t :: TcCoercion
co_t = LeftOrRight -> TcCoercion -> TcCoercion
mkTcLRCo LeftOrRight
CRight TcCoercion
co
       ; CtEvidence
evar_s <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc ( CtEvidence -> Type -> Type -> Type
mkTcEqPredLikeEv CtEvidence
ev Type
s1 Type
s2
                                     , TcCoercion -> EvTerm
evCoercion TcCoercion
co_s )
       ; CtEvidence
evar_t <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc ( CtEvidence -> Type -> Type -> Type
mkTcEqPredLikeEv CtEvidence
ev Type
t1 Type
t2
                                     , TcCoercion -> EvTerm
evCoercion TcCoercion
co_t )
       ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence
evar_t]
       ; CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
evar_s EqRel
NomEq Type
s1 Type
s2 }

  where
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

    s1k :: Type
s1k = HasDebugCallStack => Type -> Type
tcTypeKind Type
s1
    s2k :: Type
s2k = HasDebugCallStack => Type -> Type
tcTypeKind Type
s2

    Type
k1 mismatches :: Type -> Type -> Bool
`mismatches` Type
k2
      =  Type -> Bool
isForAllTy Type
k1 Bool -> Bool -> Bool
&& Bool -> Bool
not (Type -> Bool
isForAllTy Type
k2)
      Bool -> Bool -> Bool
|| Bool -> Bool
not (Type -> Bool
isForAllTy Type
k1) Bool -> Bool -> Bool
&& Type -> Bool
isForAllTy Type
k2

-----------------------
-- | Break apart an equality over a casted type
-- looking like   (ty1 |> co1) ~ ty2   (modulo a swap-flag)
canEqCast :: Bool         -- are both types rewritten?
          -> CtEvidence
          -> EqRel
          -> SwapFlag
          -> TcType -> Coercion   -- LHS (res. RHS), ty1 |> co1
          -> TcType -> TcType     -- RHS (res. LHS), ty2 both normal and pretty
          -> TcS (StopOrContinue Ct)
canEqCast :: Bool
-> CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> TcCoercion
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCast Bool
rewritten CtEvidence
ev EqRel
eq_rel SwapFlag
swapped Type
ty1 TcCoercion
co1 Type
ty2 Type
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"Decomposing cast" ([SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
                                           , forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"|>" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr TcCoercion
co1
                                           , forall a. Outputable a => a -> SDoc
ppr Type
ps_ty2 ])
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                      (Role -> Type -> TcCoercion -> Reduction
mkGReflLeftRedn Role
role Type
ty1 TcCoercion
co1)
                      (Role -> Type -> Reduction
mkReflRedn Role
role Type
ps_ty2)
       ; Bool
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
rewritten CtEvidence
new_ev EqRel
eq_rel Type
ty1 Type
ty1 Type
ty2 Type
ps_ty2 }
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

------------------------
canTyConApp :: CtEvidence -> EqRel
            -> TyCon -> [TcType]
            -> TyCon -> [TcType]
            -> TcS (StopOrContinue Ct)
-- See Note [Decomposing TyConApps]
-- Neither tc1 nor tc2 is a saturated funTyCon
canTyConApp :: CtEvidence
-> EqRel
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> TcS (StopOrContinue Ct)
canTyConApp CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Type]
tys1 TyCon
tc2 [Type]
tys2
  | TyCon
tc1 forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  , [Type]
tys1 forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
tys2
  = do { InertSet
inerts <- TcS InertSet
getTcSInerts
       ; if InertSet -> Bool
can_decompose InertSet
inerts
         then CtEvidence
-> EqRel -> TyCon -> [Type] -> [Type] -> TcS (StopOrContinue Ct)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Type]
tys1 [Type]
tys2
         else CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqFailure CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2 }

  -- See Note [Skolem abstract data] in GHC.Core.Tycon
  | TyCon -> Bool
tyConSkolem TyCon
tc1 Bool -> Bool -> Bool
|| TyCon -> Bool
tyConSkolem TyCon
tc2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canTyConApp: skolem abstract" (forall a. Outputable a => a -> SDoc
ppr TyCon
tc1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr TyCon
tc2)
       ; forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
AbstractTyConReason CtEvidence
ev) }

  -- Fail straight away for better error messages
  -- See Note [Use canEqFailure in canDecomposableTyConApp]
  | EqRel
eq_rel forall a. Eq a => a -> a -> Bool
== EqRel
ReprEq Bool -> Bool -> Bool
&& Bool -> Bool
not (TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc1 Role
Representational Bool -> Bool -> Bool
&&
                             TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc2 Role
Representational)
  = CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqFailure CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
  | Bool
otherwise
  = CtEvidence -> Type -> Type -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Type
ty1 Type
ty2
  where
    -- Reconstruct the types for error messages. This would do
    -- the wrong thing (from a pretty printing point of view)
    -- for functions, because we've lost the AnonArgFlag; but
    -- in fact we never call canTyConApp on a saturated FunTyCon
    ty1 :: Type
ty1 = TyCon -> [Type] -> Type
mkTyConApp TyCon
tc1 [Type]
tys1
    ty2 :: Type
ty2 = TyCon -> [Type] -> Type
mkTyConApp TyCon
tc2 [Type]
tys2

    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev

     -- See Note [Decomposing equality]
    can_decompose :: InertSet -> Bool
can_decompose InertSet
inerts
      =  TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 (EqRel -> Role
eqRelRole EqRel
eq_rel)
      Bool -> Bool -> Bool
|| (CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev forall a. Eq a => a -> a -> Bool
/= CtFlavour
Given Bool -> Bool -> Bool
&& forall a. Bag a -> Bool
isEmptyBag (CtLoc -> Type -> InertSet -> Bag Ct
matchableGivens CtLoc
loc Type
pred InertSet
inerts))

{-
Note [Use canEqFailure in canDecomposableTyConApp]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We must use canEqFailure, not canEqHardFailure here, because there is
the possibility of success if working with a representational equality.
Here is one case:

  type family TF a where TF Char = Bool
  data family DF a
  newtype instance DF Bool = MkDF Int

Suppose we are canonicalising (Int ~R DF (TF a)), where we don't yet
know `a`. This is *not* a hard failure, because we might soon learn
that `a` is, in fact, Char, and then the equality succeeds.

Here is another case:

  [G] Age ~R Int

where Age's constructor is not in scope. We don't want to report
an "inaccessible code" error in the context of this Given!

For example, see typecheck/should_compile/T10493, repeated here:

  import Data.Ord (Down)  -- no constructor

  foo :: Coercible (Down Int) Int => Down Int -> Int
  foo = coerce

That should compile, but only because we use canEqFailure and not
canEqHardFailure.

Note [Decomposing equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have a constraint (of any flavour and role) that looks like
T tys1 ~ T tys2, what can we conclude about tys1 and tys2? The answer,
of course, is "it depends". This Note spells it all out.

In this Note, "decomposition" refers to taking the constraint
  [fl] (T tys1 ~X T tys2)
(for some flavour fl and some role X) and replacing it with
  [fls'] (tys1 ~Xs' tys2)
where that notation indicates a list of new constraints, where the
new constraints may have different flavours and different roles.

The key property to consider is injectivity. When decomposing a Given, the
decomposition is sound if and only if T is injective in all of its type
arguments. When decomposing a Wanted, the decomposition is sound (assuming the
correct roles in the produced equality constraints), but it may be a guess --
that is, an unforced decision by the constraint solver. Decomposing Wanteds
over injective TyCons does not entail guessing. But sometimes we want to
decompose a Wanted even when the TyCon involved is not injective! (See below.)

So, in broad strokes, we want this rule:

(*) Decompose a constraint (T tys1 ~X T tys2) if and only if T is injective
at role X.

Pursuing the details requires exploring three axes:
* Flavour: Given vs. Wanted
* Role: Nominal vs. Representational
* TyCon species: datatype vs. newtype vs. data family vs. type family vs. type variable

(A type variable isn't a TyCon, of course, but it's convenient to put the AppTy case
in the same table.)

Here is a table (discussion following) detailing where decomposition of
   (T s1 ... sn) ~r (T t1 .. tn)
is allowed.  The first four lines (Data types ... type family) refer
to TyConApps with various TyCons T; the last line is for AppTy, covering
both where there is a type variable at the head and the case for an over-
saturated type family.

NOMINAL               GIVEN        WANTED                         WHERE

Datatype               YES          YES                           canTyConApp
Newtype                YES          YES                           canTyConApp
Data family            YES          YES                           canTyConApp
Type family            NO{1}        YES, in injective args{1}     canEqCanLHS2
AppTy                  YES          YES                           can_eq_app

REPRESENTATIONAL      GIVEN        WANTED

Datatype               YES          YES                           canTyConApp
Newtype                NO{2}       MAYBE{2}                canTyConApp(can_decompose)
Data family            NO{3}       MAYBE{3}                canTyConApp(can_decompose)
Type family            NO           NO                            canEqCanLHS2
AppTy                  NO{4}        NO{4}                         can_eq_nc'

{1}: Type families can be injective in some, but not all, of their arguments,
so we want to do partial decomposition. This is quite different than the way
other decomposition is done, where the decomposed equalities replace the original
one. We thus proceed much like we do with superclasses, emitting new Wanteds
when "decomposing" a partially-injective type family Wanted. Injective type
families have no corresponding evidence of their injectivity, so we cannot
decompose an injective-type-family Given.

{2}: See Note [Decomposing newtypes at representational role]

{3}: Because of the possibility of newtype instances, we must treat
data families like newtypes. See also
Note [Decomposing newtypes at representational role]. See #10534 and
test case typecheck/should_fail/T10534.

{4}: See Note [Decomposing AppTy at representational role]

   Because type variables can stand in for newtypes, we conservatively do not
   decompose AppTys over representational equality. Here are two examples that
   demonstrate why we can't:

   4a: newtype Phant a = MkPhant Int
       [W] alpha Int ~R beta Bool

   If we eventually solve alpha := Phant and beta := Phant, then we can solve
   this equality by unwrapping. But it would have been disastrous to decompose
   the wanted to produce Int ~ Bool, which is definitely insoluble.

   4b: newtype Age = MkAge Int
       [W] alpha Age ~R Maybe Int

   First, a question: if we know that ty1 ~R ty2, can we conclude that
   a ty1 ~R a ty2? Not for all a. This is precisely why we need role annotations
   on type constructors. So, if we were to decompose, we would need to
   decompose to [W] alpha ~R Maybe and [W] Age ~ Int. On the other hand, if we
   later solve alpha := Maybe, then we would decompose to [W] Age ~R Int, and
   that would be soluble.

In the implementation of can_eq_nc and friends, we don't directly pattern
match using lines like in the tables above, as those tables don't cover
all cases (what about PrimTyCon? tuples?). Instead we just ask about injectivity,
boiling the tables above down to rule (*). The exceptions to rule (*) are for
injective type families, which are handled separately from other decompositions,
and the MAYBE entries above.

Note [Decomposing newtypes at representational role]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This note discusses the 'newtype' line in the REPRESENTATIONAL table
in Note [Decomposing equality]. (At nominal role, newtypes are fully
decomposable.)

Here is a representative example of why representational equality over
newtypes is tricky:

  newtype Nt a = Mk Bool         -- NB: a is not used in the RHS,
  type role Nt representational  -- but the user gives it an R role anyway

If we have [W] Nt alpha ~R Nt beta, we *don't* want to decompose to
[W] alpha ~R beta, because it's possible that alpha and beta aren't
representationally equal. Here's another example.

  newtype Nt a = MkNt (Id a)
  type family Id a where Id a = a

  [W] Nt Int ~R Nt Age

Because of its use of a type family, Nt's parameter will get inferred to have
a nominal role. Thus, decomposing the wanted will yield [W] Int ~N Age, which
is unsatisfiable. Unwrapping, though, leads to a solution.

Conclusion:
 * Unwrap newtypes before attempting to decompose them.
   This is done in can_eq_nc'.

It all comes from the fact that newtypes aren't necessarily injective
w.r.t. representational equality.

Furthermore, as explained in Note [NthCo and newtypes] in GHC.Core.TyCo.Rep, we can't use
NthCo on representational coercions over newtypes. NthCo comes into play
only when decomposing givens.

Conclusion:
 * Do not decompose [G] N s ~R N t

Is it sensible to decompose *Wanted* constraints over newtypes?  Yes!
It's the only way we could ever prove (IO Int ~R IO Age), recalling
that IO is a newtype.

However we must be careful.  Consider

  type role Nt representational

  [G] Nt a ~R Nt b       (1)
  [W] NT alpha ~R Nt b   (2)
  [W] alpha ~ a          (3)

If we focus on (3) first, we'll substitute in (2), and now it's
identical to the given (1), so we succeed.  But if we focus on (2)
first, and decompose it, we'll get (alpha ~R b), which is not soluble.
This is exactly like the question of overlapping Givens for class
constraints: see Note [Instance and Given overlap] in GHC.Tc.Solver.Interact.

Conclusion:
  * Decompose [W] N s ~R N t  iff there no given constraint that could
    later solve it.

Note [Decomposing AppTy at representational role]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We never decompose AppTy at a representational role. For Givens, doing
so is simply unsound: the LRCo coercion former requires a nominal-roled
arguments. (See (1) for an example of why.) For Wanteds, decomposing
would be sound, but it would be a guess, and a non-confluent one at that.

Here is an example:

    [G] g1 :: a ~R b
    [W] w1 :: Maybe b ~R alpha a
    [W] w2 :: alpha ~ Maybe

Suppose we see w1 before w2. If we were to decompose, we would decompose
this to become

    [W] w3 :: Maybe ~R alpha
    [W] w4 :: b ~ a

Note that w4 is *nominal*. A nominal role here is necessary because AppCo
requires a nominal role on its second argument. (See (2) for an example of
why.) If we decomposed w1 to w3,w4, we would then get stuck, because w4
is insoluble. On the other hand, if we see w2 first, setting alpha := Maybe,
all is well, as we can decompose Maybe b ~R Maybe a into b ~R a.

Another example:

    newtype Phant x = MkPhant Int

    [W] w1 :: Phant Int ~R alpha Bool
    [W] w2 :: alpha ~ Phant

If we see w1 first, decomposing would be disastrous, as we would then try
to solve Int ~ Bool. Instead, spotting w2 allows us to simplify w1 to become

    [W] w1' :: Phant Int ~R Phant Bool

which can then (assuming MkPhant is in scope) be simplified to Int ~R Int,
and all will be well. See also Note [Unwrap newtypes first].

Bottom line: never decompose AppTy with representational roles.

(1) Decomposing a Given AppTy over a representational role is simply
unsound. For example, if we have co1 :: Phant Int ~R a Bool (for
the newtype Phant, above), then we surely don't want any relationship
between Int and Bool, lest we also have co2 :: Phant ~ a around.

(2) The role on the AppCo coercion is a conservative choice, because we don't
know the role signature of the function. For example, let's assume we could
have a representational role on the second argument of AppCo. Then, consider

    data G a where    -- G will have a nominal role, as G is a GADT
      MkG :: G Int
    newtype Age = MkAge Int

    co1 :: G ~R a        -- by assumption
    co2 :: Age ~R Int    -- by newtype axiom
    co3 = AppCo co1 co2 :: G Age ~R a Int    -- by our broken AppCo

and now co3 can be used to cast MkG to have type G Age, in violation of
the way GADTs are supposed to work (which is to use nominal equality).

-}

canDecomposableTyConAppOK :: CtEvidence -> EqRel
                          -> TyCon -> [TcType] -> [TcType]
                          -> TcS (StopOrContinue Ct)
-- Precondition: tys1 and tys2 are the same length, hence "OK"
canDecomposableTyConAppOK :: CtEvidence
-> EqRel -> TyCon -> [Type] -> [Type] -> TcS (StopOrContinue Ct)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc [Type]
tys1 [Type]
tys2
  = forall a. HasCallStack => Bool -> a -> a
assert ([Type]
tys1 forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
tys2) forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcS ()
traceTcS String
"canDecomposableTyConAppOK"
                  (forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr TyCon
tc SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [Type]
tys1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [Type]
tys2)
       ; case CtEvidence
ev of
           CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest, ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters }
                  -- new_locs and tc_roles are both infinite, so
                  -- we are guaranteed that cos has the same length
                  -- as tys1 and tys2
             -> do { [TcCoercion]
cos <- forall (m :: * -> *) a b c d e.
Monad m =>
(a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
zipWith4M (RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters) [CtLoc]
new_locs [Role]
tc_roles [Type]
tys1 [Type]
tys2
                   ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
mkTyConAppCo Role
role TyCon
tc [TcCoercion]
cos) }

           CtGiven { ctev_evar :: CtEvidence -> TcTyVar
ctev_evar = TcTyVar
evar }
             -> do { let ev_co :: TcCoercion
ev_co = TcTyVar -> TcCoercion
mkCoVarCo TcTyVar
evar
                   ; [CtEvidence]
given_evs <- CtLoc -> [(Type, EvTerm)] -> TcS [CtEvidence]
newGivenEvVars CtLoc
loc forall a b. (a -> b) -> a -> b
$
                                  [ ( Role -> Type -> Type -> Type
mkPrimEqPredRole Role
r Type
ty1 Type
ty2
                                    , TcCoercion -> EvTerm
evCoercion forall a b. (a -> b) -> a -> b
$ HasDebugCallStack => Role -> Int -> TcCoercion -> TcCoercion
mkNthCo Role
r Int
i TcCoercion
ev_co )
                                  | (Role
r, Type
ty1, Type
ty2, Int
i) <- forall a b c d. [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]
zip4 [Role]
tc_roles [Type]
tys1 [Type]
tys2 [Int
0..]
                                  , Role
r forall a. Eq a => a -> a -> Bool
/= Role
Phantom
                                  , Bool -> Bool
not (Type -> Bool
isCoercionTy Type
ty1) Bool -> Bool -> Bool
&& Bool -> Bool
not (Type -> Bool
isCoercionTy Type
ty2) ]
                   ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence]
given_evs }

    ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed TyConApp" }

  where
    loc :: CtLoc
loc        = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    role :: Role
role       = EqRel -> Role
eqRelRole EqRel
eq_rel

      -- infinite, as tyConRolesX returns an infinite tail of Nominal
    tc_roles :: [Role]
tc_roles   = Role -> TyCon -> [Role]
tyConRolesX Role
role TyCon
tc

      -- Add nuances to the location during decomposition:
      --  * if the argument is a kind argument, remember this, so that error
      --    messages say "kind", not "type". This is determined based on whether
      --    the corresponding tyConBinder is named (that is, dependent)
      --  * if the argument is invisible, note this as well, again by
      --    looking at the corresponding binder
      -- For oversaturated tycons, we need the (repeat loc) tail, which doesn't
      -- do either of these changes. (Forgetting to do so led to #16188)
      --
      -- NB: infinite in length
    new_locs :: [CtLoc]
new_locs = [ CtLoc
new_loc
               | TyConBinder
bndr <- TyCon -> [TyConBinder]
tyConBinders TyCon
tc
               , let new_loc0 :: CtLoc
new_loc0 | TyConBinder -> Bool
isNamedTyConBinder TyConBinder
bndr = CtLoc -> CtLoc
toKindLoc CtLoc
loc
                              | Bool
otherwise               = CtLoc
loc
                     new_loc :: CtLoc
new_loc  | forall tv. VarBndr tv TyConBndrVis -> Bool
isInvisibleTyConBinder TyConBinder
bndr
                              = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
updateCtLocOrigin CtLoc
new_loc0 CtOrigin -> CtOrigin
toInvisibleOrigin
                              | Bool
otherwise
                              = CtLoc
new_loc0 ]
               forall a. [a] -> [a] -> [a]
++ forall a. a -> [a]
repeat CtLoc
loc

-- | Call when canonicalizing an equality fails, but if the equality is
-- representational, there is some hope for the future.
-- Examples in Note [Use canEqFailure in canDecomposableTyConApp]
canEqFailure :: CtEvidence -> EqRel
             -> TcType -> TcType -> TcS (StopOrContinue Ct)
canEqFailure :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqFailure CtEvidence
ev EqRel
NomEq Type
ty1 Type
ty2
  = CtEvidence -> Type -> Type -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Type
ty1 Type
ty2
canEqFailure CtEvidence
ev EqRel
ReprEq Type
ty1 Type
ty2
  = do { (Reduction
redn1, RewriterSet
rewriters1) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ty1
       ; (Reduction
redn2, RewriterSet
rewriters2) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ty2
            -- We must rewrite the types before putting them in the
            -- inert set, so that we are sure to kick them out when
            -- new equalities become available
       ; String -> SDoc -> TcS ()
traceTcS String
"canEqFailure with ReprEq" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, forall a. Outputable a => a -> SDoc
ppr Reduction
redn1, forall a. Outputable a => a -> SDoc
ppr Reduction
redn2 ]
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence (RewriterSet
rewriters1 forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters2) CtEvidence
ev SwapFlag
NotSwapped Reduction
redn1 Reduction
redn2
       ; forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
ReprEqReason CtEvidence
new_ev) }

-- | Call when canonicalizing an equality fails with utterly no hope.
canEqHardFailure :: CtEvidence
                 -> TcType -> TcType -> TcS (StopOrContinue Ct)
-- See Note [Make sure that insolubles are fully rewritten]
canEqHardFailure :: CtEvidence -> Type -> Type -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Type
ty1 Type
ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqHardFailure" (forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
ty2)
       ; (Reduction
redn1, RewriterSet
rewriters1) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ty1
       ; (Reduction
redn2, RewriterSet
rewriters2) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ty2
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence (RewriterSet
rewriters1 forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters2) CtEvidence
ev SwapFlag
NotSwapped Reduction
redn1 Reduction
redn2
       ; forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
ShapeMismatchReason CtEvidence
new_ev) }

{-
Note [Decomposing TyConApps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we see (T s1 t1 ~ T s2 t2), then we can just decompose to
  (s1 ~ s2, t1 ~ t2)
and push those back into the work list.  But if
  s1 = K k1    s2 = K k2
then we will just decomopose s1~s2, and it might be better to
do so on the spot.  An important special case is where s1=s2,
and we get just Refl.

So canDecomposableTyCon is a fast-path decomposition that uses
unifyWanted etc to short-cut that work.

Note [Canonicalising type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (s1 t1) ~ ty2, how should we proceed?
The simple thing is to see if ty2 is of form (s2 t2), and
decompose.

However, over-eager decomposition gives bad error messages
for things like
   a b ~ Maybe c
   e f ~ p -> q
Suppose (in the first example) we already know a~Array.  Then if we
decompose the application eagerly, yielding
   a ~ Maybe
   b ~ c
we get an error        "Can't match Array ~ Maybe",
but we'd prefer to get "Can't match Array b ~ Maybe c".

So instead can_eq_wanted_app rewrites the LHS and RHS, in the hope of
replacing (a b) by (Array b), before using try_decompose_app to
decompose it.

Note [Make sure that insolubles are fully rewritten]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When an equality fails, we still want to rewrite the equality
all the way down, so that it accurately reflects
 (a) the mutable reference substitution in force at start of solving
 (b) any ty-binds in force at this point in solving
See Note [Rewrite insolubles] in GHC.Tc.Solver.InertSet.
And if we don't do this there is a bad danger that
GHC.Tc.Solver.applyTyVarDefaulting will find a variable
that has in fact been substituted.

Note [Do not decompose Given polytype equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider [G] (forall a. t1 ~ forall a. t2).  Can we decompose this?
No -- what would the evidence look like?  So instead we simply discard
this given evidence.

Note [No top-level newtypes on RHS of representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we're in this situation:

 work item:  [W] c1 : a ~R b
     inert:  [G] c2 : b ~R Id a

where
  newtype Id a = Id a

We want to make sure canEqCanLHS sees [W] a ~R a, after b is rewritten
and the Id newtype is unwrapped. This is assured by requiring only rewritten
types in canEqCanLHS *and* having the newtype-unwrapping check above
the tyvar check in can_eq_nc.

Note [Put touchable variables on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ticket #10009, a very nasty example:

    f :: (UnF (F b) ~ b) => F b -> ()

    g :: forall a. (UnF (F a) ~ a) => a -> ()
    g _ = f (undefined :: F a)

For g we get [G]  g1 : UnF (F a) ~ a
             [W] w1 : UnF (F beta) ~ beta
             [W] w2 : F a ~ F beta

g1 is canonical (CEqCan). It is oriented as above because a is not touchable.
See canEqTyVarFunEq.

w1 is similarly canonical, though the occurs-check in canEqTyVarFunEq is key
here.

w2 is canonical. But which way should it be oriented? As written, we'll be
stuck. When w2 is added to the inert set, nothing gets kicked out: g1 is
a Given (and Wanteds don't rewrite Givens), and w2 doesn't mention the LHS
of w2. We'll thus lose.

But if w2 is swapped around, to

    [W] w3 : F beta ~ F a

then we'll kick w1 out of the inert
set (it mentions the LHS of w3). We then rewrite w1 to

    [W] w4 : UnF (F a) ~ beta

and then, using g1, to

    [W] w5 : a ~ beta

at which point we can unify and go on to glory. (This rewriting actually
happens all at once, in the call to rewrite during canonicalisation.)

But what about the new LHS makes it better? It mentions a variable (beta)
that can appear in a Wanted -- a touchable metavariable never appears
in a Given. On the other hand, the original LHS mentioned only variables
that appear in Givens. We thus choose to put variables that can appear
in Wanteds on the left.

Ticket #12526 is another good example of this in action.

-}

---------------------
canEqCanLHS :: CtEvidence            -- ev :: lhs ~ rhs
            -> EqRel -> SwapFlag
            -> CanEqLHS              -- lhs (or, if swapped, rhs)
            -> TcType                -- lhs: pretty lhs, already rewritten
            -> TcType -> TcType      -- rhs: already rewritten
            -> TcS (StopOrContinue Ct)
canEqCanLHS :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
xi2 Type
ps_xi2
  | Type
k1 HasDebugCallStack => Type -> Type -> Bool
`tcEqType` Type
k2
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
xi2 Type
ps_xi2

  | Bool
otherwise
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
k1 Type
xi2 Type
k2

  where
    k1 :: Type
k1 = CanEqLHS -> Type
canEqLHSKind CanEqLHS
lhs1
    k2 :: Type
k2 = HasDebugCallStack => Type -> Type
tcTypeKind Type
xi2

canEqCanLHSHetero :: CtEvidence         -- :: (xi1 :: ki1) ~ (xi2 :: ki2)
                  -> EqRel -> SwapFlag
                  -> CanEqLHS           -- xi1
                  -> TcKind             -- ki1
                  -> TcType             -- xi2
                  -> TcKind             -- ki2
                  -> TcS (StopOrContinue Ct)
canEqCanLHSHetero :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ki1 Type
xi2 Type
ki2
  -- See Note [Equalities with incompatible kinds]
  = do { (CtEvidence
kind_ev, TcCoercion
kind_co) <- TcS (CtEvidence, TcCoercion)
mk_kind_eq   -- :: ki2 ~N ki1

       ; let  -- kind_co :: (ki2 :: *) ~N (ki1 :: *)   (whether swapped or not)
             lhs_redn :: Reduction
lhs_redn = Role -> Type -> Reduction
mkReflRedn Role
role Type
xi1
             rhs_redn :: Reduction
rhs_redn = Role -> Type -> TcCoercion -> Reduction
mkGReflRightRedn Role
role Type
xi2 TcCoercion
kind_co

             -- See Note [Equalities with incompatible kinds], Wrinkle (1)
             -- This will be ignored in rewriteEqEvidence if the work item is a Given
             rewriters :: RewriterSet
rewriters = TcCoercion -> RewriterSet
rewriterSetFromCo TcCoercion
kind_co

       ; String -> SDoc -> TcS ()
traceTcS String
"Hetero equality gives rise to kind equality"
           (forall a. Outputable a => a -> SDoc
ppr TcCoercion
kind_co SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> [SDoc] -> SDoc
sep [ forall a. Outputable a => a -> SDoc
ppr Type
ki2, String -> SDoc
text String
"~#", forall a. Outputable a => a -> SDoc
ppr Type
ki1 ])
       ; CtEvidence
type_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
rewriters CtEvidence
ev SwapFlag
swapped Reduction
lhs_redn Reduction
rhs_redn

       ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence
type_ev]  -- delay the type equality until after we've finished
                               -- the kind equality, which may unlock things
                               -- See Note [Equalities with incompatible kinds]

       ; CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
kind_ev EqRel
NomEq Type
ki2 Type
ki1 }
  where
    mk_kind_eq :: TcS (CtEvidence, CoercionN)
    mk_kind_eq :: TcS (CtEvidence, TcCoercion)
mk_kind_eq = case CtEvidence
ev of
      CtGiven { ctev_evar :: CtEvidence -> TcTyVar
ctev_evar = TcTyVar
evar }
        -> do { let kind_co :: TcCoercion
kind_co = TcCoercion -> TcCoercion
maybe_sym forall a b. (a -> b) -> a -> b
$ TcCoercion -> TcCoercion
mkTcKindCo (TcTyVar -> TcCoercion
mkTcCoVarCo TcTyVar
evar)  -- :: k2 ~ k1
              ; CtEvidence
kind_ev <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
kind_loc (Type
kind_pty, TcCoercion -> EvTerm
evCoercion TcCoercion
kind_co)
              ; forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence
kind_ev, HasDebugCallStack => CtEvidence -> TcCoercion
ctEvCoercion CtEvidence
kind_ev) }

      CtWanted { ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters }
        -> CtLoc
-> RewriterSet
-> Role
-> Type
-> Type
-> TcS (CtEvidence, TcCoercion)
newWantedEq CtLoc
kind_loc RewriterSet
rewriters Role
Nominal Type
ki2 Type
ki1

    xi1 :: Type
xi1      = CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs1
    loc :: CtLoc
loc      = CtEvidence -> CtLoc
ctev_loc CtEvidence
ev
    role :: Role
role     = EqRel -> Role
eqRelRole EqRel
eq_rel
    kind_loc :: CtLoc
kind_loc = Type -> Type -> CtLoc -> CtLoc
mkKindLoc Type
xi1 Type
xi2 CtLoc
loc
    kind_pty :: Type
kind_pty = Type -> Type -> Type -> Type -> Type
mkHeteroPrimEqPred Type
liftedTypeKind Type
liftedTypeKind Type
ki2 Type
ki1

    maybe_sym :: TcCoercion -> TcCoercion
maybe_sym = case SwapFlag
swapped of
          SwapFlag
IsSwapped  -> forall a. a -> a
id         -- if the input is swapped, then we already
                                   -- will have k2 ~ k1
          SwapFlag
NotSwapped -> TcCoercion -> TcCoercion
mkTcSymCo

-- guaranteed that tcTypeKind lhs == tcTypeKind rhs
canEqCanLHSHomo :: CtEvidence
                -> EqRel -> SwapFlag
                -> CanEqLHS           -- lhs (or, if swapped, rhs)
                -> TcType             -- pretty lhs
                -> TcType -> TcType   -- rhs, pretty rhs
                -> TcS (StopOrContinue Ct)
canEqCanLHSHomo :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue Ct)
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
xi2 Type
ps_xi2
  | (Type
xi2', MCoercion
mco) <- Type -> (Type, MCoercion)
split_cast_ty Type
xi2
  , Just CanEqLHS
lhs2 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
xi2'
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> CanEqLHS
-> Type
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 CanEqLHS
lhs2 (Type
ps_xi2 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco) MCoercion
mco

  | Bool
otherwise
  = CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi2

  where
    split_cast_ty :: Type -> (Type, MCoercion)
split_cast_ty (CastTy Type
ty TcCoercion
co) = (Type
ty, TcCoercion -> MCoercion
MCo TcCoercion
co)
    split_cast_ty Type
other          = (Type
other, MCoercion
MRefl)

-- This function deals with the case that both LHS and RHS are potential
-- CanEqLHSs.
canEqCanLHS2 :: CtEvidence              -- lhs ~ (rhs |> mco)
                                        -- or, if swapped: (rhs |> mco) ~ lhs
             -> EqRel -> SwapFlag
             -> CanEqLHS                -- lhs (or, if swapped, rhs)
             -> TcType                  -- pretty lhs
             -> CanEqLHS                -- rhs
             -> TcType                  -- pretty rhs
             -> MCoercion               -- :: kind(rhs) ~N kind(lhs)
             -> TcS (StopOrContinue Ct)
canEqCanLHS2 :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> CanEqLHS
-> Type
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 CanEqLHS
lhs2 Type
ps_xi2 MCoercion
mco
  | CanEqLHS
lhs1 CanEqLHS -> CanEqLHS -> Bool
`eqCanEqLHS` CanEqLHS
lhs2
    -- It must be the case that mco is reflexive
  = CtEvidence -> EqRel -> Type -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel (CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs1)

  | TyVarLHS TcTyVar
tv1 <- CanEqLHS
lhs1
  , TyVarLHS TcTyVar
tv2 <- CanEqLHS
lhs2
  , Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars (CtEvidence -> Bool
isGiven CtEvidence
ev) TcTyVar
tv1 TcTyVar
tv2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqLHS2 swapOver" (forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv2 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped)
       ; CtEvidence
new_ev <- TcS CtEvidence
do_swap
       ; CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped (TcTyVar -> CanEqLHS
TyVarLHS TcTyVar
tv2)
                                                   (Type
ps_xi1 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion
sym_mco) }

  | TyVarLHS TcTyVar
tv1 <- CanEqLHS
lhs1
  , TyFamLHS TyCon
fun_tc2 [Type]
fun_args2 <- CanEqLHS
lhs2
  = CtEvidence
-> EqRel
-> SwapFlag
-> TcTyVar
-> Type
-> TyCon
-> [Type]
-> Type
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqTyVarFunEq CtEvidence
ev EqRel
eq_rel SwapFlag
swapped TcTyVar
tv1 Type
ps_xi1 TyCon
fun_tc2 [Type]
fun_args2 Type
ps_xi2 MCoercion
mco

  | TyFamLHS TyCon
fun_tc1 [Type]
fun_args1 <- CanEqLHS
lhs1
  , TyVarLHS TcTyVar
tv2 <- CanEqLHS
lhs2
  = do { CtEvidence
new_ev <- TcS CtEvidence
do_swap
       ; CtEvidence
-> EqRel
-> SwapFlag
-> TcTyVar
-> Type
-> TyCon
-> [Type]
-> Type
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqTyVarFunEq CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped TcTyVar
tv2 Type
ps_xi2
                                                 TyCon
fun_tc1 [Type]
fun_args1 Type
ps_xi1 MCoercion
sym_mco }

  | TyFamLHS TyCon
fun_tc1 [Type]
fun_args1 <- CanEqLHS
lhs1
  , TyFamLHS TyCon
fun_tc2 [Type]
fun_args2 <- CanEqLHS
lhs2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHS2 two type families" (forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs2)

         -- emit wanted equalities for injective type families
       ; let inj_eqns :: [TypeEqn]  -- TypeEqn = Pair Type
             inj_eqns :: [Pair Type]
inj_eqns
               | EqRel
ReprEq <- EqRel
eq_rel   = []   -- injectivity applies only for nom. eqs.
               | TyCon
fun_tc1 forall a. Eq a => a -> a -> Bool
/= TyCon
fun_tc2 = []   -- if the families don't match, stop.

               | Injective [Bool]
inj <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fun_tc1
               = [ forall a. a -> a -> Pair a
Pair Type
arg1 Type
arg2
                 | (Type
arg1, Type
arg2, Bool
True) <- forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [Type]
fun_args1 [Type]
fun_args2 [Bool]
inj ]

                 -- built-in synonym families don't have an entry point
                 -- for this use case. So, we just use sfInteractInert
                 -- and pass two equal RHSs. We *could* add another entry
                 -- point, but then there would be a burden to make
                 -- sure the new entry point and existing ones were
                 -- internally consistent. This is slightly distasteful,
                 -- but it works well in practice and localises the
                 -- problem.
               | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fun_tc1
               = let ki1 :: Type
ki1 = CanEqLHS -> Type
canEqLHSKind CanEqLHS
lhs1
                     ki2 :: Type
ki2 | MCoercion
MRefl <- MCoercion
mco
                         = Type
ki1   -- just a small optimisation
                         | Bool
otherwise
                         = CanEqLHS -> Type
canEqLHSKind CanEqLHS
lhs2

                     fake_rhs1 :: Type
fake_rhs1 = Type -> Type
anyTypeOfKind Type
ki1
                     fake_rhs2 :: Type
fake_rhs2 = Type -> Type
anyTypeOfKind Type
ki2
                 in
                 BuiltInSynFamily -> [Type] -> Type -> [Type] -> Type -> [Pair Type]
sfInteractInert BuiltInSynFamily
ops [Type]
fun_args1 Type
fake_rhs1 [Type]
fun_args2 Type
fake_rhs2

               | Bool
otherwise  -- ordinary, non-injective type family
               = []

       ; case CtEvidence
ev of
           CtWanted { ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } ->
             forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (\ (Pair Type
t1 Type
t2) -> RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev) Role
Nominal Type
t1 Type
t2) [Pair Type]
inj_eqns
           CtGiven {} -> forall (m :: * -> *) a. Monad m => a -> m a
return ()
             -- See Note [No Given/Given fundeps] in GHC.Tc.Solver.Interact

       ; TcLevel
tclvl <- TcS TcLevel
getTcLevel
       ; let tvs1 :: VarSet
tvs1 = [Type] -> VarSet
tyCoVarsOfTypes [Type]
fun_args1
             tvs2 :: VarSet
tvs2 = [Type] -> VarSet
tyCoVarsOfTypes [Type]
fun_args2

             swap_for_rewriting :: Bool
swap_for_rewriting = (TcTyVar -> Bool) -> VarSet -> Bool
anyVarSet (TcLevel -> TcTyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) VarSet
tvs2 Bool -> Bool -> Bool
&&
                          -- swap 'em: Note [Put touchable variables on the left]
                                  Bool -> Bool
not ((TcTyVar -> Bool) -> VarSet -> Bool
anyVarSet (TcLevel -> TcTyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) VarSet
tvs1)
                          -- this check is just to avoid unfruitful swapping

               -- If we have F a ~ F (F a), we want to swap.
             swap_for_occurs :: Bool
swap_for_occurs
               | CheckTyEqResult -> Bool
cterHasNoProblem   forall a b. (a -> b) -> a -> b
$ TyCon -> [Type] -> Type -> CheckTyEqResult
checkTyFamEq TyCon
fun_tc2 [Type]
fun_args2
                                                   (TyCon -> [Type] -> Type
mkTyConApp TyCon
fun_tc1 [Type]
fun_args1)
               , CheckTyEqResult -> Bool
cterHasOccursCheck forall a b. (a -> b) -> a -> b
$ TyCon -> [Type] -> Type -> CheckTyEqResult
checkTyFamEq TyCon
fun_tc1 [Type]
fun_args1
                                                   (TyCon -> [Type] -> Type
mkTyConApp TyCon
fun_tc2 [Type]
fun_args2)
               = Bool
True

               | Bool
otherwise
               = Bool
False

       ; if Bool
swap_for_rewriting Bool -> Bool -> Bool
|| Bool
swap_for_occurs
         then do { CtEvidence
new_ev <- TcS CtEvidence
do_swap
                 ; CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
lhs2 (Type
ps_xi1 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion
sym_mco) }
         else TcS (StopOrContinue Ct)
finish_without_swapping }

  -- that's all the special cases. Now we just figure out which non-special case
  -- to continue to.
  | Bool
otherwise
  = TcS (StopOrContinue Ct)
finish_without_swapping

  where
    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco

    do_swap :: TcS CtEvidence
do_swap = CtEvidence
-> EqRel -> SwapFlag -> Type -> Type -> MCoercion -> TcS CtEvidence
rewriteCastedEquality CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs1) (CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs2) MCoercion
mco
    finish_without_swapping :: TcS (StopOrContinue Ct)
finish_without_swapping = CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 (Type
ps_xi2 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion
mco)


-- This function handles the case where one side is a tyvar and the other is
-- a type family application. Which to put on the left?
--   If the tyvar is a touchable meta-tyvar, put it on the left, as this may
--   be our only shot to unify.
--   Otherwise, put the function on the left, because it's generally better to
--   rewrite away function calls. This makes types smaller. And it seems necessary:
--     [W] F alpha ~ alpha
--     [W] F alpha ~ beta
--     [W] G alpha beta ~ Int   ( where we have type instance G a a = a )
--   If we end up with a stuck alpha ~ F alpha, we won't be able to solve this.
--   Test case: indexed-types/should_compile/CEqCanOccursCheck
canEqTyVarFunEq :: CtEvidence               -- :: lhs ~ (rhs |> mco)
                                            -- or (rhs |> mco) ~ lhs if swapped
                -> EqRel -> SwapFlag
                -> TyVar -> TcType          -- lhs (or if swapped rhs), pretty lhs
                -> TyCon -> [Xi] -> TcType  -- rhs (or if swapped lhs) fun and args, pretty rhs
                -> MCoercion                -- :: kind(rhs) ~N kind(lhs)
                -> TcS (StopOrContinue Ct)
canEqTyVarFunEq :: CtEvidence
-> EqRel
-> SwapFlag
-> TcTyVar
-> Type
-> TyCon
-> [Type]
-> Type
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqTyVarFunEq CtEvidence
ev EqRel
eq_rel SwapFlag
swapped TcTyVar
tv1 Type
ps_xi1 TyCon
fun_tc2 [Type]
fun_args2 Type
ps_xi2 MCoercion
mco
  = do { TouchabilityTestResult
is_touchable <- CtFlavour -> TcTyVar -> Type -> TcS TouchabilityTestResult
touchabilityTest (CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev) TcTyVar
tv1 Type
rhs
       ; if | case TouchabilityTestResult
is_touchable of { TouchabilityTestResult
Untouchable -> Bool
False; TouchabilityTestResult
_ -> Bool
True }
            , CheckTyEqResult -> Bool
cterHasNoProblem forall a b. (a -> b) -> a -> b
$
                TcTyVar -> Type -> CheckTyEqResult
checkTyVarEq TcTyVar
tv1 Type
rhs CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
`cterRemoveProblem` CheckTyEqProblem
cteTypeFamily
            -> CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (TcTyVar -> CanEqLHS
TyVarLHS TcTyVar
tv1) Type
rhs

            | Bool
otherwise
              -> do { CtEvidence
new_ev <- CtEvidence
-> EqRel -> SwapFlag -> Type -> Type -> MCoercion -> TcS CtEvidence
rewriteCastedEquality CtEvidence
ev EqRel
eq_rel SwapFlag
swapped
                                  (TcTyVar -> Type
mkTyVarTy TcTyVar
tv1) (TyCon -> [Type] -> Type
mkTyConApp TyCon
fun_tc2 [Type]
fun_args2)
                                  MCoercion
mco
                    ; CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped
                                  (TyCon -> [Type] -> CanEqLHS
TyFamLHS TyCon
fun_tc2 [Type]
fun_args2)
                                  (Type
ps_xi1 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion
sym_mco) } }
  where
    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco
    rhs :: Type
rhs = Type
ps_xi2 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion
mco

-- The RHS here is either not CanEqLHS, or it's one that we
-- want to rewrite the LHS to (as per e.g. swapOverTyVars)
canEqCanLHSFinish :: CtEvidence
                  -> EqRel -> SwapFlag
                  -> CanEqLHS             -- lhs (or, if swapped, rhs)
                  -> TcType               -- rhs (or, if swapped, lhs)
                  -> TcS (StopOrContinue Ct)
canEqCanLHSFinish :: CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Type -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
rhs
-- RHS is fully rewritten, but with type synonyms
-- preserved as much as possible
-- guaranteed that tyVarKind lhs == typeKind rhs, for (TyEq:K)
-- (TyEq:N) is checked in can_eq_nc', and (TyEq:TV) is handled in canEqCanLHS2

  = do {
          -- this performs the swap if necessary
         CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                                     (Role -> Type -> Reduction
mkReflRedn Role
role Type
lhs_ty)
                                     (Role -> Type -> Reduction
mkReflRedn Role
role Type
rhs)

     -- by now, (TyEq:K) is already satisfied
       ; forall (m :: * -> *). (HasCallStack, Applicative m) => Bool -> m ()
massert (CanEqLHS -> Type
canEqLHSKind CanEqLHS
lhs Type -> Type -> Bool
`eqType` HasDebugCallStack => Type -> Type
tcTypeKind Type
rhs)

     -- by now, (TyEq:N) is already satisfied (if applicable)
       ; forall (m :: * -> *).
(HasCallStack, Monad m) =>
m Bool -> SDoc -> m ()
assertPprM TcS Bool
ty_eq_N_OK forall a b. (a -> b) -> a -> b
$
           [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"CanEqCanLHSFinish: (TyEq:N) not satisfied"
                , String -> SDoc
text String
"rhs:" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
rhs
                ]

     -- guarantees (TyEq:OC), (TyEq:F)
     -- Must do the occurs check even on tyvar/tyvar
     -- equalities, in case have  x ~ (y :: ..x...); this is #12593.
       ; let result0 :: CheckTyEqResult
result0 = CanEqLHS -> Type -> CheckTyEqResult
checkTypeEq CanEqLHS
lhs Type
rhs CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
`cterRemoveProblem` CheckTyEqProblem
cteTypeFamily
     -- type families are OK here
     -- NB: no occCheckExpand here; see Note [Rewriting synonyms] in GHC.Tc.Solver.Rewrite

              -- a ~R# b a is soluble if b later turns out to be Identity
             result :: CheckTyEqResult
result = case EqRel
eq_rel of
                        EqRel
NomEq  -> CheckTyEqResult
result0
                        EqRel
ReprEq -> CheckTyEqResult -> CheckTyEqResult
cterSetOccursCheckSoluble CheckTyEqResult
result0

             reason :: CtIrredReason
reason = CheckTyEqResult -> CtIrredReason
NonCanonicalReason CheckTyEqResult
result

       ; if CheckTyEqResult -> Bool
cterHasNoProblem CheckTyEqResult
result
         then do { String -> SDoc -> TcS ()
traceTcS String
"CEqCan" (forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
rhs)
                 ; forall a. a -> TcS (StopOrContinue a)
continueWith (CEqCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
new_ev, cc_lhs :: CanEqLHS
cc_lhs = CanEqLHS
lhs
                                        , cc_rhs :: Type
cc_rhs = Type
rhs, cc_eq_rel :: EqRel
cc_eq_rel = EqRel
eq_rel }) }

         else do { Maybe Reduction
m_stuff <- CtEvidence
-> CheckTyEqResult -> CanEqLHS -> Type -> TcS (Maybe Reduction)
breakTyEqCycle_maybe CtEvidence
ev CheckTyEqResult
result CanEqLHS
lhs Type
rhs
                           -- See Note [Type equality cycles];
                           -- returning Nothing is the vastly common case
                 ; case Maybe Reduction
m_stuff of
                     { Maybe Reduction
Nothing ->
                         do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHSFinish can't make a canonical"
                                       (forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
rhs)
                            ; forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
reason CtEvidence
new_ev) }
                     ; Just rhs_redn :: Reduction
rhs_redn@(Reduction TcCoercion
_ Type
new_rhs) ->
              do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHSFinish breaking a cycle" forall a b. (a -> b) -> a -> b
$
                            forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
rhs
                 ; String -> SDoc -> TcS ()
traceTcS String
"new RHS:" (forall a. Outputable a => a -> SDoc
ppr Type
new_rhs)

                   -- This check is Detail (1) in the Note
                 ; if CheckTyEqResult -> Bool
cterHasOccursCheck (CanEqLHS -> Type -> CheckTyEqResult
checkTypeEq CanEqLHS
lhs Type
new_rhs)

                   then do { String -> SDoc -> TcS ()
traceTcS String
"Note [Type equality cycles] Detail (1)"
                                      (forall a. Outputable a => a -> SDoc
ppr Type
new_rhs)
                           ; forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredReason -> CtEvidence -> Ct
mkIrredCt CtIrredReason
reason CtEvidence
new_ev) }

                   else do { -- See Detail (6) of Note [Type equality cycles]
                             CtEvidence
new_new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet
                                             CtEvidence
new_ev SwapFlag
NotSwapped
                                             (Role -> Type -> Reduction
mkReflRedn Role
Nominal Type
lhs_ty)
                                             Reduction
rhs_redn

                           ; forall a. a -> TcS (StopOrContinue a)
continueWith (CEqCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
new_new_ev
                                                  , cc_lhs :: CanEqLHS
cc_lhs = CanEqLHS
lhs
                                                  , cc_rhs :: Type
cc_rhs = Type
new_rhs
                                                  , cc_eq_rel :: EqRel
cc_eq_rel = EqRel
eq_rel }) }}}}}
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

    lhs_ty :: Type
lhs_ty = CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs

    -- This is about (TyEq:N): check that we don't have a newtype
    -- whose constructor is in scope at the top-level of the RHS.
    ty_eq_N_OK :: TcS Bool
    ty_eq_N_OK :: TcS Bool
ty_eq_N_OK
      | EqRel
ReprEq <- EqRel
eq_rel
      , Just TyCon
tc <- Type -> Maybe TyCon
tyConAppTyCon_maybe Type
rhs
      , Just DataCon
con <- TyCon -> Maybe DataCon
newTyConDataCon_maybe TyCon
tc
      -- #21010: only a problem if the newtype constructor is in scope
      -- yet we didn't rewrite it away.
      = do { GlobalRdrEnv
rdr_env <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
           ; let con_in_scope :: Bool
con_in_scope = forall a. Maybe a -> Bool
isJust forall a b. (a -> b) -> a -> b
$ GlobalRdrEnv -> Name -> Maybe GlobalRdrElt
lookupGRE_Name GlobalRdrEnv
rdr_env (DataCon -> Name
dataConName DataCon
con)
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ Bool -> Bool
not Bool
con_in_scope }
      | Bool
otherwise
      = forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True

-- | Solve a reflexive equality constraint
canEqReflexive :: CtEvidence    -- ty ~ ty
               -> EqRel
               -> TcType        -- ty
               -> TcS (StopOrContinue Ct)   -- always Stop
canEqReflexive :: CtEvidence -> EqRel -> Type -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
ty
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev (TcCoercion -> EvTerm
evCoercion forall a b. (a -> b) -> a -> b
$
                               Role -> Type -> TcCoercion
mkTcReflCo (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
ty)
       ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved by reflexivity" }

rewriteCastedEquality :: CtEvidence     -- :: lhs ~ (rhs |> mco), or (rhs |> mco) ~ lhs
                      -> EqRel -> SwapFlag
                      -> TcType         -- lhs
                      -> TcType         -- rhs
                      -> MCoercion      -- mco
                      -> TcS CtEvidence -- :: (lhs |> sym mco) ~ rhs
                                        -- result is independent of SwapFlag
rewriteCastedEquality :: CtEvidence
-> EqRel -> SwapFlag -> Type -> Type -> MCoercion -> TcS CtEvidence
rewriteCastedEquality CtEvidence
ev EqRel
eq_rel SwapFlag
swapped Type
lhs Type
rhs MCoercion
mco
  = RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped Reduction
lhs_redn Reduction
rhs_redn
  where
    lhs_redn :: Reduction
lhs_redn = Role -> Type -> MCoercion -> Reduction
mkGReflRightMRedn Role
role Type
lhs MCoercion
sym_mco
    rhs_redn :: Reduction
rhs_redn = Role -> Type -> MCoercion -> Reduction
mkGReflLeftMRedn  Role
role Type
rhs MCoercion
mco

    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco
    role :: Role
role    = EqRel -> Role
eqRelRole EqRel
eq_rel

{- Note [Equalities with incompatible kinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What do we do when we have an equality

  (tv :: k1) ~ (rhs :: k2)

where k1 and k2 differ? Easy: we create a coercion that relates k1 and
k2 and use this to cast. To wit, from

  [X] (tv :: k1) ~ (rhs :: k2)

(where [X] is [G] or [W]), we go to

  [X] co :: k2 ~ k1
  [X] (tv :: k1) ~ ((rhs |> co) :: k1)

We carry on with the *kind equality*, not the type equality, because
solving the former may unlock the latter. This choice is made in
canEqCanLHSHetero. It is important: otherwise, T13135 loops.

Wrinkles:

 (1) When X is W, the new type-level wanted is effectively rewritten by the
     kind-level one. We thus include the kind-level wanted in the RewriterSet
     for the type-level one. See Note [Wanteds rewrite Wanteds] in GHC.Tc.Types.Constraint.
     This is done in canEqCanLHSHetero.

 (2) If we have [W] w :: alpha ~ (rhs |> co_hole), should we unify alpha? No.
     The problem is that the wanted w is effectively rewritten by another wanted,
     and unifying alpha effectively promotes this wanted to a given. Doing so
     means we lose track of the rewriter set associated with the wanted.

     On the other hand, w is perfectly suitable for rewriting, because of the
     way we carefully track rewriter sets.

     We thus allow w to be a CEqCan, but we prevent unification. See
     Note [Unification preconditions] in GHC.Tc.Utils.Unify.

     The only tricky part is that we must later indeed unify if/when the kind-level
     wanted gets solved. This is done in kickOutAfterFillingCoercionHole,
     which kicks out all equalities whose RHS mentions the filled-in coercion hole.
     Note that it looks for type family equalities, too, because of the use of
     unifyTest in canEqTyVarFunEq.

 (3) Suppose we have [W] (a :: k1) ~ (rhs :: k2). We duly follow the
     algorithm detailed here, producing [W] co :: k2 ~ k1, and adding
     [W] (a :: k1) ~ ((rhs |> co) :: k1) to the irreducibles. Some time
     later, we solve co, and fill in co's coercion hole. This kicks out
     the irreducible as described in (2).
     But now, during canonicalization, we see the cast
     and remove it, in canEqCast. By the time we get into canEqCanLHS, the equality
     is heterogeneous again, and the process repeats.

     To avoid this, we don't strip casts off a type if the other type
     in the equality is a CanEqLHS (the scenario above can happen with a
     type family, too. testcase: typecheck/should_compile/T13822).
     And this is an improvement regardless:
     because tyvars can, generally, unify with casted types, there's no
     reason to go through the work of stripping off the cast when the
     cast appears opposite a tyvar. This is implemented in the cast case
     of can_eq_nc'.

Historical note:

We used to do this via emitting a Derived kind equality and then parking
the heterogeneous equality as irreducible. But this new approach is much
more direct. And it doesn't produce duplicate Deriveds (as the old one did).

Note [Type synonyms and canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We treat type synonym applications as xi types, that is, they do not
count as type function applications.  However, we do need to be a bit
careful with type synonyms: like type functions they may not be
generative or injective.  However, unlike type functions, they are
parametric, so there is no problem in expanding them whenever we see
them, since we do not need to know anything about their arguments in
order to expand them; this is what justifies not having to treat them
as specially as type function applications.  The thing that causes
some subtleties is that we prefer to leave type synonym applications
*unexpanded* whenever possible, in order to generate better error
messages.

If we encounter an equality constraint with type synonym applications
on both sides, or a type synonym application on one side and some sort
of type application on the other, we simply must expand out the type
synonyms in order to continue decomposing the equality constraint into
primitive equality constraints.  For example, suppose we have

  type F a = [Int]

and we encounter the equality

  F a ~ [b]

In order to continue we must expand F a into [Int], giving us the
equality

  [Int] ~ [b]

which we can then decompose into the more primitive equality
constraint

  Int ~ b.

However, if we encounter an equality constraint with a type synonym
application on one side and a variable on the other side, we should
NOT (necessarily) expand the type synonym, since for the purpose of
good error messages we want to leave type synonyms unexpanded as much
as possible.  Hence the ps_xi1, ps_xi2 argument passed to canEqCanLHS.

Note [Type equality cycles]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this situation (from indexed-types/should_compile/GivenLoop):

  instance C (Maybe b)
  *[G] a ~ Maybe (F a)
  [W] C a

or (typecheck/should_compile/T19682b):

  instance C (a -> b)
  *[W] alpha ~ (Arg alpha -> Res alpha)
  [W] C alpha

or (typecheck/should_compile/T21515):

  type family Code a
  *[G] Code a ~ '[ '[ Head (Head (Code a)) ] ]
  [W] Code a ~ '[ '[ alpha ] ]

In order to solve the final Wanted, we must use the starred constraint
for rewriting. But note that all starred constraints have occurs-check failures,
and so we can't straightforwardly add these to the inert set and
use them for rewriting. (NB: A rigid type constructor is at the
top of all RHSs, preventing reorienting in canEqTyVarFunEq in the tyvar
cases.)

The key idea is to replace the outermost type family applications in the RHS of the
starred constraints with a fresh variable, which we'll call a cycle-breaker
variable, or cbv. Then, relate the cbv back with the original type family application
via new equality constraints. Our situations thus become:

  instance C (Maybe b)
  [G] a ~ Maybe cbv
  [G] F a ~ cbv
  [W] C a

or

  instance C (a -> b)
  [W] alpha ~ (cbv1 -> cbv2)
  [W] Arg alpha ~ cbv1
  [W] Res alpha ~ cbv2
  [W] C alpha

or

  [G] Code a ~ '[ '[ cbv ] ]
  [G] Head (Head (Code a)) ~ cbv
  [W] Code a ~ '[ '[ alpha ] ]

This transformation (creating the new types and emitting new equality
constraints) is done in breakTyEqCycle_maybe.

The details depend on whether we're working with a Given or a Wanted.

Given
-----

We emit a new Given, [G] F a ~ cbv, equating the type family application to
our new cbv. Note its orientation: The type family ends up on the left; see
commentary on canEqTyVarFunEq, which decides how to orient such cases. No
special treatment for CycleBreakerTvs is necessary. This scenario is now
easily soluble, by using the first Given to rewrite the Wanted, which can now
be solved.

(The first Given actually also rewrites the second one, giving
[G] F (Maybe cbv) ~ cbv, but this causes no trouble.)

Of course, we don't want our fresh variables leaking into e.g. error messages.
So we fill in the metavariables with their original type family applications
after we're done running the solver (in nestImplicTcS and runTcSWithEvBinds).
This is done by restoreTyVarCycles, which uses the inert_cycle_breakers field in
InertSet, which contains the pairings invented in breakTyEqCycle_maybe.

That is:

We transform
  [G] g : lhs ~ ...(F lhs)...
to
  [G] (Refl lhs) : F lhs ~ cbv      -- CEqCan
  [G] g          : lhs ~ ...cbv...  -- CEqCan

Note that
* `cbv` is a fresh cycle breaker variable.
* `cbv` is a is a meta-tyvar, but it is completely untouchable.
* We track the cycle-breaker variables in inert_cycle_breakers in InertSet
* We eventually fill in the cycle-breakers, with `cbv := F lhs`.
  No one else fills in cycle-breakers!
* The evidence for the new `F lhs ~ cbv` constraint is Refl, because we know
  this fill-in is ultimately going to happen.
* In inert_cycle_breakers, we remember the (cbv, F lhs) pair; that is, we
  remember the /original/ type.  The [G] F lhs ~ cbv constraint may be rewritten
  by other givens (eg if we have another [G] lhs ~ (b,c)), but at the end we
  still fill in with cbv := F lhs
* This fill-in is done when solving is complete, by restoreTyVarCycles
  in nestImplicTcS and runTcSWithEvBinds.

Wanted
------
The fresh cycle-breaker variables here must actually be normal, touchable
metavariables. That is, they are TauTvs. Nothing at all unusual. Repeating
the example from above, we have

  *[W] alpha ~ (Arg alpha -> Res alpha)

and we turn this into

  *[W] alpha ~ (cbv1 -> cbv2)
  [W] Arg alpha ~ cbv1
  [W] Res alpha ~ cbv2

where cbv1 and cbv2 are fresh TauTvs. Why TauTvs? See [Why TauTvs] below.

Critically, we emit the two new constraints (the last two above)
directly instead of calling unifyWanted. (Otherwise, we'd end up unifying cbv1
and cbv2 immediately, achieving nothing.)
Next, we unify alpha := cbv1 -> cbv2, having eliminated the occurs check. This
unification -- which must be the next step after breaking the cycles --
happens in the course of normal behavior of top-level
interactions, later in the solver pipeline. We know this unification will
indeed happen because breakTyEqCycle_maybe, which decides whether to apply
this logic, checks to ensure unification will succeed in its final_check.
(In particular, the LHS must be a touchable tyvar, never a type family. We don't
yet have an example of where this logic is needed with a type family, and it's
unclear how to handle this case, so we're skipping for now.) Now, we're
here (including further context from our original example, from the top of the
Note):

  instance C (a -> b)
  [W] Arg (cbv1 -> cbv2) ~ cbv1
  [W] Res (cbv1 -> cbv2) ~ cbv2
  [W] C (cbv1 -> cbv2)

The first two W constraints reduce to reflexivity and are discarded,
and the last is easily soluble.

[Why TauTvs]:
Let's look at another example (typecheck/should_compile/T19682) where we need
to unify the cbvs:

  class    (AllEqF xs ys, SameShapeAs xs ys) => AllEq xs ys
  instance (AllEqF xs ys, SameShapeAs xs ys) => AllEq xs ys

  type family SameShapeAs xs ys :: Constraint where
    SameShapeAs '[] ys      = (ys ~ '[])
    SameShapeAs (x : xs) ys = (ys ~ (Head ys : Tail ys))

  type family AllEqF xs ys :: Constraint where
    AllEqF '[]      '[]      = ()
    AllEqF (x : xs) (y : ys) = (x ~ y, AllEq xs ys)

  [W] alpha ~ (Head alpha : Tail alpha)
  [W] AllEqF '[Bool] alpha

Without the logic detailed in this Note, we're stuck here, as AllEqF cannot
reduce and alpha cannot unify. Let's instead apply our cycle-breaker approach,
just as described above. We thus invent cbv1 and cbv2 and unify
alpha := cbv1 -> cbv2, yielding (after zonking)

  [W] Head (cbv1 : cbv2) ~ cbv1
  [W] Tail (cbv1 : cbv2) ~ cbv2
  [W] AllEqF '[Bool] (cbv1 : cbv2)

The first two W constraints simplify to reflexivity and are discarded.
But the last reduces:

  [W] Bool ~ cbv1
  [W] AllEq '[] cbv2

The first of these is solved by unification: cbv1 := Bool. The second
is solved by the instance for AllEq to become

  [W] AllEqF '[] cbv2
  [W] SameShapeAs '[] cbv2

While the first of these is stuck, the second makes progress, to lead to

  [W] AllEqF '[] cbv2
  [W] cbv2 ~ '[]

This second constraint is solved by unification: cbv2 := '[]. We now
have

  [W] AllEqF '[] '[]

which reduces to

  [W] ()

which is trivially satisfiable. Hooray!

Note that we need to unify the cbvs here; if we did not, there would be
no way to solve those constraints. That's why the cycle-breakers are
ordinary TauTvs.

In all cases
------------

We detect this scenario by the following characteristics:
 - a constraint with a soluble occurs-check failure
   (as indicated by the cteSolubleOccurs bit set in a CheckTyEqResult
   from checkTypeEq)
 - and a nominal equality
 - and either
    - a Given flavour (but see also Detail (7) below)
    - a Wanted flavour, with a touchable metavariable on the left

We don't use this trick for representational equalities, as there is no
concrete use case where it is helpful (unlike for nominal equalities).
Furthermore, because function applications can be CanEqLHSs, but newtype
applications cannot, the disparities between the cases are enough that it
would be effortful to expand the idea to representational equalities. A quick
attempt, with

      data family N a b

      f :: (Coercible a (N a b), Coercible (N a b) b) => a -> b
      f = coerce

failed with "Could not match 'b' with 'b'." Further work is held off
until when we have a concrete incentive to explore this dark corner.

Details:

 (1) We don't look under foralls, at all, when substituting away type family
     applications, because doing so can never be fruitful. Recall that we
     are in a case like [G] lhs ~ forall b. ... lhs ....   Until we have a type
     family that can pull the body out from a forall (e.g. type instance F (forall b. ty) = ty),
     this will always be
     insoluble. Note also that the forall cannot be in an argument to a
     type family, or that outer type family application would already have
     been substituted away.

     However, we still must check to make sure that breakTyEqCycle_maybe actually
     succeeds in getting rid of all occurrences of the offending lhs. If
     one is hidden under a forall, this won't be true. A similar problem can
     happen if the variable appears only in a kind
     (e.g. k ~ ... (a :: k) ...). So we perform an additional check after
     performing the substitution. It is tiresome to re-run all of checkTypeEq
     here, but reimplementing just the occurs-check is even more tiresome.

     Skipping this check causes typecheck/should_fail/GivenForallLoop and
     polykinds/T18451 to loop.

 (2) Our goal here is to avoid loops in rewriting. We can thus skip looking
     in coercions, as we don't rewrite in coercions in the algorithm in
     GHC.Solver.Rewrite. (This is another reason
     we need to re-check that we've gotten rid of all occurrences of the
     offending variable.)

 (3) As we're substituting as described in this Note, we can build ill-kinded
     types. For example, if we have Proxy (F a) b, where (b :: F a), then
     replacing this with Proxy cbv b is ill-kinded. However, we will later
     set cbv := F a, and so the zonked type will be well-kinded again.
     The temporary ill-kinded type hurts no one, and avoiding this would
     be quite painfully difficult.

     Specifically, this detail does not contravene the Purely Kinded Type Invariant
     (Note [The Purely Kinded Type Invariant (PKTI)] in GHC.Tc.Gen.HsType).
     The PKTI says that we can call typeKind on any type, without failure.
     It would be violated if we, say, replaced a kind (a -> b) with a kind c,
     because an arrow kind might be consulted in piResultTys. Here, we are
     replacing one opaque type like (F a b c) with another, cbv (opaque in
     that we never assume anything about its structure, like that it has a
     result type or a RuntimeRep argument).

 (4) The evidence for the produced Givens is all just reflexive, because
     we will eventually set the cycle-breaker variable to be the type family,
     and then, after the zonk, all will be well. See also the notes at the
     end of the Given section of this Note.

 (5) The approach here is inefficient because it replaces every (outermost)
     type family application with a type variable, regardless of whether that
     particular appplication is implicated in the occurs check.  An alternative
     would be to replce only type-family applications that meantion the offending LHS.
     For instance, we could choose to
     affect only type family applications that mention the offending LHS:
     e.g. in a ~ (F b, G a), we need to replace only G a, not F b. Furthermore,
     we could try to detect cases like a ~ (F a, F a) and use the same
     tyvar to replace F a. (Cf.
     Note [Flattening type-family applications when matching instances]
     in GHC.Core.Unify, which
     goes to this extra effort.) There may be other opportunities for
     improvement. However, this is really a very small corner case.
     The investment to craft a clever,
     performant solution seems unworthwhile.

 (6) We often get the predicate associated with a constraint from its
     evidence with ctPred. We thus must not only make sure the generated
     CEqCan's fields have the updated RHS type (that is, the one produced
     by replacing type family applications with fresh variables),
     but we must also update the evidence itself. This is done by the call to rewriteEqEvidence
     in canEqCanLHSFinish.

 (7) We don't wish to apply this magic on the equalities created
     by this very same process.
     Consider this, from typecheck/should_compile/ContextStack2:

       type instance TF (a, b) = (TF a, TF b)
       t :: (a ~ TF (a, Int)) => ...

       [G] a ~ TF (a, Int)

     The RHS reduces, so we get

       [G] a ~ (TF a, TF Int)

     We then break cycles, to get

       [G] g1 :: a ~ (cbv1, cbv2)
       [G] g2 :: TF a ~ cbv1
       [G] g3 :: TF Int ~ cbv2

     g1 gets added to the inert set, as written. But then g2 becomes
     the work item. g1 rewrites g2 to become

       [G] TF (cbv1, cbv2) ~ cbv1

     which then uses the type instance to become

       [G] (TF cbv1, TF cbv2) ~ cbv1

     which looks remarkably like the Given we started with. If left
     unchecked, this will end up breaking cycles again, looping ad
     infinitum (and resulting in a context-stack reduction error,
     not an outright loop). The solution is easy: don't break cycles
     on an equality generated by breaking cycles. Instead, we mark this
     final Given as a CIrredCan with a NonCanonicalReason with the soluble
     occurs-check bit set (only).

     We track these equalities by giving them a special CtOrigin,
     CycleBreakerOrigin. This works for both Givens and Wanteds, as
     we need the logic in the W case for e.g. typecheck/should_fail/T17139.
     Because this logic needs to work for Wanteds, too, we cannot
     simply look for a CycleBreakerTv on the left: Wanteds don't use them.

 (8) We really want to do this all only when there is a soluble occurs-check
     failure, not when other problems arise (such as an impredicative
     equality like alpha ~ forall a. a -> a). That is why breakTyEqCycle_maybe
     uses cterHasOnlyProblem when looking at the result of checkTypeEq, which
     checks for many of the invariants on a CEqCan.
-}

{-
************************************************************************
*                                                                      *
                  Evidence transformation
*                                                                      *
************************************************************************
-}

data StopOrContinue a
  = ContinueWith a    -- The constraint was not solved, although it may have
                      --   been rewritten

  | Stop CtEvidence   -- The (rewritten) constraint was solved
         SDoc         -- Tells how it was solved
                      -- Any new sub-goals have been put on the work list
  deriving (forall a b. a -> StopOrContinue b -> StopOrContinue a
forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
forall (f :: * -> *).
(forall a b. (a -> b) -> f a -> f b)
-> (forall a b. a -> f b -> f a) -> Functor f
<$ :: forall a b. a -> StopOrContinue b -> StopOrContinue a
$c<$ :: forall a b. a -> StopOrContinue b -> StopOrContinue a
fmap :: forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
$cfmap :: forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
Functor)

instance Outputable a => Outputable (StopOrContinue a) where
  ppr :: StopOrContinue a -> SDoc
ppr (Stop CtEvidence
ev SDoc
s)      = String -> SDoc
text String
"Stop" SDoc -> SDoc -> SDoc
<> SDoc -> SDoc
parens SDoc
s SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
  ppr (ContinueWith a
w) = String -> SDoc
text String
"ContinueWith" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr a
w

continueWith :: a -> TcS (StopOrContinue a)
continueWith :: forall a. a -> TcS (StopOrContinue a)
continueWith = forall (m :: * -> *) a. Monad m => a -> m a
return forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a. a -> StopOrContinue a
ContinueWith

stopWith :: CtEvidence -> String -> TcS (StopOrContinue a)
stopWith :: forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
s = forall (m :: * -> *) a. Monad m => a -> m a
return (forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev (String -> SDoc
text String
s))

andWhenContinue :: TcS (StopOrContinue a)
                -> (a -> TcS (StopOrContinue b))
                -> TcS (StopOrContinue b)
andWhenContinue :: forall a b.
TcS (StopOrContinue a)
-> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
andWhenContinue TcS (StopOrContinue a)
tcs1 a -> TcS (StopOrContinue b)
tcs2
  = do { StopOrContinue a
r <- TcS (StopOrContinue a)
tcs1
       ; case StopOrContinue a
r of
           Stop CtEvidence
ev SDoc
s       -> forall (m :: * -> *) a. Monad m => a -> m a
return (forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev SDoc
s)
           ContinueWith a
ct -> a -> TcS (StopOrContinue b)
tcs2 a
ct }
infixr 0 `andWhenContinue`    -- allow chaining with ($)

rewriteEvidence :: RewriterSet  -- ^ See Note [Wanteds rewrite Wanteds]
                                -- in GHC.Tc.Types.Constraint
                -> CtEvidence   -- ^ old evidence
                -> Reduction    -- ^ new predicate + coercion, of type <type of old evidence> ~ new predicate
                -> TcS (StopOrContinue CtEvidence)
-- Returns Just new_ev iff either (i)  'co' is reflexivity
--                             or (ii) 'co' is not reflexivity, and 'new_pred' not cached
-- In either case, there is nothing new to do with new_ev
{-
     rewriteEvidence old_ev new_pred co
Main purpose: create new evidence for new_pred;
              unless new_pred is cached already
* Returns a new_ev : new_pred, with same wanted/given flag as old_ev
* If old_ev was wanted, create a binding for old_ev, in terms of new_ev
* If old_ev was given, AND not cached, create a binding for new_ev, in terms of old_ev
* Returns Nothing if new_ev is already cached

        Old evidence    New predicate is               Return new evidence
        flavour                                        of same flavor
        -------------------------------------------------------------------
        Wanted          Already solved or in inert     Nothing
                        Not                            Just new_evidence

        Given           Already in inert               Nothing
                        Not                            Just new_evidence

Note [Rewriting with Refl]
~~~~~~~~~~~~~~~~~~~~~~~~~~
If the coercion is just reflexivity then you may re-use the same
variable.  But be careful!  Although the coercion is Refl, new_pred
may reflect the result of unification alpha := ty, so new_pred might
not _look_ the same as old_pred, and it's vital to proceed from now on
using new_pred.

The rewriter preserves type synonyms, so they should appear in new_pred
as well as in old_pred; that is important for good error messages.

If we are rewriting with Refl, then there are no new rewriters to add to
the rewriter set. We check this with an assertion.
 -}


rewriteEvidence :: RewriterSet
-> CtEvidence -> Reduction -> TcS (StopOrContinue CtEvidence)
rewriteEvidence RewriterSet
rewriters CtEvidence
old_ev (Reduction TcCoercion
co Type
new_pred)
  | TcCoercion -> Bool
isTcReflCo TcCoercion
co -- See Note [Rewriting with Refl]
  = forall a. HasCallStack => Bool -> a -> a
assert (RewriterSet -> Bool
isEmptyRewriterSet RewriterSet
rewriters) forall a b. (a -> b) -> a -> b
$
    forall a. a -> TcS (StopOrContinue a)
continueWith (HasDebugCallStack => CtEvidence -> Type -> CtEvidence
setCtEvPredType CtEvidence
old_ev Type
new_pred)

rewriteEvidence RewriterSet
rewriters ev :: CtEvidence
ev@(CtGiven { ctev_evar :: CtEvidence -> TcTyVar
ctev_evar = TcTyVar
old_evar, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc })
                (Reduction TcCoercion
co Type
new_pred)
  = forall a. HasCallStack => Bool -> a -> a
assert (RewriterSet -> Bool
isEmptyRewriterSet RewriterSet
rewriters) forall a b. (a -> b) -> a -> b
$ -- this is a Given, not a wanted
    do { CtEvidence
new_ev <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc (Type
new_pred, EvTerm
new_tm)
       ; forall a. a -> TcS (StopOrContinue a)
continueWith CtEvidence
new_ev }
  where
    -- mkEvCast optimises ReflCo
    new_tm :: EvTerm
new_tm = EvExpr -> TcCoercion -> EvTerm
mkEvCast (TcTyVar -> EvExpr
evId TcTyVar
old_evar)
                (Role -> Role -> TcCoercion -> TcCoercion
tcDowngradeRole Role
Representational (CtEvidence -> Role
ctEvRole CtEvidence
ev) TcCoercion
co)

rewriteEvidence RewriterSet
new_rewriters
                ev :: CtEvidence
ev@(CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest
                             , ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc
                             , ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters })
                (Reduction TcCoercion
co Type
new_pred)
  = do { MaybeNew
mb_new_ev <- CtLoc -> RewriterSet -> Type -> TcS MaybeNew
newWanted CtLoc
loc RewriterSet
rewriters' Type
new_pred
       ; forall (m :: * -> *). (HasCallStack, Applicative m) => Bool -> m ()
massert (TcCoercion -> Role
tcCoercionRole TcCoercion
co forall a. Eq a => a -> a -> Bool
== CtEvidence -> Role
ctEvRole CtEvidence
ev)
       ; TcEvDest -> EvTerm -> TcS ()
setWantedEvTerm TcEvDest
dest
            (EvExpr -> TcCoercion -> EvTerm
mkEvCast (MaybeNew -> EvExpr
getEvExpr MaybeNew
mb_new_ev)
                      (Role -> Role -> TcCoercion -> TcCoercion
tcDowngradeRole Role
Representational (CtEvidence -> Role
ctEvRole CtEvidence
ev) (TcCoercion -> TcCoercion
mkSymCo TcCoercion
co)))
       ; case MaybeNew
mb_new_ev of
            Fresh  CtEvidence
new_ev -> forall a. a -> TcS (StopOrContinue a)
continueWith CtEvidence
new_ev
            Cached EvExpr
_      -> forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Cached wanted" }
  where
    rewriters' :: RewriterSet
rewriters' = RewriterSet
rewriters forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
new_rewriters


rewriteEqEvidence :: RewriterSet        -- New rewriters
                                        -- See GHC.Tc.Types.Constraint
                                        -- Note [Wanteds rewrite Wanteds]
                  -> CtEvidence         -- Old evidence :: olhs ~ orhs (not swapped)
                                        --              or orhs ~ olhs (swapped)
                  -> SwapFlag
                  -> Reduction          -- lhs_co :: olhs ~ nlhs
                  -> Reduction          -- rhs_co :: orhs ~ nrhs
                  -> TcS CtEvidence     -- Of type nlhs ~ nrhs
-- With reductions (Reduction lhs_co nlhs) (Reduction rhs_co nrhs),
-- rewriteEqEvidence yields, for a given equality (Given g olhs orhs):
-- If not swapped
--      g1 : nlhs ~ nrhs = sym lhs_co ; g ; rhs_co
-- If swapped
--      g1 : nlhs ~ nrhs = sym lhs_co ; Sym g ; rhs_co
--
-- For a wanted equality (Wanted w), we do the dual thing:
-- New  w1 : nlhs ~ nrhs
-- If not swapped
--      w : olhs ~ orhs = lhs_co ; w1 ; sym rhs_co
-- If swapped
--      w : orhs ~ olhs = rhs_co ; sym w1 ; sym lhs_co
--
-- It's all a form of rewriteEvidence, specialised for equalities
rewriteEqEvidence :: RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
new_rewriters CtEvidence
old_ev SwapFlag
swapped (Reduction TcCoercion
lhs_co Type
nlhs) (Reduction TcCoercion
rhs_co Type
nrhs)
  | SwapFlag
NotSwapped <- SwapFlag
swapped
  , TcCoercion -> Bool
isTcReflCo TcCoercion
lhs_co      -- See Note [Rewriting with Refl]
  , TcCoercion -> Bool
isTcReflCo TcCoercion
rhs_co
  = forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => CtEvidence -> Type -> CtEvidence
setCtEvPredType CtEvidence
old_ev Type
new_pred)

  | CtGiven { ctev_evar :: CtEvidence -> TcTyVar
ctev_evar = TcTyVar
old_evar } <- CtEvidence
old_ev
  = do { let new_tm :: EvTerm
new_tm = TcCoercion -> EvTerm
evCoercion ( TcCoercion -> TcCoercion
mkTcSymCo TcCoercion
lhs_co
                                  TcCoercion -> TcCoercion -> TcCoercion
`mkTcTransCo` SwapFlag -> TcCoercion -> TcCoercion
maybeTcSymCo SwapFlag
swapped (TcTyVar -> TcCoercion
mkTcCoVarCo TcTyVar
old_evar)
                                  TcCoercion -> TcCoercion -> TcCoercion
`mkTcTransCo` TcCoercion
rhs_co)
       ; CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc' (Type
new_pred, EvTerm
new_tm) }

  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest
             , ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } <- CtEvidence
old_ev
  , let rewriters' :: RewriterSet
rewriters' = RewriterSet
rewriters forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
new_rewriters
  = do { (CtEvidence
new_ev, TcCoercion
hole_co) <- CtLoc
-> RewriterSet
-> Role
-> Type
-> Type
-> TcS (CtEvidence, TcCoercion)
newWantedEq CtLoc
loc' RewriterSet
rewriters'
                                          (CtEvidence -> Role
ctEvRole CtEvidence
old_ev) Type
nlhs Type
nrhs
       ; let co :: TcCoercion
co = SwapFlag -> TcCoercion -> TcCoercion
maybeTcSymCo SwapFlag
swapped forall a b. (a -> b) -> a -> b
$
                  TcCoercion
lhs_co
                  TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion
hole_co
                  TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion -> TcCoercion
mkTcSymCo TcCoercion
rhs_co
       ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co
       ; String -> SDoc -> TcS ()
traceTcS String
"rewriteEqEvidence" ([SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr CtEvidence
old_ev
                                            , forall a. Outputable a => a -> SDoc
ppr Type
nlhs
                                            , forall a. Outputable a => a -> SDoc
ppr Type
nrhs
                                            , forall a. Outputable a => a -> SDoc
ppr TcCoercion
co
                                            , forall a. Outputable a => a -> SDoc
ppr RewriterSet
new_rewriters ])
       ; forall (m :: * -> *) a. Monad m => a -> m a
return CtEvidence
new_ev }

#if __GLASGOW_HASKELL__ <= 810
  | otherwise
  = panic "rewriteEvidence"
#endif
  where
    new_pred :: Type
new_pred = CtEvidence -> Type -> Type -> Type
mkTcEqPredLikeEv CtEvidence
old_ev Type
nlhs Type
nrhs

      -- equality is like a type class. Bumping the depth is necessary because
      -- of recursive newtypes, where "reducing" a newtype can actually make
      -- it bigger. See Note [Newtypes can blow the stack].
    loc :: CtLoc
loc      = CtEvidence -> CtLoc
ctEvLoc CtEvidence
old_ev
    loc' :: CtLoc
loc'     = CtLoc -> CtLoc
bumpCtLocDepth CtLoc
loc

{-
************************************************************************
*                                                                      *
              Unification
*                                                                      *
************************************************************************

Note [unifyWanted]
~~~~~~~~~~~~~~~~~~
When decomposing equalities we often create new wanted constraints for
(s ~ t).  But what if s=t?  Then it'd be faster to return Refl right away.

Rather than making an equality test (which traverses the structure of the
type, perhaps fruitlessly), unifyWanted traverses the common structure, and
bales out when it finds a difference by creating a new Wanted constraint.
But where it succeeds in finding common structure, it just builds a coercion
to reflect it.
-}

unifyWanted :: RewriterSet -> CtLoc
            -> Role -> TcType -> TcType -> TcS Coercion
-- Return coercion witnessing the equality of the two types,
-- emitting new work equalities where necessary to achieve that
-- Very good short-cut when the two types are equal, or nearly so
-- See Note [unifyWanted]
-- The returned coercion's role matches the input parameter
unifyWanted :: RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
Phantom Type
ty1 Type
ty2
  = do { TcCoercion
kind_co <- RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
Nominal (HasDebugCallStack => Type -> Type
tcTypeKind Type
ty1) (HasDebugCallStack => Type -> Type
tcTypeKind Type
ty2)
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion -> Type -> Type -> TcCoercion
mkPhantomCo TcCoercion
kind_co Type
ty1 Type
ty2) }

unifyWanted RewriterSet
rewriters CtLoc
loc Role
role Type
orig_ty1 Type
orig_ty2
  = Type -> Type -> TcS TcCoercion
go Type
orig_ty1 Type
orig_ty2
  where
    go :: Type -> Type -> TcS TcCoercion
go Type
ty1 Type
ty2 | Just Type
ty1' <- Type -> Maybe Type
tcView Type
ty1 = Type -> Type -> TcS TcCoercion
go Type
ty1' Type
ty2
    go Type
ty1 Type
ty2 | Just Type
ty2' <- Type -> Maybe Type
tcView Type
ty2 = Type -> Type -> TcS TcCoercion
go Type
ty1 Type
ty2'

    go (FunTy AnonArgFlag
_ Type
w1 Type
s1 Type
t1) (FunTy AnonArgFlag
_ Type
w2 Type
s2 Type
t2)
      = do { TcCoercion
co_s <- RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
role Type
s1 Type
s2
           ; TcCoercion
co_t <- RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
role Type
t1 Type
t2
           ; TcCoercion
co_w <- RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
Nominal Type
w1 Type
w2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> TcCoercion -> TcCoercion -> TcCoercion -> TcCoercion
mkFunCo Role
role TcCoercion
co_w TcCoercion
co_s TcCoercion
co_t) }
    go (TyConApp TyCon
tc1 [Type]
tys1) (TyConApp TyCon
tc2 [Type]
tys2)
      | TyCon
tc1 forall a. Eq a => a -> a -> Bool
== TyCon
tc2, [Type]
tys1 forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
tys2
      , TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 Role
role -- don't look under newtypes at Rep equality
      = do { [TcCoercion]
cos <- forall (m :: * -> *) a b c d.
Monad m =>
(a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d]
zipWith3M (RewriterSet -> CtLoc -> Role -> Type -> Type -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc)
                              (Role -> TyCon -> [Role]
tyConRolesX Role
role TyCon
tc1) [Type]
tys1 [Type]
tys2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
mkTyConAppCo Role
role TyCon
tc1 [TcCoercion]
cos) }

    go ty1 :: Type
ty1@(TyVarTy TcTyVar
tv) Type
ty2
      = do { Maybe Type
mb_ty <- TcTyVar -> TcS (Maybe Type)
isFilledMetaTyVar_maybe TcTyVar
tv
           ; case Maybe Type
mb_ty of
                Just Type
ty1' -> Type -> Type -> TcS TcCoercion
go Type
ty1' Type
ty2
                Maybe Type
Nothing   -> Type -> Type -> TcS TcCoercion
bale_out Type
ty1 Type
ty2}
    go Type
ty1 ty2 :: Type
ty2@(TyVarTy TcTyVar
tv)
      = do { Maybe Type
mb_ty <- TcTyVar -> TcS (Maybe Type)
isFilledMetaTyVar_maybe TcTyVar
tv
           ; case Maybe Type
mb_ty of
                Just Type
ty2' -> Type -> Type -> TcS TcCoercion
go Type
ty1 Type
ty2'
                Maybe Type
Nothing   -> Type -> Type -> TcS TcCoercion
bale_out Type
ty1 Type
ty2 }

    go ty1 :: Type
ty1@(CoercionTy {}) (CoercionTy {})
      = forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Type -> TcCoercion
mkReflCo Role
role Type
ty1) -- we just don't care about coercions!

    go Type
ty1 Type
ty2 = Type -> Type -> TcS TcCoercion
bale_out Type
ty1 Type
ty2

    bale_out :: Type -> Type -> TcS TcCoercion
bale_out Type
ty1 Type
ty2
       | Type
ty1 HasDebugCallStack => Type -> Type -> Bool
`tcEqType` Type
ty2 = forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Type -> TcCoercion
mkTcReflCo Role
role Type
ty1)
        -- Check for equality; e.g. a ~ a, or (m a) ~ (m a)
       | Bool
otherwise = CtLoc -> RewriterSet -> Role -> Type -> Type -> TcS TcCoercion
emitNewWantedEq CtLoc
loc RewriterSet
rewriters Role
role Type
orig_ty1 Type
orig_ty2