{-# LANGUAGE MultiWayIf #-}

-- | Solving Class constraints CDictCan
module GHC.Tc.Solver.Dict (
  solveDict, solveDictNC,
  checkInstanceOK,
  matchLocalInst, chooseInstance,
  makeSuperClasses, mkStrictSuperClasses,
  solveCallStack    -- For GHC.Tc.Solver
  ) where

import GHC.Prelude

import GHC.Tc.Errors.Types
import GHC.Tc.Instance.FunDeps
import GHC.Tc.Instance.Class( safeOverlap, matchEqualityInst )
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Origin
import GHC.Tc.Types.EvTerm( evCallStack )
import GHC.Tc.Solver.InertSet
import GHC.Tc.Solver.Monad
import GHC.Tc.Solver.Types
import GHC.Tc.Utils.TcType
import GHC.Tc.Utils.Unify( uType )

import GHC.Hs.Type( HsIPName(..) )

import GHC.Core
import GHC.Core.Type
import GHC.Core.InstEnv     ( DFunInstType )
import GHC.Core.Class
import GHC.Core.Predicate
import GHC.Core.Multiplicity ( scaledThing )
import GHC.Core.Unify ( ruleMatchTyKiX )

import GHC.Types.Name.Set
import GHC.Types.Var
import GHC.Types.Id( mkTemplateLocals )
import GHC.Types.Var.Set
import GHC.Types.SrcLoc
import GHC.Types.Var.Env

import GHC.Utils.Monad ( concatMapM, foldlM )
import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Utils.Misc

import GHC.Data.Bag

import GHC.Driver.DynFlags

import qualified GHC.LanguageExtensions as LangExt

import Data.Maybe ( listToMaybe, mapMaybe, isJust )
import Data.Void( Void )

import Control.Monad.Trans.Maybe( MaybeT, runMaybeT )
import Control.Monad.Trans.Class( lift )
import Control.Monad( mzero )


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

solveDictNC :: CtEvidence -> Class -> [Type] -> SolverStage Void
-- NC: this comes from CNonCanonical or CIrredCan
-- Precondition: already rewritten by inert set
solveDictNC :: CtEvidence -> Class -> [Type] -> SolverStage Void
solveDictNC CtEvidence
ev Class
cls [Type]
tys
  = do { TcS () -> SolverStage ()
forall a. TcS a -> SolverStage a
simpleStage (TcS () -> SolverStage ()) -> TcS () -> SolverStage ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc -> TcS ()
traceTcS String
"solveDictNC" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys) SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev)
       ; DictCt
dict_ct <- CtEvidence -> Class -> [Type] -> SolverStage DictCt
canDictCt CtEvidence
ev Class
cls [Type]
tys
       ; DictCt -> SolverStage Void
solveDict DictCt
dict_ct }

solveDict :: DictCt -> SolverStage Void
-- Preconditions: `tys` are already rewritten by the inert set
solveDict :: DictCt -> SolverStage Void
solveDict dict_ct :: DictCt
dict_ct@(DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev, di_cls :: DictCt -> Class
di_cls = Class
cls, di_tys :: DictCt -> [Type]
di_tys = [Type]
tys })
  | Class -> Bool
isEqualityClass Class
cls
  = CtEvidence -> Class -> [Type] -> SolverStage Void
solveEqualityDict CtEvidence
ev Class
cls [Type]
tys

  | Bool
otherwise
  = Bool -> SDoc -> SolverStage Void -> SolverStage Void
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (CtEvidence -> Role
ctEvRole CtEvidence
ev Role -> Role -> Bool
forall a. Eq a => a -> a -> Bool
== Role
Nominal) (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys) (SolverStage Void -> SolverStage Void)
-> SolverStage Void -> SolverStage Void
forall a b. (a -> b) -> a -> b
$
    do { TcS () -> SolverStage ()
forall a. TcS a -> SolverStage a
simpleStage (TcS () -> SolverStage ()) -> TcS () -> SolverStage ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc -> TcS ()
traceTcS String
"solveDict" (DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_ct)

       ; DictCt -> SolverStage ()
tryInertDicts DictCt
dict_ct
       ; DictCt -> SolverStage ()
tryInstances DictCt
dict_ct

       -- Try fundeps /after/ tryInstances:
       --     see (DFL2) in Note [Do fundeps last]
       ; DictCt -> SolverStage ()
doLocalFunDepImprovement DictCt
dict_ct
           -- doLocalFunDepImprovement does StartAgain if there
           -- are any fundeps: see (DFL1) in Note [Do fundeps last]

       ; DictCt -> SolverStage ()
doTopFunDepImprovement DictCt
dict_ct

       ; DictCt -> SolverStage ()
tryLastResortProhibitedSuperClass DictCt
dict_ct
       ; TcS () -> SolverStage ()
forall a. TcS a -> SolverStage a
simpleStage (DictCt -> TcS ()
updInertDicts DictCt
dict_ct)
       ; CtEvidence -> String -> SolverStage Void
forall a. CtEvidence -> String -> SolverStage a
stopWithStage (DictCt -> CtEvidence
dictCtEvidence DictCt
dict_ct) String
"Kept inert DictCt" }

updInertDicts :: DictCt -> TcS ()
updInertDicts :: DictCt -> TcS ()
updInertDicts dict_ct :: DictCt
dict_ct@(DictCt { di_cls :: DictCt -> Class
di_cls = Class
cls, di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev, di_tys :: DictCt -> [Type]
di_tys = [Type]
tys })
  = do { String -> SDoc -> TcS ()
traceTcS String
"Adding inert dict" (DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_ct SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls  SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys)

       ; if |  CtEvidence -> Bool
isGiven CtEvidence
ev, Just (Type
str_ty, Type
_) <- Class -> [Type] -> Maybe (Type, Type)
isIPPred_maybe Class
cls [Type]
tys
            -> -- See (SIP1) and (SIP2) in Note [Shadowing of implicit parameters]
               -- Update /both/ inert_cans /and/ inert_solved_dicts.
               (InertSet -> InertSet) -> TcS ()
updInertSet ((InertSet -> InertSet) -> TcS ())
-> (InertSet -> InertSet) -> TcS ()
forall a b. (a -> b) -> a -> b
$ \ inerts :: InertSet
inerts@(IS { inert_cans :: InertSet -> InertCans
inert_cans = InertCans
ics, inert_solved_dicts :: InertSet -> DictMap DictCt
inert_solved_dicts = DictMap DictCt
solved }) ->
               InertSet
inerts { inert_cans         = updDicts (filterDicts (not_ip_for str_ty)) ics
                      , inert_solved_dicts = filterDicts (not_ip_for str_ty) solved }
            |  Bool
otherwise
            -> () -> TcS ()
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ()

       -- Add the new constraint to the inert set
       ; (InertCans -> InertCans) -> TcS ()
updInertCans ((DictMap DictCt -> DictMap DictCt) -> InertCans -> InertCans
updDicts (DictCt -> DictMap DictCt -> DictMap DictCt
addDict DictCt
dict_ct)) }
  where
    not_ip_for :: Type -> DictCt -> Bool
    not_ip_for :: Type -> DictCt -> Bool
not_ip_for Type
str_ty (DictCt { di_cls :: DictCt -> Class
di_cls = Class
cls, di_tys :: DictCt -> [Type]
di_tys = [Type]
tys })
      = Bool -> Bool
not (Type -> Class -> [Type] -> Bool
mentionsIP Type
str_ty Class
cls [Type]
tys)

canDictCt :: CtEvidence -> Class -> [Type] -> SolverStage DictCt
-- Once-only processing of Dict constraints:
--   * expand superclasses
--   * deal with CallStack
canDictCt :: CtEvidence -> Class -> [Type] -> SolverStage DictCt
canDictCt CtEvidence
ev Class
cls [Type]
tys
  | CtEvidence -> Bool
isGiven CtEvidence
ev  -- See Note [Eagerly expand given superclasses]
  = TcS (StopOrContinue DictCt) -> SolverStage DictCt
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue DictCt) -> SolverStage DictCt)
-> TcS (StopOrContinue DictCt) -> SolverStage DictCt
forall a b. (a -> b) -> a -> b
$
    do { DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; [Ct]
sc_cts <- ScDepth
-> CtEvidence -> [EvVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses (DynFlags -> ScDepth
givensFuel DynFlags
dflags) CtEvidence
ev [] [] Class
cls [Type]
tys
         -- givensFuel dflags: See Note [Expanding Recursive Superclasses and ExpansionFuel]
       ; Cts -> TcS ()
emitWork ([Ct] -> Cts
forall a. [a] -> Bag a
listToBag [Ct]
sc_cts)

       ; DictCt -> TcS (StopOrContinue DictCt)
forall a. a -> TcS (StopOrContinue a)
continueWith (DictCt { di_ev :: CtEvidence
di_ev = CtEvidence
ev, di_cls :: Class
di_cls = Class
cls
                              , di_tys :: [Type]
di_tys = [Type]
tys, di_pend_sc :: ScDepth
di_pend_sc = ScDepth
doNotExpand }) }
         -- doNotExpand: We have already expanded superclasses for /this/ dict
         -- so set the fuel to doNotExpand to avoid repeating expansion

  | 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
  = TcS (StopOrContinue DictCt) -> SolverStage DictCt
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue DictCt) -> SolverStage DictCt)
-> TcS (StopOrContinue DictCt) -> SolverStage DictCt
forall a b. (a -> b) -> a -> b
$
    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
CtEvidence -> EvExpr
ctEvExpr CtEvidence
new_ev)
       ; CtEvidence -> EvCallStack -> TcS ()
solveCallStack CtEvidence
ev EvCallStack
ev_cs

       ; DictCt -> TcS (StopOrContinue DictCt)
forall a. a -> TcS (StopOrContinue a)
continueWith (DictCt { di_ev :: CtEvidence
di_ev = CtEvidence
new_ev, di_cls :: Class
di_cls = Class
cls
                              , di_tys :: [Type]
di_tys = [Type]
tys, di_pend_sc :: ScDepth
di_pend_sc = ScDepth
doNotExpand }) }
         -- doNotExpand: No superclasses for class CallStack
         -- See invariants in CDictCan.cc_pend_sc

  | Bool
otherwise
  = TcS (StopOrContinue DictCt) -> SolverStage DictCt
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue DictCt) -> SolverStage DictCt)
-> TcS (StopOrContinue DictCt) -> SolverStage DictCt
forall a b. (a -> b) -> a -> b
$
    do { DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; let fuel :: ScDepth
fuel | Class -> Bool
classHasSCs Class
cls = DynFlags -> ScDepth
wantedsFuel DynFlags
dflags
                  | Bool
otherwise       = ScDepth
doNotExpand
                  -- See Invariants in `CCDictCan.cc_pend_sc`
       ; DictCt -> TcS (StopOrContinue DictCt)
forall a. a -> TcS (StopOrContinue a)
continueWith (DictCt { di_ev :: CtEvidence
di_ev = CtEvidence
ev, di_cls :: Class
di_cls = Class
cls
                              , di_tys :: [Type]
di_tys = [Type]
tys, di_pend_sc :: ScDepth
di_pend_sc = ScDepth
fuel }) }
  where
    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    orig :: CtOrigin
orig = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc
    pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev

solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
-- Also called from GHC.Tc.Solver when defaulting call stacks
solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
solveCallStack CtEvidence
ev EvCallStack
ev_cs
  -- 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]
  = do { EvExpr
cs_tm <- EvCallStack -> TcS EvExpr
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 -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev Bool
True EvTerm
ev_tm }


{- Note [Shadowing of implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we add a new /given/ implicit parameter to the inert set, it /replaces/
any existing givens for the same implicit parameter.  This makes a difference
in two places:

* In `GHC.Tc.Solver.InertSet.solveOneFromTheOther`, be careful when we have
   (?x :: ty) in the inert set and an identical (?x :: ty) as the work item.

* In `updInertDicts` in this module, when adding [G] (?x :: ty), remove  any
  existing [G] (?x :: ty'), regardless of ty'.

* Wrinkle (SIP1): we must be careful of superclasses.  Consider
     f,g :: (?x::Int, C a) => a -> a
     f v = let ?x = 4 in g v

  The call to 'g' gives rise to a Wanted constraint (?x::Int, C a).
  We must /not/ solve this from the Given (?x::Int, C a), because of
  the intervening binding for (?x::Int).  #14218.

  We deal with this by arranging that when we add [G] (?x::ty) we delete
  * from the inert_cans, and
  * from the inert_solved_dicts
  any existing [G] (?x::ty) /and/ any [G] D tys, where (D tys) has a superclass
  with (?x::ty).  See Note [Local implicit parameters] in GHC.Core.Predicate.

  An important special case is constraint tuples like [G] (% ?x::ty, Eq a %).
  But it could happen for `class xx => D xx where ...` and the constraint D
  (?x :: int).  This corner (constraint-kinded variables instantiated with
  implicit parameter constraints) is not well explorered.

  Example in #14218, and #23761

  The code that accounts for (SIP1) is in updInertDicts; in particular the call to
  GHC.Core.Predicate.mentionsIP.

* Wrinkle (SIP2): we must apply this update semantics for `inert_solved_dicts`
  as well as `inert_cans`.
  You might think that wouldn't be necessary, because an element of
  `inert_solved_dicts` is never an implicit parameter (see
  Note [Solved dictionaries] in GHC.Tc.Solver.InertSet).
  While that is true, dictionaries in `inert_solved_dicts` may still have
  implicit parameters as a /superclass/! For example:

    class c => C c where ...
    f :: C (?x::Int) => blah

  Now (C (?x::Int)) has a superclass (?x::Int). This may look exotic, but it
  happens particularly for constraint tuples, like `(% ?x::Int, Eq a %)`.

Example 1:

Suppose we have (typecheck/should_compile/ImplicitParamFDs)
  flub :: (?x :: Int) => (Int, Integer)
  flub = (?x, let ?x = 5 in ?x)
When we are checking the last ?x occurrence, we guess its type to be a fresh
unification variable alpha and emit an (IP "x" alpha) constraint. But the given
(?x :: Int) has been translated to an IP "x" Int constraint, which has a
functional dependency from the name to the type. So if that (?x::Int) is still
in the inert set, we'd get a fundep interaction that tells us that alpha ~ Int,
and we get a type error. This is bad.  The "replacement" semantics stops this
happening.

Example 2:

f :: (?x :: Char) => Char
f = let ?x = 'a' in ?x

The "let ?x = ..." generates an implication constraint of the form:

?x :: Char => ?x :: Char

Furthermore, the signature for `f` also generates an implication
constraint, so we end up with the following nested implication:

?x :: Char => (?x :: Char => ?x :: Char)

Note that the wanted (?x :: Char) constraint may be solved in two incompatible
ways: either by using the parameter from the signature, or by using the local
definition.  Our intention is that the local definition should "shadow" the
parameter of the signature.  The "replacement" semantics for implicit parameters
does this.

Example 3:

Similarly, consider
   f :: (?x::a) => Bool -> a

   g v = let ?x::Int = 3
         in (f v, let ?x::Bool = True in f v)

This should probably be well typed, with
   g :: Bool -> (Int, Bool)

So the inner binding for ?x::Bool *overrides* the outer one.

See ticket #17104 for a rather tricky example of this overriding
behaviour.

All this works for the normal cases but it has an odd side effect in
some pathological programs like this:

    -- This is accepted, the second parameter shadows
    f1 :: (?x :: Int, ?x :: Char) => Char
    f1 = ?x

    -- This is rejected, the second parameter shadows
    f2 :: (?x :: Int, ?x :: Char) => Int
    f2 = ?x

Both of these are actually wrong:  when we try to use either one,
we'll get two incompatible wanted constraints (?x :: Int, ?x :: Char),
which would lead to an error.

I can think of two ways to fix this:

  1. Simply disallow multiple constraints for the same implicit
    parameter---this is never useful, and it can be detected completely
    syntactically.

  2. Move the shadowing machinery to the location where we nest
     implications, and add some code here that will produce an
     error if we get multiple givens for the same implicit parameter.
-}


{- ******************************************************************************
*                                                                               *
                   solveEqualityDict
*                                                                               *
****************************************************************************** -}

{- Note [Solving equality classes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (~), which behaves as if it was defined like this:
  class a ~# b => a ~ b
  instance a ~# b => a ~ b
There are two more similar "equality classes" like this.  The full list is
  * (~)         eqTyCon
  * (~~)        heqTyCon
  * Coercible   coercibleTyCon
(See Note [The equality types story] in GHC.Builtin.Types.Prim.)

(EQC1) For Givens, when expanding the superclasses of a equality class,
  we can /replace/ the constraint with its superclasses (which, remember, are
  equally powerful) rather than /adding/ them. This can make a huge difference.
  Consider T17836, which has a constraint like
      forall b,c. a ~ (b,c) =>
        forall d,e. c ~ (d,e) =>
          ...etc...
  If we just /add/ the superclasses of [G] g1:a ~ (b,c), we'll put
  [G] g1:(a~(b,c)) in the inert set and emit [G] g2:a ~# (b,c).  That will
  kick out g1, and it'll be re-inserted as [G] g1':(b,c)~(b,c) which does
  no good to anyone.  When the implication is deeply nested, this has
  quadratic cost, and no benefit.  Just replace!

  (This can have a /big/ effect: test T17836 involves deeply-nested GADT
  pattern matching. Its compile-time allocation decreased by 40% when
  I added the "replace" rather than "add" semantics.)

(EQC2) Faced with [W] t1 ~ t2, it's always OK to reduce it to [W] t1 ~# t2,
  without worrying about Note [Instance and Given overlap].  Why?  Because
  if we had [G] s1 ~ s2, then we'd get the superclass [G] s1 ~# s2, and
  so the reduction of the [W] constraint does not risk losing any solutions.

  On the other hand, it can be fatal to /fail/ to reduce such equalities
  on the grounds of Note [Instance and Given overlap], because many good
  things flow from [W] t1 ~# t2.

Conclusion: we have a special solver pipeline for equality-class constraints,
`solveEqualityDict`.  It aggressively decomposes the boxed equality constraint
into an unboxed coercion, both for Givens and Wanteds, and /replaces/ the
boxed equality constraint with the unboxed one, so that the inert set never
contains the boxed one.

Note [Solving tuple constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I tried treating tuple constraints, such as (% Eq a, Show a %), rather like
equality-class constraints (see Note [Solving equality classes]). That is, by
eagerly decomposing tuple-constraints into their component (Eq a) and (Show a).

But discarding the tuple Given (which "replacing" does) means that we may
have to reconstruct it for a recursive call.  For example
    f :: (% Eq a, Show a %) => blah
    f x = ....(f x')....
If we decomposed eagerly we'd get
    f = \(d : (% Eq a, Show a %)).
         let de = fst d
             ds = snd d
         in ....(f (% de, ds %))...
and the optimiser may not be clever enough to transform (f (% de, ds %)) into
(f d). See #10359 and its test case, and #23398.  (This issue is less pressing for
equality classes because they have to be unpacked strictly, so CSE-ing away
the reconstruction works fine.

So at the moment we don't decompose tuple constraints eagerly; instead we mostly
just treat them like other constraints.
* Given tuples are decomposed via their superclasses, in `canDictCt`.  So
    [G] (% Eq a, Show a %)
  has superclasses
    [G] Eq a,  [G] Show a

* Wanted tuples are decomposed via a built-in "instance". See
  `GHC.Tc.Instance.Class.matchCTuple`

There is a bit of special treatment: search for isCTupleClass.
-}

solveEqualityDict :: CtEvidence -> Class -> [Type] -> SolverStage Void
-- Precondition: (isEqualityClass cls) True, so cls is (~), (~~), or Coercible
solveEqualityDict :: CtEvidence -> Class -> [Type] -> SolverStage Void
solveEqualityDict CtEvidence
ev Class
cls [Type]
tys
  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest } <- CtEvidence
ev
  = TcS (StopOrContinue Void) -> SolverStage Void
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue Void) -> SolverStage Void)
-> TcS (StopOrContinue Void) -> SolverStage Void
forall a b. (a -> b) -> a -> b
$
    do { let (DataCon
data_con, Role
role, Type
t1, Type
t2) = Class -> [Type] -> (DataCon, Role, Type, Type)
matchEqualityInst Class
cls [Type]
tys
         -- Unify t1~t2, putting anything that can't be solved
         -- immediately into the work list
       ; (TcCoercion
co, Cts
_, [EvVar]
_) <- CtEvidence
-> Role
-> (UnifyEnv -> TcM TcCoercion)
-> TcS (TcCoercion, Cts, [EvVar])
forall a.
CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [EvVar])
wrapUnifierTcS CtEvidence
ev Role
role ((UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [EvVar]))
-> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [EvVar])
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
                       UnifyEnv -> Type -> Type -> TcM TcCoercion
uType UnifyEnv
uenv Type
t1 Type
t2
         -- Set  d :: (t1~t2) = Eq# co
       ; TcEvDest -> Bool -> EvTerm -> TcS ()
setWantedEvTerm TcEvDest
dest Bool
True (EvTerm -> TcS ()) -> EvTerm -> TcS ()
forall a b. (a -> b) -> a -> b
$
         DataCon -> [Type] -> [EvExpr] -> EvTerm
evDataConApp DataCon
data_con [Type]
tys [TcCoercion -> EvExpr
forall b. TcCoercion -> Expr b
Coercion TcCoercion
co]
       ; CtEvidence -> String -> TcS (StopOrContinue Void)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved wanted lifted equality" }

  | CtGiven { ctev_evar :: CtEvidence -> EvVar
ctev_evar = EvVar
ev_id, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc } <- CtEvidence
ev
  , [EvVar
sel_id] <- Class -> [EvVar]
classSCSelIds Class
cls  -- Equality classes have just one superclass
  = TcS (StopOrContinue Void) -> SolverStage Void
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue Void) -> SolverStage Void)
-> TcS (StopOrContinue Void) -> SolverStage Void
forall a b. (a -> b) -> a -> b
$
    do { let sc_pred :: Type
sc_pred = EvVar -> [Type] -> Type
classMethodInstTy EvVar
sel_id [Type]
tys
             ev_expr :: EvTerm
ev_expr = EvExpr -> EvTerm
EvExpr (EvExpr -> EvTerm) -> EvExpr -> EvTerm
forall a b. (a -> b) -> a -> b
$ EvVar -> EvExpr
forall b. EvVar -> Expr b
Var EvVar
sel_id EvExpr -> [Type] -> EvExpr
forall b. Expr b -> [Type] -> Expr b
`mkTyApps` [Type]
tys EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App` EvVar -> EvExpr
evId EvVar
ev_id
       ; CtEvidence
given_ev <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc (Type
sc_pred, EvTerm
ev_expr)
       ; Ct -> TcS (StopOrContinue Void)
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (CtEvidence -> Ct
mkNonCanonical CtEvidence
given_ev) }
  | Bool
otherwise
  = String -> SDoc -> SolverStage Void
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"solveEqualityDict" (Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls)

{- ******************************************************************************
*                                                                               *
                   interactDict
*                                                                               *
*********************************************************************************

Note [Shortcut solving]
~~~~~~~~~~~~~~~~~~~~~~~
When we interact a [W] constraint with a [G] constraint that solves it, there is
a possibility that we could produce better code if instead we solved from a
top-level instance declaration (See #12791, #5835). For example:

    class M a b where m :: a -> b

    type C a b = (Num a, M a b)

    f :: C Int b => b -> Int -> Int
    f _ x = x + 1

The body of `f` requires a [W] `Num Int` instance. We could solve this
constraint from the givens because we have `C Int b` and that provides us a
solution for `Num Int`. This would let us produce core like the following
(with -O2):

    f :: forall b. C Int b => b -> Int -> Int
    f = \ (@ b) ($d(%,%) :: C Int b) _ (eta1 :: Int) ->
        + @ Int
          (GHC.Classes.$p1(%,%) @ (Num Int) @ (M Int b) $d(%,%))
          eta1
          A.f1

This is bad! We could do /much/ better if we solved [W] `Num Int` directly
from the instance that we have in scope:

    f :: forall b. C Int b => b -> Int -> Int
    f = \ (@ b) _ _ (x :: Int) ->
        case x of { GHC.Types.I# x1 -> GHC.Types.I# (GHC.Prim.+# x1 1#) }

** NB: It is important to emphasize that all this is purely an optimization:
** exactly the same programs should typecheck with or without this
** procedure.

Solving fully
~~~~~~~~~~~~~
There is a reason why the solver does not simply try to solve such
constraints with top-level instances. If the solver finds a relevant
instance declaration in scope, that instance may require a context
that can't be solved for. A good example of this is:

    f :: Ord [a] => ...
    f x = ..Need Eq [a]...

If we have instance `Eq a => Eq [a]` in scope and we tried to use it, we would
be left with the obligation to solve the constraint Eq a, which we cannot. So we
must be conservative in our attempt to use an instance declaration to solve the
[W] constraint we're interested in.

Our rule is that we try to solve all of the instance's subgoals
recursively all at once. Precisely: We only attempt to solve
constraints of the form `C1, ... Cm => C t1 ... t n`, where all the Ci
are themselves class constraints of the form `C1', ... Cm' => C' t1'
... tn'` and we only succeed if the entire tree of constraints is
solvable from instances.

An example that succeeds:

    class Eq a => C a b | b -> a where
      m :: b -> a

    f :: C [Int] b => b -> Bool
    f x = m x == []

We solve for `Eq [Int]`, which requires `Eq Int`, which we also have. This
produces the following core:

    f :: forall b. C [Int] b => b -> Bool
    f = \ (@ b) ($dC :: C [Int] b) (x :: b) ->
        GHC.Classes.$fEq[]_$s$c==
          (m @ [Int] @ b $dC x) (GHC.Types.[] @ Int)

An example that fails:

    class Eq a => C a b | b -> a where
      m :: b -> a

    f :: C [a] b => b -> Bool
    f x = m x == []

Which, because solving `Eq [a]` demands `Eq a` which we cannot solve, produces:

    f :: forall a b. C [a] b => b -> Bool
    f = \ (@ a) (@ b) ($dC :: C [a] b) (eta :: b) ->
        ==
          @ [a]
          (A.$p1C @ [a] @ b $dC)
          (m @ [a] @ b $dC eta)
          (GHC.Types.[] @ a)

Note [Shortcut solving: type families]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have (#13943)
  class Take (n :: Nat) where ...
  instance {-# OVERLAPPING #-}                    Take 0 where ..
  instance {-# OVERLAPPABLE #-} (Take (n - 1)) => Take n where ..

And we have [W] Take 3.  That only matches one instance so we get
[W] Take (3-1).  Really we should now rewrite to reduce the (3-1) to 2, and
so on -- but that is reproducing yet more of the solver.  Sigh.  For now,
we just give up (remember all this is just an optimisation).

But we must not just naively try to lookup (Take (3-1)) in the
InstEnv, or it'll (wrongly) appear not to match (Take 0) and get a
unique match on the (Take n) instance.  That leads immediately to an
infinite loop.  Hence the check that 'preds' have no type families
(isTyFamFree).

Note [Shortcut solving: incoherence]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This optimization relies on coherence of dictionaries to be correct. When we
cannot assume coherence because of IncoherentInstances then this optimization
can change the behavior of the user's code.

The following four modules produce a program whose output would change depending
on whether we apply this optimization when IncoherentInstances is in effect:

=========
    {-# LANGUAGE MultiParamTypeClasses #-}
    module A where

    class A a where
      int :: a -> Int

    class A a => C a b where
      m :: b -> a -> a

=========
    {-# LANGUAGE FlexibleInstances     #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    module B where

    import A

    instance A a where
      int _ = 1

    instance C a [b] where
      m _ = id

=========
    {-# LANGUAGE FlexibleContexts      #-}
    {-# LANGUAGE FlexibleInstances     #-}
    {-# LANGUAGE IncoherentInstances   #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    module C where

    import A

    instance A Int where
      int _ = 2

    instance C Int [Int] where
      m _ = id

    intC :: C Int a => a -> Int -> Int
    intC _ x = int x

=========
    module Main where

    import A
    import B
    import C

    main :: IO ()
    main = print (intC [] (0::Int))

The output of `main` if we avoid the optimization under the effect of
IncoherentInstances is `1`. If we were to do the optimization, the output of
`main` would be `2`.

Note [Shortcut try_solve_from_instance]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The workhorse of the short-cut solver is
    try_solve_from_instance :: (EvBindMap, DictMap CtEvidence)
                            -> CtEvidence       -- Solve this
                            -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
Note that:

* The CtEvidence is the goal to be solved

* The MaybeT manages early failure if we find a subgoal that
  cannot be solved from instances.

* The (EvBindMap, DictMap CtEvidence) is an accumulating purely-functional
  state that allows try_solve_from_instance to augment the evidence
  bindings and inert_solved_dicts as it goes.

  If it succeeds, we commit all these bindings and solved dicts to the
  main TcS InertSet.  If not, we abandon it all entirely.

Passing along the solved_dicts important for two reasons:

* We need to be able to handle recursive super classes. The
  solved_dicts state  ensures that we remember what we have already
  tried to solve to avoid looping.

* As #15164 showed, it can be important to exploit sharing between
  goals. E.g. To solve G we may need G1 and G2. To solve G1 we may need H;
  and to solve G2 we may need H. If we don't spot this sharing we may
  solve H twice; and if this pattern repeats we may get exponentially bad
  behaviour.

Note [No Given/Given fundeps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We do not create constraints from:
* Given/Given interactions via functional dependencies or type family
  injectivity annotations.
* Given/instance fundep interactions via functional dependencies or
  type family injectivity annotations.

In this Note, all these interactions are called just "fundeps".

We ingore such fundeps for several reasons:

1. These fundeps will never serve a purpose in accepting more
   programs: Given constraints do not contain metavariables that could
   be unified via exploring fundeps. They *could* be useful in
   discovering inaccessible code. However, the constraints will be
   Wanteds, and as such will cause errors (not just warnings) if they
   go unsolved. Maybe there is a clever way to get the right
   inaccessible code warnings, but the path forward is far from
   clear. #12466 has further commentary.

2. Furthermore, here is a case where a Given/instance interaction is actively
   harmful (from dependent/should_compile/RaeJobTalk):

       type family a == b :: Bool
       type family Not a = r | r -> a where
         Not False = True
         Not True  = False

       [G] Not (a == b) ~ True

   Reacting this Given with the equations for Not produces

      [W] a == b ~ False

   This is indeed a true consequence, and would make sense as a fresh Given.
   But we don't have a way to produce evidence for fundeps, as a Wanted it
   is /harmful/: we can't prove it, and so we'll report an error and reject
   the program. (Previously fundeps gave rise to Deriveds, which
   carried no evidence, so it didn't matter that they could not be proved.)

3. #20922 showed a subtle different problem with Given/instance fundeps.
      type family ZipCons (as :: [k]) (bssx :: [[k]]) = (r :: [[k]]) | r -> as bssx where
        ZipCons (a ': as) (bs ': bss) = (a ': bs) ': ZipCons as bss
        ...

      tclevel = 4
      [G] ZipCons is1 iss ~ (i : is2) : jss

   (The tclevel=4 means that this Given is at level 4.)  The fundep tells us that
   'iss' must be of form (is2 : beta[4]) where beta[4] is a fresh unification
   variable; we don't know what type it stands for. So we would emit
      [W] iss ~ is2 : beta

   Again we can't prove that equality; and worse we'll rewrite iss to
   (is2:beta) in deeply nested constraints inside this implication,
   where beta is untouchable (under other equality constraints), leading
   to other insoluble constraints.

The bottom line: since we have no evidence for them, we should ignore Given/Given
and Given/instance fundeps entirely.
-}

tryInertDicts :: DictCt -> SolverStage ()
tryInertDicts :: DictCt -> SolverStage ()
tryInertDicts DictCt
dict_ct
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$ do { InertCans
inerts <- TcS InertCans
getInertCans
               ; InertCans -> DictCt -> TcS (StopOrContinue ())
try_inert_dicts InertCans
inerts DictCt
dict_ct }

try_inert_dicts :: InertCans -> DictCt -> TcS (StopOrContinue ())
try_inert_dicts :: InertCans -> DictCt -> TcS (StopOrContinue ())
try_inert_dicts InertCans
inerts dict_w :: DictCt
dict_w@(DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev_w, di_cls :: DictCt -> Class
di_cls = Class
cls, di_tys :: DictCt -> [Type]
di_tys = [Type]
tys })
  | Just DictCt
dict_i <- InertCans -> CtLoc -> Class -> [Type] -> Maybe DictCt
lookupInertDict InertCans
inerts (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w) Class
cls [Type]
tys
  , let ev_i :: CtEvidence
ev_i  = DictCt -> CtEvidence
dictCtEvidence DictCt
dict_i
        loc_i :: CtLoc
loc_i = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_i
        loc_w :: CtLoc
loc_w = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
  = -- There is a matching dictionary in the inert set
    do { -- First to try to solve it /completely/ from top level instances
         -- See Note [Shortcut solving]
         DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; Bool
short_cut_worked <- DynFlags -> CtEvidence -> CtEvidence -> TcS Bool
shortCutSolver DynFlags
dflags CtEvidence
ev_w CtEvidence
ev_i
       ; if Bool
short_cut_worked
         then CtEvidence -> String -> TcS (StopOrContinue ())
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev_w String
"interactDict/solved from instance"

         -- Next see if we are in "loopy-superclass" land.  If so,
         -- we don't want to replace the (Given) inert with the
         -- (Wanted) work-item, or vice versa; we want to hang on
         -- to both, and try to solve the work-item via an instance.
         -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
         else if CtLoc -> CtLoc -> Bool
prohibitedSuperClassSolve CtLoc
loc_i CtLoc
loc_w
         then () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()
         else
    do { -- The short-cut solver didn't fire, and loopy superclasses
         -- are dealt with, so we can either solve
         -- the inert from the work-item or vice-versa.
       ; case Ct -> Ct -> InteractResult
solveOneFromTheOther (DictCt -> Ct
CDictCan DictCt
dict_i) (DictCt -> Ct
CDictCan DictCt
dict_w) of
           InteractResult
KeepInert -> do { String -> SDoc -> TcS ()
traceTcS String
"lookupInertDict:KeepInert" (DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_w)
                           ; CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_w Bool
True (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev_i)
                           ; StopOrContinue () -> TcS (StopOrContinue ())
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (StopOrContinue () -> TcS (StopOrContinue ()))
-> StopOrContinue () -> TcS (StopOrContinue ())
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc -> StopOrContinue ()
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev_w (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Dict equal" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_w) }
           InteractResult
KeepWork  -> do { String -> SDoc -> TcS ()
traceTcS String
"lookupInertDict:KeepWork" (DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_w)
                           ; CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_i Bool
True (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev_w)
                           ; (InertCans -> InertCans) -> TcS ()
updInertCans ((DictMap DictCt -> DictMap DictCt) -> InertCans -> InertCans
updDicts ((DictMap DictCt -> DictMap DictCt) -> InertCans -> InertCans)
-> (DictMap DictCt -> DictMap DictCt) -> InertCans -> InertCans
forall a b. (a -> b) -> a -> b
$ DictCt -> DictMap DictCt -> DictMap DictCt
forall a. DictCt -> DictMap a -> DictMap a
delDict DictCt
dict_w)
                           ; () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () } } }

  | Bool
otherwise
  = do { String -> SDoc -> TcS ()
traceTcS String
"tryInertDicts:no" (DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_w SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys)
       ; () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }

-- See Note [Shortcut solving]
shortCutSolver :: DynFlags
               -> CtEvidence -- Work item
               -> CtEvidence -- Inert we want to try to replace
               -> TcS Bool   -- True <=> success
shortCutSolver :: DynFlags -> CtEvidence -> CtEvidence -> TcS Bool
shortCutSolver DynFlags
dflags CtEvidence
ev_w CtEvidence
ev_i
  | CtEvidence -> Bool
isWanted CtEvidence
ev_w
  , CtEvidence -> Bool
isGiven CtEvidence
ev_i
    -- We are about to solve a [W] constraint from a [G] constraint. We take
    -- a moment to see if we can get a better solution using an instance.
    -- Note that we only do this for the sake of performance. Exactly the same
    -- programs should typecheck regardless of whether we take this step or
    -- not. See Note [Shortcut solving]

  , Bool -> Bool
not (Type -> Bool
isIPLikePred (CtEvidence -> Type
ctEvPred CtEvidence
ev_w))   -- Not for implicit parameters (#18627)

  , Bool -> Bool
not (Extension -> DynFlags -> Bool
xopt Extension
LangExt.IncoherentInstances DynFlags
dflags)
    -- If IncoherentInstances is on then we cannot rely on coherence of proofs
    -- in order to justify this optimization: The proof provided by the
    -- [G] constraint's superclass may be different from the top-level proof.
    -- See Note [Shortcut solving: incoherence]

  , GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_SolveConstantDicts DynFlags
dflags
    -- Enabled by the -fsolve-constant-dicts flag

  = do { EvBindsVar
ev_binds_var <- TcS EvBindsVar
getTcEvBindsVar
       ; EvBindMap
ev_binds <- Bool -> SDoc -> TcS EvBindMap -> TcS EvBindMap
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (Bool -> Bool
not (EvBindsVar -> Bool
isCoEvBindsVar EvBindsVar
ev_binds_var )) (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev_w) (TcS EvBindMap -> TcS EvBindMap) -> TcS EvBindMap -> TcS EvBindMap
forall a b. (a -> b) -> a -> b
$
                     EvBindsVar -> TcS EvBindMap
getTcEvBindsMap EvBindsVar
ev_binds_var
       ; DictMap DictCt
solved_dicts <- TcS (DictMap DictCt)
getSolvedDicts

       ; Maybe (EvBindMap, DictMap DictCt)
mb_stuff <- MaybeT TcS (EvBindMap, DictMap DictCt)
-> TcS (Maybe (EvBindMap, DictMap DictCt))
forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT (MaybeT TcS (EvBindMap, DictMap DictCt)
 -> TcS (Maybe (EvBindMap, DictMap DictCt)))
-> MaybeT TcS (EvBindMap, DictMap DictCt)
-> TcS (Maybe (EvBindMap, DictMap DictCt))
forall a b. (a -> b) -> a -> b
$
                     (EvBindMap, DictMap DictCt)
-> CtEvidence -> MaybeT TcS (EvBindMap, DictMap DictCt)
try_solve_from_instance (EvBindMap
ev_binds, DictMap DictCt
solved_dicts) CtEvidence
ev_w

       ; case Maybe (EvBindMap, DictMap DictCt)
mb_stuff of
           Maybe (EvBindMap, DictMap DictCt)
Nothing -> Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
           Just (EvBindMap
ev_binds', DictMap DictCt
solved_dicts')
              -> do { EvBindsVar -> EvBindMap -> TcS ()
setTcEvBindsMap EvBindsVar
ev_binds_var EvBindMap
ev_binds'
                    ; DictMap DictCt -> TcS ()
setSolvedDicts DictMap DictCt
solved_dicts'
                    ; Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True } }

  | Bool
otherwise
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
  where
    -- This `CtLoc` is used only to check the well-staged condition of any
    -- candidate DFun. Our subgoals all have the same stage as our root
    -- [W] constraint so it is safe to use this while solving them.
    loc_w :: CtLoc
loc_w = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w

    try_solve_from_instance   -- See Note [Shortcut try_solve_from_instance]
      :: (EvBindMap, DictMap DictCt) -> CtEvidence
      -> MaybeT TcS (EvBindMap, DictMap DictCt)
    try_solve_from_instance :: (EvBindMap, DictMap DictCt)
-> CtEvidence -> MaybeT TcS (EvBindMap, DictMap DictCt)
try_solve_from_instance (EvBindMap
ev_binds, DictMap DictCt
solved_dicts) CtEvidence
ev
      | let pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
ev
      , ClassPred Class
cls [Type]
tys <- Type -> Pred
classifyPredType Type
pred
      = do { ClsInstResult
inst_res <- TcS ClsInstResult -> MaybeT TcS ClsInstResult
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS ClsInstResult -> MaybeT TcS ClsInstResult)
-> TcS ClsInstResult -> MaybeT TcS ClsInstResult
forall a b. (a -> b) -> a -> b
$ DynFlags -> Bool -> Class -> [Type] -> TcS ClsInstResult
matchGlobalInst DynFlags
dflags Bool
True Class
cls [Type]
tys
           ; case ClsInstResult
inst_res of
               OneInst { cir_new_theta :: ClsInstResult -> [Type]
cir_new_theta   = [Type]
preds
                       , cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                       , cir_canonical :: ClsInstResult -> Bool
cir_canonical   = Bool
canonical
                       , cir_what :: ClsInstResult -> InstanceWhat
cir_what        = InstanceWhat
what }
                 | InstanceWhat -> Bool
safeOverlap InstanceWhat
what
                 , (Type -> Bool) -> [Type] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all Type -> Bool
isTyFamFree [Type]
preds  -- Note [Shortcut solving: type families]
                 -> do { let dict_ct :: DictCt
dict_ct = DictCt { di_ev :: CtEvidence
di_ev = CtEvidence
ev, di_cls :: Class
di_cls = Class
cls
                                              , di_tys :: [Type]
di_tys = [Type]
tys, di_pend_sc :: ScDepth
di_pend_sc = ScDepth
doNotExpand }
                             solved_dicts' :: DictMap DictCt
solved_dicts' = DictCt -> DictMap DictCt -> DictMap DictCt
addSolvedDict DictCt
dict_ct DictMap DictCt
solved_dicts
                             -- solved_dicts': it is important that we add our goal
                             -- to the cache before we solve! Otherwise we may end
                             -- up in a loop while solving recursive dictionaries.

                       ; TcS () -> MaybeT TcS ()
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS () -> MaybeT TcS ()) -> TcS () -> MaybeT TcS ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc -> TcS ()
traceTcS String
"shortCutSolver: found instance" ([Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
preds)
                       ; CtLoc
loc' <- TcS CtLoc -> MaybeT TcS CtLoc
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS CtLoc -> MaybeT TcS CtLoc) -> TcS CtLoc -> MaybeT TcS CtLoc
forall a b. (a -> b) -> a -> b
$ CtLoc -> InstanceWhat -> Type -> TcS CtLoc
checkInstanceOK (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev) InstanceWhat
what Type
pred
                       ; TcS () -> MaybeT TcS ()
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS () -> MaybeT TcS ()) -> TcS () -> MaybeT TcS ()
forall a b. (a -> b) -> a -> b
$ CtLoc -> Type -> TcS ()
checkReductionDepth CtLoc
loc' Type
pred


                       ; [MaybeNew]
evc_vs <- (Type -> MaybeT TcS MaybeNew) -> [Type] -> MaybeT TcS [MaybeNew]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtEvidence
-> CtLoc -> DictMap DictCt -> Type -> MaybeT TcS MaybeNew
new_wanted_cached CtEvidence
ev CtLoc
loc' DictMap DictCt
solved_dicts') [Type]
preds
                                  -- Emit work for subgoals but use our local cache
                                  -- so we can solve recursive dictionaries.

                       ; let ev_tm :: EvTerm
ev_tm     = [EvExpr] -> EvTerm
mk_ev ((MaybeNew -> EvExpr) -> [MaybeNew] -> [EvExpr]
forall a b. (a -> b) -> [a] -> [b]
map MaybeNew -> EvExpr
getEvExpr [MaybeNew]
evc_vs)
                             ev_binds' :: EvBindMap
ev_binds' = EvBindMap -> EvBind -> EvBindMap
extendEvBinds EvBindMap
ev_binds (EvBind -> EvBindMap) -> EvBind -> EvBindMap
forall a b. (a -> b) -> a -> b
$
                                         EvVar -> Bool -> EvTerm -> EvBind
mkWantedEvBind (CtEvidence -> EvVar
ctEvEvId CtEvidence
ev) Bool
canonical EvTerm
ev_tm

                       ; ((EvBindMap, DictMap DictCt)
 -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap DictCt))
-> (EvBindMap, DictMap DictCt)
-> [CtEvidence]
-> MaybeT TcS (EvBindMap, DictMap DictCt)
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM (EvBindMap, DictMap DictCt)
-> CtEvidence -> MaybeT TcS (EvBindMap, DictMap DictCt)
try_solve_from_instance (EvBindMap
ev_binds', DictMap DictCt
solved_dicts') ([CtEvidence] -> MaybeT TcS (EvBindMap, DictMap DictCt))
-> [CtEvidence] -> MaybeT TcS (EvBindMap, DictMap DictCt)
forall a b. (a -> b) -> a -> b
$
                         [MaybeNew] -> [CtEvidence]
freshGoals [MaybeNew]
evc_vs }

               ClsInstResult
_ -> MaybeT TcS (EvBindMap, DictMap DictCt)
forall a. MaybeT TcS a
forall (m :: * -> *) a. MonadPlus m => m a
mzero }

      | Bool
otherwise
      = MaybeT TcS (EvBindMap, DictMap DictCt)
forall a. MaybeT TcS a
forall (m :: * -> *) a. MonadPlus m => m a
mzero


    -- Use a local cache of solved dicts while emitting EvVars for new work
    -- We bail out of the entire computation if we need to emit an EvVar for
    -- a subgoal that isn't a ClassPred.
    new_wanted_cached :: CtEvidence -> CtLoc
                      -> DictMap DictCt -> TcPredType -> MaybeT TcS MaybeNew
    new_wanted_cached :: CtEvidence
-> CtLoc -> DictMap DictCt -> Type -> MaybeT TcS MaybeNew
new_wanted_cached CtEvidence
ev_w CtLoc
loc DictMap DictCt
cache Type
pty
      | ClassPred Class
cls [Type]
tys <- Type -> Pred
classifyPredType Type
pty
      = TcS MaybeNew -> MaybeT TcS MaybeNew
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS MaybeNew -> MaybeT TcS MaybeNew)
-> TcS MaybeNew -> MaybeT TcS MaybeNew
forall a b. (a -> b) -> a -> b
$ case DictMap DictCt -> CtLoc -> Class -> [Type] -> Maybe DictCt
forall a. DictMap a -> CtLoc -> Class -> [Type] -> Maybe a
findDict DictMap DictCt
cache CtLoc
loc_w Class
cls [Type]
tys of
          Just DictCt
dict_ct -> MaybeNew -> TcS MaybeNew
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (MaybeNew -> TcS MaybeNew) -> MaybeNew -> TcS MaybeNew
forall a b. (a -> b) -> a -> b
$ EvExpr -> MaybeNew
Cached (HasDebugCallStack => CtEvidence -> EvExpr
CtEvidence -> EvExpr
ctEvExpr (DictCt -> CtEvidence
dictCtEvidence DictCt
dict_ct))
          Maybe DictCt
Nothing      -> CtEvidence -> MaybeNew
Fresh (CtEvidence -> MaybeNew) -> TcS CtEvidence -> TcS MaybeNew
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> CtLoc -> RewriterSet -> Type -> TcS CtEvidence
newWantedNC CtLoc
loc (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
ev_w) Type
pty
      | Bool
otherwise = MaybeT TcS MaybeNew
forall a. MaybeT TcS a
forall (m :: * -> *) a. MonadPlus m => m a
mzero

{- *******************************************************************
*                                                                    *
         Top-level reaction for class constraints (CDictCan)
*                                                                    *
**********************************************************************-}

tryInstances :: DictCt -> SolverStage ()
tryInstances :: DictCt -> SolverStage ()
tryInstances DictCt
dict_ct
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$ do { InertSet
inerts <- TcS InertSet
getInertSet
               ; InertSet -> DictCt -> TcS (StopOrContinue ())
try_instances InertSet
inerts DictCt
dict_ct }

try_instances :: InertSet -> DictCt -> TcS (StopOrContinue ())
-- Try to use type-class instance declarations to simplify the constraint
try_instances :: InertSet -> DictCt -> TcS (StopOrContinue ())
try_instances InertSet
inerts work_item :: DictCt
work_item@(DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev, di_cls :: DictCt -> Class
di_cls = Class
cls
                                       , di_tys :: DictCt -> [Type]
di_tys = [Type]
xis })
  | CtEvidence -> Bool
isGiven CtEvidence
ev   -- Never use instances for Given constraints
  = () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()
     -- See Note [No Given/Given fundeps]

  | Just CtEvidence
solved_ev <- InertSet -> CtLoc -> Class -> [Type] -> Maybe CtEvidence
lookupSolvedDict InertSet
inerts CtLoc
dict_loc Class
cls [Type]
xis   -- Cached
  = do { CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev Bool
True (CtEvidence -> EvTerm
ctEvTerm CtEvidence
solved_ev)
       ; CtEvidence -> String -> TcS (StopOrContinue ())
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Dict/Top (cached)" }

  | Bool
otherwise  -- Wanted, but not cached
   = do { DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
        ; ClsInstResult
lkup_res <- DynFlags
-> InertSet -> Class -> [Type] -> CtLoc -> TcS ClsInstResult
matchClassInst DynFlags
dflags InertSet
inerts Class
cls [Type]
xis CtLoc
dict_loc
        ; case ClsInstResult
lkup_res of
               OneInst { cir_what :: ClsInstResult -> InstanceWhat
cir_what = InstanceWhat
what }
                  -> do { InstanceWhat -> DictCt -> TcS ()
insertSafeOverlapFailureTcS InstanceWhat
what DictCt
work_item
                        ; InstanceWhat -> DictCt -> TcS ()
updSolvedDicts InstanceWhat
what DictCt
work_item
                        ; CtEvidence -> ClsInstResult -> TcS (StopOrContinue ())
forall a. CtEvidence -> ClsInstResult -> TcS (StopOrContinue a)
chooseInstance CtEvidence
ev ClsInstResult
lkup_res }
               ClsInstResult
_  -> -- NoInstance or NotSure
                     -- We didn't solve it; so try functional dependencies
                     () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }
   where
     dict_loc :: CtLoc
dict_loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

chooseInstance :: CtEvidence -> ClsInstResult -> TcS (StopOrContinue a)
chooseInstance :: forall a. CtEvidence -> ClsInstResult -> TcS (StopOrContinue a)
chooseInstance CtEvidence
work_item
               (OneInst { cir_new_theta :: ClsInstResult -> [Type]
cir_new_theta   = [Type]
theta
                        , cir_what :: ClsInstResult -> InstanceWhat
cir_what        = InstanceWhat
what
                        , cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                        , cir_canonical :: ClsInstResult -> Bool
cir_canonical   = Bool
canonical })
  = do { String -> SDoc -> TcS ()
traceTcS String
"doTopReact/found instance for" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
work_item
       ; CtLoc
deeper_loc <- CtLoc -> InstanceWhat -> Type -> TcS CtLoc
checkInstanceOK CtLoc
loc InstanceWhat
what Type
pred
       ; CtLoc -> Type -> TcS ()
checkReductionDepth CtLoc
deeper_loc Type
pred
       ; TcS Bool -> SDoc -> TcS ()
forall (m :: * -> *).
(HasCallStack, Monad m) =>
m Bool -> SDoc -> m ()
assertPprM (TcS EvBindsVar
getTcEvBindsVar TcS EvBindsVar -> (EvBindsVar -> TcS Bool) -> TcS Bool
forall a b. TcS a -> (a -> TcS b) -> TcS b
forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>>= Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bool -> TcS Bool)
-> (EvBindsVar -> Bool) -> EvBindsVar -> TcS Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. Bool -> Bool
not (Bool -> Bool) -> (EvBindsVar -> Bool) -> EvBindsVar -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. EvBindsVar -> Bool
isCoEvBindsVar)
                    (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
work_item)
       ; [MaybeNew]
evc_vars <- (Type -> TcS MaybeNew) -> [Type] -> TcS [MaybeNew]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtLoc -> RewriterSet -> Type -> TcS MaybeNew
newWanted CtLoc
deeper_loc (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
work_item)) [Type]
theta
       ; CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
work_item Bool
canonical ([EvExpr] -> EvTerm
mk_ev ((MaybeNew -> EvExpr) -> [MaybeNew] -> [EvExpr]
forall a b. (a -> b) -> [a] -> [b]
map MaybeNew -> EvExpr
getEvExpr [MaybeNew]
evc_vars))
       ; [CtEvidence] -> TcS ()
emitWorkNC ([MaybeNew] -> [CtEvidence]
freshGoals [MaybeNew]
evc_vars)
       ; CtEvidence -> String -> TcS (StopOrContinue a)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
work_item String
"Dict/Top (solved wanted)" }
  where
     pred :: Type
pred = CtEvidence -> Type
ctEvPred CtEvidence
work_item
     loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
work_item

chooseInstance CtEvidence
work_item ClsInstResult
lookup_res
  = String -> SDoc -> TcS (StopOrContinue a)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"chooseInstance" (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
work_item SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
lookup_res)

checkInstanceOK :: CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc
-- Check that it's OK to use this instance:
--    (a) the use is well staged in the Template Haskell sense
-- Returns the CtLoc to used for sub-goals
-- Probably also want to call checkReductionDepth
checkInstanceOK :: CtLoc -> InstanceWhat -> Type -> TcS CtLoc
checkInstanceOK CtLoc
loc InstanceWhat
what Type
pred
  = do { CtLoc -> InstanceWhat -> Type -> TcS ()
checkWellStagedDFun CtLoc
loc InstanceWhat
what Type
pred
       ; CtLoc -> TcS CtLoc
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return CtLoc
deeper_loc }
  where
     deeper_loc :: CtLoc
deeper_loc = CtLoc -> CtLoc
zap_origin (CtLoc -> CtLoc
bumpCtLocDepth CtLoc
loc)
     origin :: CtOrigin
origin     = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc

     zap_origin :: CtLoc -> CtLoc
zap_origin CtLoc
loc  -- After applying an instance we can set ScOrigin to
                     -- NotNakedSc, so that prohibitedSuperClassSolve never fires
                     -- See Note [Solving superclass constraints] in
                     -- GHC.Tc.TyCl.Instance, (sc1).
       | ScOrigin ClsInstOrQC
what NakedScFlag
_ <- CtOrigin
origin
       = CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin CtLoc
loc (ClsInstOrQC -> NakedScFlag -> CtOrigin
ScOrigin ClsInstOrQC
what NakedScFlag
NotNakedSc)
       | Bool
otherwise
       = CtLoc
loc

matchClassInst :: DynFlags -> InertSet
               -> Class -> [Type]
               -> CtLoc -> TcS ClsInstResult
matchClassInst :: DynFlags
-> InertSet -> Class -> [Type] -> CtLoc -> TcS ClsInstResult
matchClassInst DynFlags
dflags InertSet
inerts Class
clas [Type]
tys CtLoc
loc
-- First check whether there is an in-scope Given that could
-- match this constraint.  In that case, do not use any instance
-- whether top level, or local quantified constraints.
-- See Note [Instance and Given overlap]
  | Bool -> Bool
not (Extension -> DynFlags -> Bool
xopt Extension
LangExt.IncoherentInstances DynFlags
dflags)
  , Bool -> Bool
not (Class -> Bool
isCTupleClass Class
clas)
        -- It is always safe to unpack constraint tuples
        -- And if we don't do so, we may never solve it at all
        -- See Note [Solving tuple constraints]
  , Bool -> Bool
not (InertSet -> CtLoc -> Class -> [Type] -> Bool
noMatchableGivenDicts InertSet
inerts CtLoc
loc Class
clas [Type]
tys)
  = do { String -> SDoc -> TcS ()
traceTcS String
"Delaying instance application" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
           [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Work item:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Class -> [Type] -> SDoc
pprClassPred Class
clas [Type]
tys ]
       ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NotSure }

  | Bool
otherwise
  = do { String -> SDoc -> TcS ()
traceTcS String
"matchClassInst" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"pred =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
pred SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Char -> SDoc
forall doc. IsLine doc => Char -> doc
char Char
'{'
       ; ClsInstResult
local_res <- Type -> CtLoc -> TcS ClsInstResult
matchLocalInst Type
pred CtLoc
loc
       ; case ClsInstResult
local_res of
           OneInst {} ->  -- See Note [Local instances and incoherence]
                do { String -> SDoc -> TcS ()
traceTcS String
"} matchClassInst local match" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
local_res
                   ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
local_res }

           ClsInstResult
NotSure -> -- In the NotSure case for local instances
                      -- we don't want to try global instances
                do { String -> SDoc -> TcS ()
traceTcS String
"} matchClassInst local not sure" SDoc
forall doc. IsOutput doc => doc
empty
                   ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
local_res }

           ClsInstResult
NoInstance  -- No local instances, so try global ones
              -> do { ClsInstResult
global_res <- DynFlags -> Bool -> Class -> [Type] -> TcS ClsInstResult
matchGlobalInst DynFlags
dflags Bool
False Class
clas [Type]
tys
                    ; String -> SDoc -> TcS ()
traceTcS String
"} matchClassInst global result" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
global_res
                    ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
global_res } }
  where
    pred :: Type
pred = Class -> [Type] -> Type
mkClassPred Class
clas [Type]
tys

{- Note [Instance and Given overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example, from the OutsideIn(X) paper:
       instance P x => Q [x]
       instance (x ~ y) => R y [x]

       wob :: forall a b. (Q [b], R b a) => a -> Int

       g :: forall a. Q [a] => [a] -> Int
       g x = wob x

From 'g' we get the implication constraint:
            forall a. Q [a] => (Q [beta], R beta [a])
If we react (Q [beta]) with its top-level axiom, we end up with a
(P beta), which we have no way of discharging. On the other hand,
if we react R beta [a] with the top-level we get  (beta ~ a), which
is solvable and can help us rewrite (Q [beta]) to (Q [a]) which is
now solvable by the given Q [a].

The partial solution is that:
  In matchClassInst (and thus in topReact), we return a matching
  instance only when there is no Given in the inerts which is
  unifiable to this particular dictionary.

  We treat any meta-tyvar as "unifiable" for this purpose,
  *including* untouchable ones.  But not skolems like 'a' in
  the implication constraint above.

The end effect is that, much as we do for overlapping instances, we
delay choosing a class instance if there is a possibility of another
instance OR a given to match our constraint later on. This fixes
tickets #4981 and #5002.

Other notes:

* The check is done *first*, so that it also covers classes
  with built-in instance solving, such as
     - constraint tuples
     - natural numbers
     - Typeable

* See also Note [What might equal later?] in GHC.Tc.Solver.InertSet.

* The given-overlap problem is arguably not easy to appear in practice
  due to our aggressive prioritization of equality solving over other
  constraints, but it is possible. I've added a test case in
  typecheck/should-compile/GivenOverlapping.hs

* Another "live" example is #10195; another is #10177.

* We ignore the overlap problem if -XIncoherentInstances is in force:
  see #6002 for a worked-out example where this makes a
  difference.

* Moreover notice that our goals here are different than the goals of
  the top-level overlapping checks. There we are interested in
  validating the following principle:

      If we inline a function f at a site where the same global
      instance environment is available as the instance environment at
      the definition site of f then we should get the same behaviour.

  But for the Given Overlap check our goal is just related to completeness of
  constraint solving.

* The solution is only a partial one.  Consider the above example with
       g :: forall a. Q [a] => [a] -> Int
       g x = let v = wob x
             in v
  and suppose we have -XNoMonoLocalBinds, so that we attempt to find the most
  general type for 'v'.  When generalising v's type we'll simplify its
  Q [alpha] constraint, but we don't have Q [a] in the 'givens', so we
  will use the instance declaration after all. #11948 was a case
  in point.

All of this is disgustingly delicate, so to discourage people from writing
simplifiable class givens, we warn about signatures that contain them;
see GHC.Tc.Validity Note [Simplifiable given constraints].


Note [Local instances and incoherence]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f :: forall b c. (Eq b, forall a. Eq a => Eq (c a))
                 => c b -> Bool
   f x = x==x

We get [W] Eq (c b), and we must use the local instance to solve it.

BUT that wanted also unifies with the top-level Eq [a] instance,
and Eq (Maybe a) etc.  We want the local instance to "win", otherwise
we can't solve the wanted at all.  So we mark it as Incohherent.
According to Note [Rules for instance lookup] in GHC.Core.InstEnv, that'll
make it win even if there are other instances that unify.

Moreover this is not a hack!  The evidence for this local instance
will be constructed by GHC at a call site... from the very instances
that unify with it here.  It is not like an incoherent user-written
instance which might have utterly different behaviour.

Consider  f :: Eq a => blah.  If we have [W] Eq a, we certainly
get it from the Eq a context, without worrying that there are
lots of top-level instances that unify with [W] Eq a!  We'll use
those instances to build evidence to pass to f. That's just the
nullary case of what's happening here.
-}

matchLocalInst :: TcPredType -> CtLoc -> TcS ClsInstResult
-- Look up the predicate in Given quantified constraints,
-- which are effectively just local instance declarations.
matchLocalInst :: Type -> CtLoc -> TcS ClsInstResult
matchLocalInst Type
pred CtLoc
loc
  = do { inerts :: InertSet
inerts@(IS { inert_cans :: InertSet -> InertCans
inert_cans = InertCans
ics }) <- TcS InertSet
getInertSet
       ; case InertSet
-> [QCInst]
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
match_local_inst InertSet
inerts (InertCans -> [QCInst]
inert_insts InertCans
ics) of
          { ([], []) -> do { String -> SDoc -> TcS ()
traceTcS String
"No local instance for" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
pred)
                           ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance }
          ; ([(CtEvidence, [DFunInstType])]
matches, [(CtEvidence, [DFunInstType])]
unifs) ->
    do { [InstDFun]
matches <- ((CtEvidence, [DFunInstType]) -> TcS InstDFun)
-> [(CtEvidence, [DFunInstType])] -> TcS [InstDFun]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtEvidence, [DFunInstType]) -> TcS InstDFun
mk_instDFun [(CtEvidence, [DFunInstType])]
matches
       ; [InstDFun]
unifs   <- ((CtEvidence, [DFunInstType]) -> TcS InstDFun)
-> [(CtEvidence, [DFunInstType])] -> TcS [InstDFun]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtEvidence, [DFunInstType]) -> TcS InstDFun
mk_instDFun [(CtEvidence, [DFunInstType])]
unifs
         -- See Note [Use only the best matching quantified constraint]
       ; case [InstDFun] -> Maybe InstDFun
dominatingMatch [InstDFun]
matches of
          { Just (EvVar
dfun_id, [Type]
tys, [Type]
theta)
            | (InstDFun -> Bool) -> [InstDFun] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (([Type]
theta [Type] -> [Type] -> Bool
`impliedBySCs`) ([Type] -> Bool) -> (InstDFun -> [Type]) -> InstDFun -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. InstDFun -> [Type]
forall a b c. (a, b, c) -> c
thdOf3) [InstDFun]
unifs
            ->
            do { let result :: ClsInstResult
result = OneInst { cir_new_theta :: [Type]
cir_new_theta   = [Type]
theta
                                      , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = EvVar -> [Type] -> [EvExpr] -> EvTerm
evDFunApp EvVar
dfun_id [Type]
tys
                                      , cir_canonical :: Bool
cir_canonical   = Bool
True
                                      , cir_what :: InstanceWhat
cir_what        = InstanceWhat
LocalInstance }
               ; String -> SDoc -> TcS ()
traceTcS String
"Best local instance found:" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                  [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"pred:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
pred
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"result:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
result
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"matches:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
matches
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"unifs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
unifs ]
               ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
result }

          ; Maybe InstDFun
mb_best ->
            do { String -> SDoc -> TcS ()
traceTcS String
"Multiple local instances; not committing to any"
                  (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"pred:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
pred
                         , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"matches:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
matches
                         , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"unifs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
unifs
                         , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"best_match:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Maybe InstDFun -> SDoc
forall a. Outputable a => a -> SDoc
ppr Maybe InstDFun
mb_best ]
               ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NotSure }}}}}
  where
    pred_tv_set :: TyCoVarSet
pred_tv_set = Type -> TyCoVarSet
tyCoVarsOfType Type
pred

    mk_instDFun :: (CtEvidence, [DFunInstType]) -> TcS InstDFun
    mk_instDFun :: (CtEvidence, [DFunInstType]) -> TcS InstDFun
mk_instDFun (CtEvidence
ev, [DFunInstType]
tys) =
      let dfun_id :: EvVar
dfun_id = CtEvidence -> EvVar
ctEvEvId CtEvidence
ev
      in do { ([Type]
tys, [Type]
theta) <- EvVar -> [DFunInstType] -> TcS ([Type], [Type])
instDFunType (CtEvidence -> EvVar
ctEvEvId CtEvidence
ev) [DFunInstType]
tys
            ; InstDFun -> TcS InstDFun
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (EvVar
dfun_id, [Type]
tys, [Type]
theta) }

    -- Compute matching and unifying local instances
    match_local_inst :: InertSet
                     -> [QCInst]
                     -> ( [(CtEvidence, [DFunInstType])]
                        , [(CtEvidence, [DFunInstType])] )
    match_local_inst :: InertSet
-> [QCInst]
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
match_local_inst InertSet
_inerts []
      = ([], [])
    match_local_inst InertSet
inerts (qci :: QCInst
qci@(QCI { qci_tvs :: QCInst -> [EvVar]
qci_tvs  = [EvVar]
qtvs
                                      , qci_pred :: QCInst -> Type
qci_pred = Type
qpred
                                      , qci_ev :: QCInst -> CtEvidence
qci_ev   = CtEvidence
qev })
                            :[QCInst]
qcis)
      | let in_scope :: InScopeSet
in_scope = TyCoVarSet -> InScopeSet
mkInScopeSet (TyCoVarSet
qtv_set TyCoVarSet -> TyCoVarSet -> TyCoVarSet
`unionVarSet` TyCoVarSet
pred_tv_set)
      , Just TvSubstEnv
tv_subst <- TyCoVarSet
-> RnEnv2 -> TvSubstEnv -> Type -> Type -> Maybe TvSubstEnv
ruleMatchTyKiX TyCoVarSet
qtv_set (InScopeSet -> RnEnv2
mkRnEnv2 InScopeSet
in_scope)
                                        TvSubstEnv
emptyTvSubstEnv Type
qpred Type
pred
      , let match :: (CtEvidence, [DFunInstType])
match = (CtEvidence
qev, (EvVar -> DFunInstType) -> [EvVar] -> [DFunInstType]
forall a b. (a -> b) -> [a] -> [b]
map (TvSubstEnv -> EvVar -> DFunInstType
forall a. VarEnv a -> EvVar -> Maybe a
lookupVarEnv TvSubstEnv
tv_subst) [EvVar]
qtvs)
      = ((CtEvidence, [DFunInstType])
match(CtEvidence, [DFunInstType])
-> [(CtEvidence, [DFunInstType])] -> [(CtEvidence, [DFunInstType])]
forall a. a -> [a] -> [a]
:[(CtEvidence, [DFunInstType])]
matches, [(CtEvidence, [DFunInstType])]
unifs)

      | Bool
otherwise
      = Bool
-> SDoc
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TyCoVarSet -> TyCoVarSet -> Bool
disjointVarSet TyCoVarSet
qtv_set (Type -> TyCoVarSet
tyCoVarsOfType Type
pred))
                  (QCInst -> SDoc
forall a. Outputable a => a -> SDoc
ppr QCInst
qci SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
pred)
            -- ASSERT: unification relies on the
            -- quantified variables being fresh
        ([(CtEvidence, [DFunInstType])]
matches, Maybe (CtEvidence, [DFunInstType])
this_unif Maybe (CtEvidence, [DFunInstType])
-> [(CtEvidence, [DFunInstType])] -> [(CtEvidence, [DFunInstType])]
forall {a}. Maybe a -> [a] -> [a]
`combine` [(CtEvidence, [DFunInstType])]
unifs)
      where
        qloc :: CtLoc
qloc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
qev
        qtv_set :: TyCoVarSet
qtv_set = [EvVar] -> TyCoVarSet
mkVarSet [EvVar]
qtvs
        ([(CtEvidence, [DFunInstType])]
matches, [(CtEvidence, [DFunInstType])]
unifs) = InertSet
-> [QCInst]
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
match_local_inst InertSet
inerts [QCInst]
qcis
        this_unif :: Maybe (CtEvidence, [DFunInstType])
this_unif
          | Just Subst
subst <- InertSet -> Type -> CtLoc -> Type -> CtLoc -> Maybe Subst
mightEqualLater InertSet
inerts Type
qpred CtLoc
qloc Type
pred CtLoc
loc
          = (CtEvidence, [DFunInstType]) -> Maybe (CtEvidence, [DFunInstType])
forall a. a -> Maybe a
Just (CtEvidence
qev, (EvVar -> DFunInstType) -> [EvVar] -> [DFunInstType]
forall a b. (a -> b) -> [a] -> [b]
map  (Subst -> EvVar -> DFunInstType
lookupTyVar Subst
subst) [EvVar]
qtvs)
          | Bool
otherwise
          = Maybe (CtEvidence, [DFunInstType])
forall a. Maybe a
Nothing

        combine :: Maybe a -> [a] -> [a]
combine Maybe a
Nothing  [a]
us = [a]
us
        combine (Just a
u) [a]
us = a
u a -> [a] -> [a]
forall a. a -> [a] -> [a]
: [a]
us

-- | Instance dictionary function and type.
type InstDFun = (DFunId, [TcType], TcThetaType)

-- | Try to find a local quantified instance that dominates all others,
-- i.e. which has a weaker instance context than all the others.
--
-- See Note [Use only the best matching quantified constraint].
dominatingMatch :: [InstDFun] -> Maybe InstDFun
dominatingMatch :: [InstDFun] -> Maybe InstDFun
dominatingMatch [InstDFun]
matches =
  [InstDFun] -> Maybe InstDFun
forall a. [a] -> Maybe a
listToMaybe ([InstDFun] -> Maybe InstDFun) -> [InstDFun] -> Maybe InstDFun
forall a b. (a -> b) -> a -> b
$ ((InstDFun, [InstDFun]) -> Maybe InstDFun)
-> [(InstDFun, [InstDFun])] -> [InstDFun]
forall a b. (a -> Maybe b) -> [a] -> [b]
mapMaybe ((InstDFun -> [InstDFun] -> Maybe InstDFun)
-> (InstDFun, [InstDFun]) -> Maybe InstDFun
forall a b c. (a -> b -> c) -> (a, b) -> c
uncurry InstDFun -> [InstDFun] -> Maybe InstDFun
go) ([InstDFun] -> [(InstDFun, [InstDFun])]
forall a. [a] -> [(a, [a])]
holes [InstDFun]
matches)
  -- listToMaybe: arbitrarily pick any one context that is weaker than
  -- all others, e.g. so that we can handle [Eq a, Num a] vs [Num a, Eq a]
  -- (see test case T22223).

  where
    go :: InstDFun -> [InstDFun] -> Maybe InstDFun
    go :: InstDFun -> [InstDFun] -> Maybe InstDFun
go InstDFun
this [] = InstDFun -> Maybe InstDFun
forall a. a -> Maybe a
Just InstDFun
this
    go this :: InstDFun
this@(EvVar
_,[Type]
_,[Type]
this_theta) ((EvVar
_,[Type]
_,[Type]
other_theta):[InstDFun]
others)
      | [Type]
this_theta [Type] -> [Type] -> Bool
`impliedBySCs` [Type]
other_theta
      = InstDFun -> [InstDFun] -> Maybe InstDFun
go InstDFun
this [InstDFun]
others
      | Bool
otherwise
      = Maybe InstDFun
forall a. Maybe a
Nothing

-- | Whether a collection of constraints is implied by another collection,
-- according to a simple superclass check.
--
-- See Note [When does a quantified instance dominate another?].
impliedBySCs :: TcThetaType -> TcThetaType -> Bool
impliedBySCs :: [Type] -> [Type] -> Bool
impliedBySCs [Type]
c1 [Type]
c2 = (Type -> Bool) -> [Type] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all Type -> Bool
in_c2 [Type]
c1
  where
    in_c2 :: TcPredType -> Bool
    in_c2 :: Type -> Bool
in_c2 Type
pred = (Type -> Bool) -> [Type] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
any (Type
pred HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType`) [Type]
c2_expanded

    c2_expanded :: [TcPredType]  -- Includes all superclasses
    c2_expanded :: [Type]
c2_expanded = [ Type
q | Type
p <- [Type]
c2, Type
q <- Type
p Type -> [Type] -> [Type]
forall a. a -> [a] -> [a]
: Type -> [Type]
transSuperClasses Type
p ]


{- Note [When does a quantified instance dominate another?]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When matching local quantified instances, it's useful to be able to pick
the one with the weakest precondition, e.g. if one has both

  [G] d1: forall a b. ( Eq a, Num b, C a b  ) => D a b
  [G] d2: forall a  .                C a Int  => D a Int
  [W] {w}: D a Int

Then it makes sense to use d2 to solve w, as doing so we end up with a strictly
weaker proof obligation of `C a Int`, compared to `(Eq a, Num Int, C a Int)`
were we to use d1.

In theory, to compute whether one context implies another, we would need to
recursively invoke the constraint solver. This is expensive, so we instead do
a simple check using superclasses, implemented in impliedBySCs.

Examples:

 - [Eq a] is implied by [Ord a]
 - [Ord a] is not implied by [Eq a],
 - any context is implied by itself,
 - the empty context is implied by any context.

Note [Use only the best matching quantified constraint]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#20582) the ambiguity check for
  (forall a. Ord (m a), forall a. Semigroup a => Eq (m a)) => m Int

Because of eager expansion of given superclasses, we get
  [G] d1: forall a. Ord (m a)
  [G] d2: forall a. Eq (m a)
  [G] d3: forall a. Semigroup a => Eq (m a)

  [W] {w1}: forall a. Ord (m a)
  [W] {w2}: forall a. Semigroup a => Eq (m a)

The first wanted is solved straightforwardly. But the second wanted
matches *two* local instances: d2 and d3. Our general rule around multiple local
instances is that we refuse to commit to any of them. However, that
means that our type fails the ambiguity check. That's bad: the type
is perfectly fine. (This actually came up in the wild, in the streamly
library.)

The solution is to prefer local instances which are easier to prove, meaning
that they have a weaker precondition. In this case, the empty context
of d2 is a weaker constraint than the "Semigroup a" context of d3, so we prefer
using it when proving w2. This allows us to pass the ambiguity check here.

Our criterion for solving a Wanted by matching local quantified instances is
thus as follows:

  - There is a matching local quantified instance that dominates all others
    matches, in the sense of [When does a quantified instance dominate another?].
    Any such match do, we pick it arbitrarily (the T22223 example below says why).
  - This local quantified instance also dominates all the unifiers, as we
    wouldn't want to commit to a single match when we might have multiple,
    genuinely different matches after further unification takes place.

Some other examples:


  #15244:

    f :: (C g, D g) => ....
    class S g => C g where ...
    class S g => D g where ...
    class (forall a. Eq a => Eq (g a)) => S g where ...

  Here, in f's RHS, there are two identical quantified constraints
  available, one via the superclasses of C and one via the superclasses
  of D. Given that each implies the other, we pick one arbitrarily.


  #22216:

    class Eq a
    class Eq a => Ord a
    class (forall b. Eq b => Eq (f b)) => Eq1 f
    class (Eq1 f, forall b. Ord b => Ord (f b)) => Ord1 f

  Suppose we have

    [G] d1: Ord1 f
    [G] d2: Eq a
    [W] {w}: Eq (f a)

  Superclass expansion of d1 gives us:

    [G] d3 : Eq1 f
    [G] d4 : forall b. Ord b => Ord (f b)

  expanding d4 and d5 gives us, respectively:

    [G] d5 : forall b. Eq  b => Eq (f b)
    [G] d6 : forall b. Ord b => Eq (f b)

  Now we have two matching local instances that we could use when solving the
  Wanted. However, it's obviously silly to use d6, given that d5 provides us with
  as much information, with a strictly weaker precondition. So we pick d5 to solve
  w. If we chose d6, we would get [W] Ord a, which in this case we can't solve.


  #22223:

    [G] forall a b. (Eq a, Ord b) => C a b
    [G] forall a b. (Ord b, Eq a) => C a b
    [W] C x y

  Here we should be free to pick either quantified constraint, as they are
  equivalent up to re-ordering of the constraints in the context.
  See also Note [Do not add duplicate quantified instances]
  in GHC.Tc.Solver.Monad.

Test cases:
  typecheck/should_compile/T20582
  quantified-constraints/T15244
  quantified-constraints/T22216{a,b,c,d,e}
  quantified-constraints/T22223

Historical note: a previous solution was to instead pick the local instance
with the least superclass depth (see Note [Replacement vs keeping]),
but that doesn't work for the example from #22216.
-}

{- *******************************************************************
*                                                                    *
         Last resort prohibited superclass
*                                                                    *
**********************************************************************-}

tryLastResortProhibitedSuperClass :: DictCt -> SolverStage ()
-- ^ As a last resort, we TEMPORARILY allow a prohibited superclass solve,
-- emitting a loud warning when doing so: we might be creating non-terminating
-- evidence (as we are in T22912 for example).
--
-- See Note [Migrating away from loopy superclass solving] in GHC.Tc.TyCl.Instance.
tryLastResortProhibitedSuperClass :: DictCt -> SolverStage ()
tryLastResortProhibitedSuperClass DictCt
dict_ct
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$ do { InertCans
inerts <- TcS InertCans
getInertCans
               ; InertCans -> DictCt -> TcS (StopOrContinue ())
last_resort InertCans
inerts DictCt
dict_ct }

last_resort :: InertCans -> DictCt -> TcS (StopOrContinue ())
last_resort :: InertCans -> DictCt -> TcS (StopOrContinue ())
last_resort InertCans
inerts (DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev_w, di_cls :: DictCt -> Class
di_cls = Class
cls, di_tys :: DictCt -> [Type]
di_tys = [Type]
xis })
  | let loc_w :: CtLoc
loc_w  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
        orig_w :: CtOrigin
orig_w = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc_w
  , ScOrigin ClsInstOrQC
_ NakedScFlag
NakedSc <- CtOrigin
orig_w   -- work_item is definitely Wanted
  , Just DictCt
ct_i <- InertCans -> CtLoc -> Class -> [Type] -> Maybe DictCt
lookupInertDict InertCans
inerts CtLoc
loc_w Class
cls [Type]
xis
  , let ev_i :: CtEvidence
ev_i = DictCt -> CtEvidence
dictCtEvidence DictCt
ct_i
  , CtEvidence -> Bool
isGiven CtEvidence
ev_i
  = do { CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_w Bool
True (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev_i)
       ; CtLoc -> TcRnMessage -> TcS ()
ctLocWarnTcS CtLoc
loc_w (TcRnMessage -> TcS ()) -> TcRnMessage -> TcS ()
forall a b. (a -> b) -> a -> b
$
         CtLoc -> Type -> TcRnMessage
TcRnLoopySuperclassSolve CtLoc
loc_w (CtEvidence -> Type
ctEvPred CtEvidence
ev_w)
       ; StopOrContinue () -> TcS (StopOrContinue ())
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (StopOrContinue () -> TcS (StopOrContinue ()))
-> StopOrContinue () -> TcS (StopOrContinue ())
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc -> StopOrContinue ()
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev_w (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Loopy superclass") }
  | Bool
otherwise
  = () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()


{- *********************************************************************
*                                                                      *
*          Functional dependencies, instantiation of equations
*                                                                      *
************************************************************************

When we spot an equality arising from a functional dependency,
we now use that equality (a "wanted") to rewrite the work-item
constraint right away.  This avoids two dangers

 Danger 1: If we send the original constraint on down the pipeline
           it may react with an instance declaration, and in delicate
           situations (when a Given overlaps with an instance) that
           may produce new insoluble goals: see #4952

 Danger 2: If we don't rewrite the constraint, it may re-react
           with the same thing later, and produce the same equality
           again --> termination worries.

To achieve this required some refactoring of GHC.Tc.Instance.FunDeps (nicer
now!).

Note [FunDep and implicit parameter reactions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Currently, our story of interacting two dictionaries (or a dictionary
and top-level instances) for functional dependencies, and implicit
parameters, is that we simply produce new Wanted equalities.  So for example

        class D a b | a -> b where ...
    Inert:
        d1 :g D Int Bool
    WorkItem:
        d2 :w D Int alpha

    We generate the extra work item
        cv :w alpha ~ Bool
    where 'cv' is currently unused.  However, this new item can perhaps be
    spontaneously solved to become given and react with d2,
    discharging it in favour of a new constraint d2' thus:
        d2' :w D Int Bool
        d2 := d2' |> D Int cv
    Now d2' can be discharged from d1

We could be more aggressive and try to *immediately* solve the dictionary
using those extra equalities.

If that were the case with the same inert set and work item we might discard
d2 directly:

        cv :w alpha ~ Bool
        d2 := d1 |> D Int cv

But in general it's a bit painful to figure out the necessary coercion,
so we just take the first approach. Here is a better example. Consider:
    class C a b c | a -> b
And:
     [Given]  d1 : C T Int Char
     [Wanted] d2 : C T beta Int
In this case, it's *not even possible* to solve the wanted immediately.
So we should simply output the functional dependency and add this guy
[but NOT its superclasses] back in the worklist. Even worse:
     [Given] d1 : C T Int beta
     [Wanted] d2: C T beta Int
Then it is solvable, but its very hard to detect this on the spot.

It's exactly the same with implicit parameters, except that the
"aggressive" approach would be much easier to implement.

Note [Fundeps with instances, and equality orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This Note describes a delicate interaction that constrains the orientation of
equalities. This one is about fundeps, but the /exact/ same thing arises for
type-family injectivity constraints: see Note [Improvement orientation].

doTopFunDepImprovement compares the constraint with all the instance
declarations, to see if we can produce any equalities. E.g
   class C2 a b | a -> b
   instance C Int Bool
Then the constraint (C Int ty) generates the equality [W] ty ~ Bool.

There is a nasty corner in #19415 which led to the typechecker looping:
   class C s t b | s -> t
   instance ... => C (T kx x) (T ky y) Int
   T :: forall k. k -> Type

   work_item: dwrk :: C (T @ka (a::ka)) (T @kb0 (b0::kb0)) Char
      where kb0, b0 are unification vars

   ==> {doTopFunDepImprovement: compare work_item with instance,
        generate /fresh/ unification variables kfresh0, yfresh0,
        emit a new Wanted, and add dwrk to inert set}

   Suppose we emit this new Wanted from the fundep:
       [W] T kb0 (b0::kb0) ~ T kfresh0 (yfresh0::kfresh0)

   ==> {solve that equality kb0 := kfresh0, b0 := yfresh0}
   Now kick out dwrk, since it mentions kb0
   But now we are back to the start!  Loop!

NB1: This example relies on an instance that does not satisfy the
     coverage condition (although it may satisfy the weak coverage
     condition), and hence whose fundeps generate fresh unification
     variables.  Not satisfying the coverage condition is known to
     lead to termination trouble, but in this case it's plain silly.

NB2: In this example, the third parameter to C ensures that the
     instance doesn't actually match the Wanted, so we can't use it to
     solve the Wanted

We solve the problem by (#21703):

    carefully orienting the new Wanted so that all the
    freshly-generated unification variables are on the LHS.

    Thus we call unifyWanteds on
       T kfresh0 (yfresh0::kfresh0) ~ T kb0 (b0::kb0)
    and /NOT/
       T kb0 (b0::kb0) ~ T kfresh0 (yfresh0::kfresh0)

Now we'll unify kfresh0:=kb0, yfresh0:=b0, and all is well.  The general idea
is that we want to preferentially eliminate those freshly-generated
unification variables, rather than unifying older variables, which causes
kick-out etc.

Keeping younger variables on the left also gives very minor improvement in
the compiler performance by having less kick-outs and allocations (-0.1% on
average).  Indeed Historical Note [Eliminate younger unification variables]
in GHC.Tc.Utils.Unify describes an earlier attempt to do so systematically,
apparently now in abeyance.

But this is is a delicate solution. We must take care to /preserve/
orientation during solving. Wrinkles:

(W1) We start with
       [W] T kfresh0 (yfresh0::kfresh0) ~ T kb0 (b0::kb0)
     Decompose to
       [W] kfresh0 ~ kb0
       [W] (yfresh0::kfresh0) ~ (b0::kb0)
     Preserve orientiation when decomposing!!

(W2) Suppose we happen to tackle the second Wanted from (W1)
     first. Then in canEqCanLHSHetero we emit a /kind/ equality, as
     well as a now-homogeneous type equality
       [W] kco : kfresh0 ~ kb0
       [W] (yfresh0::kfresh0) ~ (b0::kb0) |> (sym kco)
     Preserve orientation in canEqCanLHSHetero!!  (Failing to
     preserve orientation here was the immediate cause of #21703.)

(W3) There is a potential interaction with the swapping done by
     GHC.Tc.Utils.Unify.swapOverTyVars.  We think it's fine, but it's
     a slight worry.  See especially Note [TyVar/TyVar orientation] in
     that module.

The trouble is that "preserving orientation" is a rather global invariant,
and sometimes we definitely do want to swap (e.g. Int ~ alpha), so we don't
even have a precise statement of what the invariant is.  The advantage
of the preserve-orientation plan is that it is extremely cheap to implement,
and apparently works beautifully.

--- Alternative plan (1) ---
Rather than have an ill-defined invariant, another possiblity is to
elminate those fresh unification variables at birth, when generating
the new fundep-inspired equalities.

The key idea is to call `instFlexiX` in `emitFunDepWanteds` on only those
type variables that are guaranteed to give us some progress. This means we
have to locally (without calling emitWanteds) identify the type variables
that do not give us any progress.  In the above example, we _know_ that
emitting the two wanteds `kco` and `co` is fruitless.

  Q: How do we identify such no-ops?

  1. Generate a matching substitution from LHS to RHS
        ɸ = [kb0 :-> k0, b0 :->  y0]
  2. Call `instFlexiX` on only those type variables that do not appear in the domain of ɸ
        ɸ' = instFlexiX ɸ (tvs - domain ɸ)
  3. Apply ɸ' on LHS and then call emitWanteds
        unifyWanteds ... (subst ɸ' LHS) RHS

Why will this work?  The matching substitution ɸ will be a best effort
substitution that gives us all the easy solutions. It can be generated with
modified version of `Core/Unify.unify_tys` where we run it in a matching mode
and never generate `SurelyApart` and always return a `MaybeApart Subst`
instead.

The same alternative plan would work for type-family injectivity constraints:
see Note [Improvement orientation] in GHC.Tc.Solver.Equality.
--- End of Alternative plan (1) ---

--- Alternative plan (2) ---
We could have a new flavour of TcTyVar (like `TauTv`, `TyVarTv` etc; see GHC.Tc.Utils.TcType.MetaInfo)
for the fresh unification variables introduced by functional dependencies.  Say `FunDepTv`.  Then in
GHC.Tc.Utils.Unify.swapOverTyVars we could arrange to keep a `FunDepTv` on the left if possible.
Looks possible, but it's one more complication.
--- End of Alternative plan (2) ---


--- Historical note: Failed Alternative Plan (3) ---
Previously we used a flag `cc_fundeps` in `CDictCan`. It would flip to False
once we used a fun dep to hint the solver to break and to stop emitting more
wanteds.  This solution was not complete, and caused a failures while trying
to solve for transitive functional dependencies (test case: T21703)
-- End of Historical note: Failed Alternative Plan (3) --

Note [Do fundeps last]
~~~~~~~~~~~~~~~~~~~~~~
Consider T4254b:
  class FD a b | a -> b where { op :: a -> b }

  instance FD Int Bool

  foo :: forall a b. (a~Int,FD a b) => a -> Bool
  foo = op

(DFL1) Try local fundeps first.
  From the ambiguity check on the type signature we get
    [G] FD Int b
    [W] FD Int beta
  Interacting these gives beta:=b; then we start again and solve without
  trying fundeps between the new [W] FD Int b and the top-level instance.
  If we did, we'd generate [W] b ~ Bool, which fails.

(DFL2) Try solving from top-level instances before fundeps
  From the definition `foo = op` we get
    [G] FD Int b
    [W] FD Int Bool
  We solve this from the top level instance before even trying fundeps.
  If we did try fundeps, we'd generate [W] b ~ Bool, which fails.


Note [Weird fundeps]
~~~~~~~~~~~~~~~~~~~~
Consider   class Het a b | a -> b where
              het :: m (f c) -> a -> m b

           class GHet (a :: * -> *) (b :: * -> *) | a -> b
           instance            GHet (K a) (K [a])
           instance Het a b => GHet (K a) (K b)

The two instances don't actually conflict on their fundeps,
although it's pretty strange.  So they are both accepted. Now
try   [W] GHet (K Int) (K Bool)
This triggers fundeps from both instance decls;
      [W] K Bool ~ K [a]
      [W] K Bool ~ K beta
And there's a risk of complaining about Bool ~ [a].  But in fact
the Wanted matches the second instance, so we never get as far
as the fundeps.

#7875 is a case in point.
-}

doLocalFunDepImprovement :: DictCt -> SolverStage ()
-- Add wanted constraints from type-class functional dependencies.
doLocalFunDepImprovement :: DictCt -> SolverStage ()
doLocalFunDepImprovement dict_ct :: DictCt
dict_ct@(DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
work_ev, di_cls :: DictCt -> Class
di_cls = Class
cls })
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$
    do { InertCans
inerts <- TcS InertCans
getInertCans
       ; Bool
imp <- (Bool -> DictCt -> TcS Bool) -> Bool -> Bag DictCt -> TcS Bool
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM Bool -> DictCt -> TcS Bool
add_fds Bool
False (DictMap DictCt -> Class -> Bag DictCt
forall a. DictMap a -> Class -> Bag a
findDictsByClass (InertCans -> DictMap DictCt
inert_dicts InertCans
inerts) Class
cls)
       ; if Bool
imp then Ct -> TcS (StopOrContinue ())
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (DictCt -> Ct
CDictCan DictCt
dict_ct)
                     else () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }
  where
    work_pred :: Type
work_pred = CtEvidence -> Type
ctEvPred CtEvidence
work_ev
    work_loc :: CtLoc
work_loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
work_ev

    add_fds :: Bool -> DictCt -> TcS Bool
    add_fds :: Bool -> DictCt -> TcS Bool
add_fds Bool
so_far (DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
inert_ev })
      | CtEvidence -> Bool
isGiven CtEvidence
work_ev Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
inert_ev
        -- Do not create FDs from Given/Given interactions: See Note [No Given/Given fundeps]
      = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
so_far
      | Bool
otherwise
      = do { String -> SDoc -> TcS ()
traceTcS String
"doLocalFunDepImprovement" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat
                [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
work_ev
                , CtLoc -> SDoc
pprCtLoc CtLoc
work_loc, Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
work_loc)
                , CtLoc -> SDoc
pprCtLoc CtLoc
inert_loc, Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
inert_loc)
                , CtLoc -> SDoc
pprCtLoc CtLoc
derived_loc, Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
derived_loc) ])

           ; Bool
unifs <- CtEvidence -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS Bool
emitFunDepWanteds CtEvidence
work_ev ([FunDepEqn (CtLoc, RewriterSet)] -> TcS Bool)
-> [FunDepEqn (CtLoc, RewriterSet)] -> TcS Bool
forall a b. (a -> b) -> a -> b
$
                      (CtLoc, RewriterSet)
-> Type -> Type -> [FunDepEqn (CtLoc, RewriterSet)]
forall loc. loc -> Type -> Type -> [FunDepEqn loc]
improveFromAnother (CtLoc
derived_loc, RewriterSet
inert_rewriters)
                                         Type
inert_pred Type
work_pred
           ; Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bool
so_far Bool -> Bool -> Bool
|| Bool
unifs)
        }
      where
        inert_pred :: Type
inert_pred = CtEvidence -> Type
ctEvPred CtEvidence
inert_ev
        inert_loc :: CtLoc
inert_loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
inert_ev
        inert_rewriters :: RewriterSet
inert_rewriters = CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
inert_ev
        derived_loc :: CtLoc
derived_loc = CtLoc
work_loc { ctl_depth  = ctl_depth work_loc `maxSubGoalDepth`
                                              ctl_depth inert_loc
                               , ctl_origin = FunDepOrigin1 work_pred
                                                            (ctLocOrigin work_loc)
                                                            (ctLocSpan work_loc)
                                                            inert_pred
                                                            (ctLocOrigin inert_loc)
                                                            (ctLocSpan inert_loc) }

doTopFunDepImprovement :: DictCt -> SolverStage ()
-- Try to functional-dependency improvement between the constraint
-- and the top-level instance declarations
-- See Note [Fundeps with instances, and equality orientation]
-- See also Note [Weird fundeps]
doTopFunDepImprovement :: DictCt -> SolverStage ()
doTopFunDepImprovement dict_ct :: DictCt
dict_ct@(DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev, di_cls :: DictCt -> Class
di_cls = Class
cls, di_tys :: DictCt -> [Type]
di_tys = [Type]
xis })
  | CtEvidence -> Bool
isGiven CtEvidence
ev     -- No improvement for Givens
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$ () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()
  | Bool
otherwise
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcS ()
traceTcS String
"try_fundeps" (DictCt -> SDoc
forall a. Outputable a => a -> SDoc
ppr DictCt
dict_ct)
       ; InstEnvs
instEnvs <- TcS InstEnvs
getInstEnvs
       ; let fundep_eqns :: [FunDepEqn (CtLoc, RewriterSet)]
fundep_eqns = InstEnvs
-> (Type -> SrcSpan -> (CtLoc, RewriterSet))
-> Class
-> [Type]
-> [FunDepEqn (CtLoc, RewriterSet)]
forall loc.
InstEnvs
-> (Type -> SrcSpan -> loc) -> Class -> [Type] -> [FunDepEqn loc]
improveFromInstEnv InstEnvs
instEnvs Type -> SrcSpan -> (CtLoc, RewriterSet)
mk_ct_loc Class
cls [Type]
xis
       ; Bool
imp <- CtEvidence -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS Bool
emitFunDepWanteds CtEvidence
ev [FunDepEqn (CtLoc, RewriterSet)]
fundep_eqns
       ; if Bool
imp then Ct -> TcS (StopOrContinue ())
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (DictCt -> Ct
CDictCan DictCt
dict_ct)
                     else () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }
  where
     dict_pred :: Type
dict_pred   = Class -> [Type] -> Type
mkClassPred Class
cls [Type]
xis
     dict_loc :: CtLoc
dict_loc    = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
     dict_origin :: CtOrigin
dict_origin = CtLoc -> CtOrigin
ctLocOrigin CtLoc
dict_loc

     mk_ct_loc :: PredType   -- From instance decl
               -> SrcSpan    -- also from instance deol
               -> (CtLoc, RewriterSet)
     mk_ct_loc :: Type -> SrcSpan -> (CtLoc, RewriterSet)
mk_ct_loc Type
inst_pred SrcSpan
inst_loc
       = ( CtLoc
dict_loc { ctl_origin = FunDepOrigin2 dict_pred dict_origin
                                                 inst_pred inst_loc }
         , RewriterSet
emptyRewriterSet )


{- *********************************************************************
*                                                                      *
*                      Superclasses
*                                                                      *
********************************************************************* -}

{- 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 field 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 > 0
(i.e. isPendingScDict holds).
See Note [Expanding Recursive Superclasses and ExpansionFuel]

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 -XNoUndecidableSuperClasses
  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 cc_pend_sc has pending
expansion fuel.)
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.InertSet.  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.

See Note [Instance and Given overlap].

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 hence 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]
-- 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 constraint's superclass will consume a unit of fuel
-- Preconditions on `cts`: 1. They are either `CDictCan` or `CQuantCan`
--                         2. Their fuel (stored in cc_pend_sc or qci_pend_sc) is > 0
makeSuperClasses :: [Ct] -> TcS [Ct]
makeSuperClasses [Ct]
cts = (Ct -> TcS [Ct]) -> [Ct] -> TcS [Ct]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f a -> m [b]
concatMapM Ct -> TcS [Ct]
go [Ct]
cts
  where
    go :: Ct -> TcS [Ct]
go (CDictCan (DictCt { di_ev :: DictCt -> CtEvidence
di_ev = CtEvidence
ev, di_cls :: DictCt -> Class
di_cls = Class
cls, di_tys :: DictCt -> [Type]
di_tys = [Type]
tys, di_pend_sc :: DictCt -> ScDepth
di_pend_sc = ScDepth
fuel }))
      = ScDepth -> TcS [Ct] -> TcS [Ct]
forall a. ScDepth -> a -> a
assertFuelPreconditionStrict ScDepth
fuel (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$ -- fuel needs to be more than 0 always
        ScDepth
-> CtEvidence -> [EvVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses ScDepth
fuel 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, qci_pend_sc :: QCInst -> ScDepth
qci_pend_sc = ScDepth
fuel }))
      = Bool -> SDoc -> TcS [Ct] -> TcS [Ct]
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (Type -> Bool
isClassPred Type
pred) (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
pred) (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$  -- The cts should all have
                                                   -- class pred heads
        ScDepth -> TcS [Ct] -> TcS [Ct]
forall a. ScDepth -> a -> a
assertFuelPreconditionStrict ScDepth
fuel (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$ -- fuel needs to be more than 0 always
        ScDepth
-> CtEvidence -> [EvVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses ScDepth
fuel CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys
      where
        ([EvVar]
tvs, [Type]
theta, Class
cls, [Type]
tys) = Type -> ([EvVar], [Type], Class, [Type])
tcSplitDFunTy (CtEvidence -> Type
ctEvPred CtEvidence
ev)
    go Ct
ct = String -> SDoc -> TcS [Ct]
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"makeSuperClasses" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct)

mkStrictSuperClasses
    :: ExpansionFuel -> 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
-- Precondition: fuel > 0
-- Postcondition: fuel for recursive superclass ct is one unit less than cls fuel
mkStrictSuperClasses :: ScDepth
-> CtEvidence -> [EvVar] -> [Type] -> Class -> [Type] -> TcS [Ct]
mkStrictSuperClasses ScDepth
fuel CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys
  = ScDepth
-> NameSet
-> CtEvidence
-> [EvVar]
-> [Type]
-> Class
-> [Type]
-> TcS [Ct]
mk_strict_superclasses (ScDepth -> ScDepth
consumeFuel ScDepth
fuel) (Name -> NameSet
unitNameSet (Class -> Name
className Class
cls))
                           CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys

mk_strict_superclasses :: ExpansionFuel -> 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
-- The caller of this function is supposed to perform fuel book keeping
-- Precondition: fuel >= 0
mk_strict_superclasses :: ScDepth
-> NameSet
-> CtEvidence
-> [EvVar]
-> [Type]
-> Class
-> [Type]
-> TcS [Ct]
mk_strict_superclasses ScDepth
_ NameSet
_ CtEvidence
_ [EvVar]
_ [Type]
_ Class
cls [Type]
_
  | Class -> Bool
isEqualityClass Class
cls
  = [Ct] -> TcS [Ct]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return []

mk_strict_superclasses ScDepth
fuel NameSet
rec_clss
                       ev :: CtEvidence
ev@(CtGiven { ctev_evar :: CtEvidence -> EvVar
ctev_evar = EvVar
evar, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc })
                       [EvVar]
tvs [Type]
theta Class
cls [Type]
tys
  = -- Given case
    do { String -> SDoc -> TcS ()
traceTcS String
"mk_strict" (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ CtOrigin -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc))
       ; (EvVar -> TcS [Ct]) -> [EvVar] -> TcS [Ct]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f a -> m [b]
concatMapM EvVar -> TcS [Ct]
do_one_given (Class -> [EvVar]
classSCSelIds Class
cls) }
  where
    dict_ids :: [EvVar]
dict_ids  = [Type] -> [EvVar]
mkTemplateLocals [Type]
theta
    this_size :: PatersonSize
this_size = Class -> [Type] -> PatersonSize
pSizeClassPred Class
cls [Type]
tys

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

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

        all_tvs :: [EvVar]
all_tvs       = [EvVar]
tvs [EvVar] -> [EvVar] -> [EvVar]
forall a. [a] -> [a] -> [a]
`chkAppend` [EvVar]
sc_tvs
        all_theta :: [Type]
all_theta     = [Type]
theta [Type] -> [Type] -> [Type]
forall a. [a] -> [a] -> [a]
`chkAppend` ((Scaled Type -> Type) -> [Scaled Type] -> [Type]
forall a b. (a -> b) -> [a] -> [b]
map Scaled Type -> Type
forall a. Scaled a -> a
scaledThing [Scaled Type]
sc_theta)
        swizzled_pred :: Type
swizzled_pred = [EvVar] -> [Type] -> Type -> Type
HasDebugCallStack => [EvVar] -> [Type] -> Type -> Type
mkInfSigmaTy [EvVar]
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 (EvExpr -> EvTerm) -> EvExpr -> EvTerm
forall a b. (a -> b) -> a -> b
$
          [EvVar] -> EvExpr -> EvExpr
forall b. [b] -> Expr b -> Expr b
mkLams [EvVar]
all_tvs (EvExpr -> EvExpr) -> EvExpr -> EvExpr
forall a b. (a -> b) -> a -> b
$
          [EvVar] -> EvExpr -> EvExpr
forall b. [b] -> Expr b -> Expr b
mkLams [EvVar]
dict_ids (EvExpr -> EvExpr) -> EvExpr -> EvExpr
forall a b. (a -> b) -> a -> b
$
          EvVar -> EvExpr
forall b. EvVar -> Expr b
Var EvVar
sel_id
            EvExpr -> [Type] -> EvExpr
forall b. Expr b -> [Type] -> Expr b
`mkTyApps` [Type]
tys
            EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App` (EvVar -> EvExpr
evId EvVar
evar EvExpr -> [EvVar] -> EvExpr
forall b. Expr b -> [EvVar] -> Expr b
`mkVarApps` ([EvVar]
tvs [EvVar] -> [EvVar] -> [EvVar]
forall a. [a] -> [a] -> [a]
++ [EvVar]
dict_ids))
            EvExpr -> [EvVar] -> EvExpr
forall b. Expr b -> [EvVar] -> Expr b
`mkVarApps` [EvVar]
sc_tvs

    sc_loc :: CtLoc
sc_loc | Class -> Bool
isCTupleClass Class
cls = CtLoc
loc
           | Bool
otherwise         = CtLoc
loc { ctl_origin = mk_sc_origin (ctLocOrigin loc) }
           -- isCTupleClass: we don't want tuples to mess up the size calculations
           -- of Note [Solving superclass constraints]. For tuple predicates, this
           -- matters, because their size can be large, and we don't want to add a
           -- big class to the size of the dictionaries in the chain. When we get
           -- down to a base predicate, we'll include its size. See #10335.
           -- See Note [Solving tuple constraints]

    -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
    -- for explanation of GivenSCOrigin and Note [Replacement vs keeping] in
    -- GHC.Tc.Solver.InertSet for why we need depths
    mk_sc_origin :: CtOrigin -> CtOrigin
    mk_sc_origin :: CtOrigin -> CtOrigin
mk_sc_origin (GivenSCOrigin SkolemInfoAnon
skol_info ScDepth
sc_depth Bool
already_blocked)
      = SkolemInfoAnon -> ScDepth -> Bool -> CtOrigin
GivenSCOrigin SkolemInfoAnon
skol_info (ScDepth
sc_depth ScDepth -> ScDepth -> ScDepth
forall a. Num a => a -> a -> a
+ ScDepth
1)
                      (Bool
already_blocked Bool -> Bool -> Bool
|| SkolemInfoAnon -> Bool
newly_blocked SkolemInfoAnon
skol_info)

    mk_sc_origin (GivenOrigin SkolemInfoAnon
skol_info)
      = -- These cases do not already have a superclass constraint: depth starts at 1
        SkolemInfoAnon -> ScDepth -> Bool -> CtOrigin
GivenSCOrigin SkolemInfoAnon
skol_info ScDepth
1 (SkolemInfoAnon -> Bool
newly_blocked SkolemInfoAnon
skol_info)

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

    newly_blocked :: SkolemInfoAnon -> Bool
newly_blocked (InstSkol ClsInstOrQC
_ PatersonSize
head_size) = Maybe PatersonCondFailure -> Bool
forall a. Maybe a -> Bool
isJust (PatersonSize
this_size PatersonSize -> PatersonSize -> Maybe PatersonCondFailure
`ltPatersonSize` PatersonSize
head_size)
    newly_blocked SkolemInfoAnon
_                      = Bool
False

-- Wanted case
mk_strict_superclasses ScDepth
fuel NameSet
rec_clss CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys
  | (Type -> Bool) -> [Type] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all Type -> Bool
noFreeVarsOfType [Type]
tys
  = [Ct] -> TcS [Ct]
forall a. a -> TcS a
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.
  = Bool -> SDoc -> TcS [Ct] -> TcS [Ct]
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr ([EvVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [EvVar]
tvs Bool -> Bool -> Bool
&& [Type] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Type]
theta) ([EvVar] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [EvVar]
tvs SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
theta) (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$
    (Type -> TcS [Ct]) -> [Type] -> TcS [Ct]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f 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" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys) SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> 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
           ; ScDepth
-> NameSet -> CtEvidence -> [EvVar] -> [Type] -> Type -> TcS [Ct]
mk_superclasses ScDepth
fuel 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 :: ExpansionFuel -> NameSet -> CtEvidence
                -> [TyVar] -> ThetaType -> PredType -> TcS [Ct]
-- Return this constraint, plus its superclasses, if any
-- Precondition: fuel >= 0
mk_superclasses :: ScDepth
-> NameSet -> CtEvidence -> [EvVar] -> [Type] -> Type -> TcS [Ct]
mk_superclasses ScDepth
fuel NameSet
rec_clss CtEvidence
ev [EvVar]
tvs [Type]
theta Type
pred
  | ClassPred Class
cls [Type]
tys <- Type -> Pred
classifyPredType Type
pred
  = ScDepth -> TcS [Ct] -> TcS [Ct]
forall a. ScDepth -> a -> a
assertFuelPrecondition ScDepth
fuel (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$
    ScDepth
-> NameSet
-> CtEvidence
-> [EvVar]
-> [Type]
-> Class
-> [Type]
-> TcS [Ct]
mk_superclasses_of ScDepth
fuel NameSet
rec_clss CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys

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

mk_superclasses_of :: ExpansionFuel -> NameSet -> CtEvidence
                   -> [TyVar] -> ThetaType -> Class -> [Type]
                   -> TcS [Ct]
-- Always return this class constraint,
-- and expand its superclasses
-- Precondition: fuel >= 0
mk_superclasses_of :: ScDepth
-> NameSet
-> CtEvidence
-> [EvVar]
-> [Type]
-> Class
-> [Type]
-> TcS [Ct]
mk_superclasses_of ScDepth
fuel NameSet
rec_clss CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys
  | Bool
loop_found = do { String -> SDoc -> TcS ()
traceTcS String
"mk_superclasses_of: loop" (Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys)
                    ; ScDepth -> TcS [Ct] -> TcS [Ct]
forall a. ScDepth -> a -> a
assertFuelPrecondition ScDepth
fuel (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$ [Ct] -> TcS [Ct]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return [ScDepth -> Ct
mk_this_ct ScDepth
fuel] }
                                                  -- cc_pend_sc of returning ct = fuel
  | Bool
otherwise  = do { String -> SDoc -> TcS ()
traceTcS String
"mk_superclasses_of" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys
                                                          , Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> Bool
isCTupleClass Class
cls)
                                                          , NameSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr NameSet
rec_clss
                                                          ])
                    ; [Ct]
sc_cts <- ScDepth -> TcS [Ct] -> TcS [Ct]
forall a. ScDepth -> a -> a
assertFuelPrecondition ScDepth
fuel (TcS [Ct] -> TcS [Ct]) -> TcS [Ct] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$
                                ScDepth
-> NameSet
-> CtEvidence
-> [EvVar]
-> [Type]
-> Class
-> [Type]
-> TcS [Ct]
mk_strict_superclasses ScDepth
fuel NameSet
rec_clss' CtEvidence
ev [EvVar]
tvs [Type]
theta Class
cls [Type]
tys
                    ; [Ct] -> TcS [Ct]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (ScDepth -> Ct
mk_this_ct ScDepth
doNotExpand Ct -> [Ct] -> [Ct]
forall a. a -> [a] -> [a]
: [Ct]
sc_cts) }
                                      -- doNotExpand: we have expanded this cls's superclasses, so
                                      -- exhaust the associated constraint's fuel,
                                      -- to avoid duplicate work
  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

    mk_this_ct :: ExpansionFuel -> Ct
    -- We can't use CNonCanonical here because we need to tradk the fuel
    mk_this_ct :: ScDepth -> Ct
mk_this_ct ScDepth
fuel | [EvVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [EvVar]
tvs, [Type] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Type]
theta
                    = DictCt -> Ct
CDictCan (DictCt { di_ev :: CtEvidence
di_ev = CtEvidence
ev, di_cls :: Class
di_cls = Class
cls
                                       , di_tys :: [Type]
di_tys = [Type]
tys, di_pend_sc :: ScDepth
di_pend_sc = ScDepth
fuel })
                    -- NB: If there is a loop, we cut off, so we have not
                    --     added the superclasses, hence cc_pend_sc = fuel
                    | Bool
otherwise
                    = QCInst -> Ct
CQuantCan (QCI { qci_tvs :: [EvVar]
qci_tvs = [EvVar]
tvs, qci_pred :: Type
qci_pred = Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys
                                     , qci_ev :: CtEvidence
qci_ev = CtEvidence
ev, qci_pend_sc :: ScDepth
qci_pend_sc = ScDepth
fuel })


{- 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.Equality.tryQCsEqCt.

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.Equality.tryQCsEqCt
which looks for primitive equalities specially in the quantified
constraints.

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