{-# LANGUAGE CPP                 #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections       #-}
{-# LANGUAGE BlockArguments      #-}
{-# LANGUAGE RecursiveDo         #-}

{-# OPTIONS_GHC -Wno-incomplete-uni-patterns   #-}
{-# OPTIONS_GHC -Wno-incomplete-record-updates #-}

{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
-}

-- | Type subsumption and unification
module GHC.Tc.Utils.Unify (
  -- Full-blown subsumption
  tcWrapResult, tcWrapResultO, tcWrapResultMono,
  tcTopSkolemise, tcSkolemiseScoped, tcSkolemiseExpType,
  tcSubType, tcSubTypeSigma, tcSubTypePat, tcSubTypeDS,
  tcSubTypeAmbiguity, tcSubMult,
  checkConstraints, checkTvConstraints,
  buildImplicationFor, buildTvImplication, emitResidualTvConstraint,

  -- Various unifications
  unifyType, unifyKind, unifyExpectedType,
  uType, promoteTcType,
  swapOverTyVars, canSolveByUnification,

  --------------------------------
  -- Holes
  tcInfer,
  matchExpectedListTy,
  matchExpectedTyConApp,
  matchExpectedAppTy,
  matchExpectedFunTys,
  matchExpectedFunKind,
  matchActualFunTySigma, matchActualFunTysRho,

  checkTyVarEq, checkTyFamEq, checkTypeEq

  ) where

#include "GhclibHsVersions.h"

import GHC.Prelude

import GHC.Hs
import GHC.Core.TyCo.Rep
import GHC.Core.TyCo.Ppr( debugPprType )
import GHC.Tc.Utils.TcMType
import GHC.Tc.Utils.Monad
import GHC.Tc.Utils.TcType
import GHC.Tc.Utils.Env
import GHC.Core.Type
import GHC.Core.Coercion
import GHC.Core.Multiplicity
import qualified GHC.LanguageExtensions as LangExt

import GHC.Tc.Types.Evidence
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Origin

import GHC.Tc.Utils.Instantiate
import GHC.Core.TyCon
import GHC.Builtin.Types
import GHC.Types.Name( Name, isSystemName )
import GHC.Types.Var as Var
import GHC.Types.Var.Set
import GHC.Types.Var.Env
import GHC.Utils.Error
import GHC.Driver.Session
import GHC.Types.Basic
import GHC.Data.Bag
import GHC.Data.FastString( fsLit )
import GHC.Utils.Misc
import GHC.Utils.Outputable as Outputable
import GHC.Utils.Panic

import GHC.Exts      ( inline )
import Control.Monad
import qualified Data.Semigroup as S ( (<>) )

{- *********************************************************************
*                                                                      *
              matchActualFunTys
*                                                                      *
********************************************************************* -}

-- | matchActualFunTySigma does looks for just one function arrow
--   returning an uninstantiated sigma-type
matchActualFunTySigma
  :: SDoc -- See Note [Herald for matchExpectedFunTys]
  -> Maybe SDoc                    -- The thing with type TcSigmaType
  -> (Arity, [Scaled TcSigmaType]) -- Total number of value args in the call, and
                                   -- types of values args to which function has
                                   --   been applied already (reversed)
                                   -- Both are used only for error messages)
  -> TcRhoType                     -- Type to analyse: a TcRhoType
  -> TcM (HsWrapper, Scaled TcSigmaType, TcSigmaType)
-- The /argument/ is a RhoType
-- The /result/   is an (uninstantiated) SigmaType
--
-- See Note [matchActualFunTy error handling] for the first three arguments

-- If   (wrap, arg_ty, res_ty) = matchActualFunTySigma ... fun_ty
-- then wrap :: fun_ty ~> (arg_ty -> res_ty)
-- and NB: res_ty is an (uninstantiated) SigmaType

matchActualFunTySigma :: SDoc
-> Maybe SDoc
-> (Int, [Scaled Type])
-> Type
-> TcM (HsWrapper, Scaled Type, Type)
matchActualFunTySigma SDoc
herald Maybe SDoc
mb_thing (Int, [Scaled Type])
err_info Type
fun_ty
  = ASSERT2( isRhoTy fun_ty, ppr fun_ty )
    Type -> TcM (HsWrapper, Scaled Type, Type)
go Type
fun_ty
  where
    -- Does not allocate unnecessary meta variables: if the input already is
    -- a function, we just take it apart.  Not only is this efficient,
    -- it's important for higher rank: the argument might be of form
    --              (forall a. ty) -> other
    -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
    -- hide the forall inside a meta-variable
    go :: TcRhoType   -- The type we're processing, perhaps after
                      -- expanding any type synonym
       -> TcM (HsWrapper, Scaled TcSigmaType, TcSigmaType)
    go :: Type -> TcM (HsWrapper, Scaled Type, Type)
go Type
ty | Just Type
ty' <- Type -> Maybe Type
tcView Type
ty = Type -> TcM (HsWrapper, Scaled Type, Type)
go Type
ty'

    go (FunTy { ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
af, ft_mult :: Type -> Type
ft_mult = Type
w, ft_arg :: Type -> Type
ft_arg = Type
arg_ty, ft_res :: Type -> Type
ft_res = Type
res_ty })
      = ASSERT( af == VisArg )
        forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, forall a. Type -> a -> Scaled a
Scaled Type
w Type
arg_ty, Type
res_ty)

    go ty :: Type
ty@(TyVarTy TcTyVar
tv)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
      = do { MetaDetails
cts <- TcTyVar -> TcM MetaDetails
readMetaTyVar TcTyVar
tv
           ; case MetaDetails
cts of
               Indirect Type
ty' -> Type -> TcM (HsWrapper, Scaled Type, Type)
go Type
ty'
               MetaDetails
Flexi        -> Type -> TcM (HsWrapper, Scaled Type, Type)
defer Type
ty }

       -- In all other cases we bale out into ordinary unification
       -- However unlike the meta-tyvar case, we are sure that the
       -- number of arguments doesn't match arity of the original
       -- type, so we can add a bit more context to the error message
       -- (cf #7869).
       --
       -- It is not always an error, because specialized type may have
       -- different arity, for example:
       --
       -- > f1 = f2 'a'
       -- > f2 :: Monad m => m Bool
       -- > f2 = undefined
       --
       -- But in that case we add specialized type into error context
       -- anyway, because it may be useful. See also #9605.
    go Type
ty = forall a. (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
addErrCtxtM (Type -> TidyEnv -> TcM (TidyEnv, SDoc)
mk_ctxt Type
ty) (Type -> TcM (HsWrapper, Scaled Type, Type)
defer Type
ty)

    ------------
    defer :: Type -> TcM (HsWrapper, Scaled Type, Type)
defer Type
fun_ty
      = do { Type
arg_ty <- TcM Type
newOpenFlexiTyVarTy
           ; Type
res_ty <- TcM Type
newOpenFlexiTyVarTy
           ; Type
mult <- Type -> TcM Type
newFlexiTyVarTy Type
multiplicityTy
           ; let unif_fun_ty :: Type
unif_fun_ty = Type -> Type -> Type -> Type
mkVisFunTy Type
mult Type
arg_ty Type
res_ty
           ; Coercion
co <- Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType Maybe SDoc
mb_thing Type
fun_ty Type
unif_fun_ty
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsWrapper
mkWpCastN Coercion
co, forall a. Type -> a -> Scaled a
Scaled Type
mult Type
arg_ty, Type
res_ty) }

    ------------
    mk_ctxt :: TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
    mk_ctxt :: Type -> TidyEnv -> TcM (TidyEnv, SDoc)
mk_ctxt Type
res_ty TidyEnv
env = TidyEnv
-> SDoc -> [Scaled Type] -> Type -> Int -> TcM (TidyEnv, SDoc)
mkFunTysMsg TidyEnv
env SDoc
herald (forall a. [a] -> [a]
reverse [Scaled Type]
arg_tys_so_far)
                                     Type
res_ty Int
n_val_args_in_call
    (Int
n_val_args_in_call, [Scaled Type]
arg_tys_so_far) = (Int, [Scaled Type])
err_info

{- Note [matchActualFunTy error handling]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
matchActualFunTySigma is made much more complicated by the
desire to produce good error messages. Consider the application
    f @Int x y
In GHC.Tc.Gen.Expr.tcArgs we deal with visible type arguments,
and then call matchActualFunTysPart for each individual value
argument. It, in turn, must instantiate any type/dictionary args,
before looking for an arrow type.

But if it doesn't find an arrow type, it wants to generate a message
like "f is applied to two arguments but its type only has one".
To do that, it needs to know about the args that tcArgs has already
munched up -- hence passing in n_val_args_in_call and arg_tys_so_far;
and hence also the accumulating so_far arg to 'go'.

This allows us (in mk_ctxt) to construct f's /instantiated/ type,
with just the values-arg arrows, which is what we really want
in the error message.

Ugh!
-}

-- Like 'matchExpectedFunTys', but used when you have an "actual" type,
-- for example in function application
matchActualFunTysRho :: SDoc   -- See Note [Herald for matchExpectedFunTys]
                     -> CtOrigin
                     -> Maybe SDoc  -- the thing with type TcSigmaType
                     -> Arity
                     -> TcSigmaType
                     -> TcM (HsWrapper, [Scaled TcSigmaType], TcRhoType)
-- If    matchActualFunTysRho n ty = (wrap, [t1,..,tn], res_ty)
-- then  wrap : ty ~> (t1 -> ... -> tn -> res_ty)
--       and res_ty is a RhoType
-- NB: the returned type is top-instantiated; it's a RhoType
matchActualFunTysRho :: SDoc
-> CtOrigin
-> Maybe SDoc
-> Int
-> Type
-> TcM (HsWrapper, [Scaled Type], Type)
matchActualFunTysRho SDoc
herald CtOrigin
ct_orig Maybe SDoc
mb_thing Int
n_val_args_wanted Type
fun_ty
  = Int
-> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
go Int
n_val_args_wanted [] Type
fun_ty
  where
    go :: Int
-> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
go Int
n [Scaled Type]
so_far Type
fun_ty
      | Bool -> Bool
not (Type -> Bool
isRhoTy Type
fun_ty)
      = do { (HsWrapper
wrap1, Type
rho) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
topInstantiate CtOrigin
ct_orig Type
fun_ty
           ; (HsWrapper
wrap2, [Scaled Type]
arg_tys, Type
res_ty) <- Int
-> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
go Int
n [Scaled Type]
so_far Type
rho
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap2 HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap1, [Scaled Type]
arg_tys, Type
res_ty) }

    go Int
0 [Scaled Type]
_ Type
fun_ty = forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, [], Type
fun_ty)

    go Int
n [Scaled Type]
so_far Type
fun_ty
      = do { (HsWrapper
wrap_fun1, Scaled Type
arg_ty1, Type
res_ty1) <- SDoc
-> Maybe SDoc
-> (Int, [Scaled Type])
-> Type
-> TcM (HsWrapper, Scaled Type, Type)
matchActualFunTySigma
                                                 SDoc
herald Maybe SDoc
mb_thing
                                                 (Int
n_val_args_wanted, [Scaled Type]
so_far)
                                                 Type
fun_ty
           ; (HsWrapper
wrap_res, [Scaled Type]
arg_tys, Type
res_ty)   <- Int
-> [Scaled Type] -> Type -> TcM (HsWrapper, [Scaled Type], Type)
go (Int
nforall a. Num a => a -> a -> a
-Int
1) (Scaled Type
arg_ty1forall a. a -> [a] -> [a]
:[Scaled Type]
so_far) Type
res_ty1
           ; let wrap_fun2 :: HsWrapper
wrap_fun2 = HsWrapper -> HsWrapper -> Scaled Type -> Type -> SDoc -> HsWrapper
mkWpFun HsWrapper
idHsWrapper HsWrapper
wrap_res Scaled Type
arg_ty1 Type
res_ty SDoc
doc
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap_fun2 HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap_fun1, Scaled Type
arg_ty1forall a. a -> [a] -> [a]
:[Scaled Type]
arg_tys, Type
res_ty) }
      where
        doc :: SDoc
doc = String -> SDoc
text String
"When inferring the argument type of a function with type" SDoc -> SDoc -> SDoc
<+>
              SDoc -> SDoc
quotes (forall a. Outputable a => a -> SDoc
ppr Type
fun_ty)


{-
************************************************************************
*                                                                      *
             matchExpected functions
*                                                                      *
************************************************************************

Note [Herald for matchExpectedFunTys]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'herald' always looks like:
   "The equation(s) for 'f' have"
   "The abstraction (\x.e) takes"
   "The section (+ x) expects"
   "The function 'f' is applied to"

This is used to construct a message of form

   The abstraction `\Just 1 -> ...' takes two arguments
   but its type `Maybe a -> a' has only one

   The equation(s) for `f' have two arguments
   but its type `Maybe a -> a' has only one

   The section `(f 3)' requires 'f' to take two arguments
   but its type `Int -> Int' has only one

   The function 'f' is applied to two arguments
   but its type `Int -> Int' has only one

When visible type applications (e.g., `f @Int 1 2`, as in #13902) enter the
picture, we have a choice in deciding whether to count the type applications as
proper arguments:

   The function 'f' is applied to one visible type argument
     and two value arguments
   but its type `forall a. a -> a` has only one visible type argument
     and one value argument

Or whether to include the type applications as part of the herald itself:

   The expression 'f @Int' is applied to two arguments
   but its type `Int -> Int` has only one

The latter is easier to implement and is arguably easier to understand, so we
choose to implement that option.

Note [matchExpectedFunTys]
~~~~~~~~~~~~~~~~~~~~~~~~~~
matchExpectedFunTys checks that a sigma has the form
of an n-ary function.  It passes the decomposed type to the
thing_inside, and returns a wrapper to coerce between the two types

It's used wherever a language construct must have a functional type,
namely:
        A lambda expression
        A function definition
     An operator section

This function must be written CPS'd because it needs to fill in the
ExpTypes produced for arguments before it can fill in the ExpType
passed in.

-}

-- Use this one when you have an "expected" type.
-- This function skolemises at each polytype.
matchExpectedFunTys :: forall a.
                       SDoc   -- See Note [Herald for matchExpectedFunTys]
                    -> UserTypeCtxt
                    -> Arity
                    -> ExpRhoType      -- Skolemised
                    -> ([Scaled ExpSigmaType] -> ExpRhoType -> TcM a)
                    -> TcM (HsWrapper, a)
-- If    matchExpectedFunTys n ty = (_, wrap)
-- then  wrap : (t1 -> ... -> tn -> ty_r) ~> ty,
--   where [t1, ..., tn], ty_r are passed to the thing_inside
matchExpectedFunTys :: forall a.
SDoc
-> UserTypeCtxt
-> Int
-> ExpRhoType
-> ([Scaled ExpRhoType] -> ExpRhoType -> TcM a)
-> TcM (HsWrapper, a)
matchExpectedFunTys SDoc
herald UserTypeCtxt
ctx Int
arity ExpRhoType
orig_ty [Scaled ExpRhoType] -> ExpRhoType -> TcM a
thing_inside
  = case ExpRhoType
orig_ty of
      Check Type
ty -> [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
go [] Int
arity Type
ty
      ExpRhoType
_        -> [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
defer [] Int
arity ExpRhoType
orig_ty
  where
    -- Skolemise any foralls /before/ the zero-arg case
    -- so that we guarantee to return a rho-type
    go :: [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
go [Scaled ExpRhoType]
acc_arg_tys Int
n Type
ty
      | ([TcTyVar]
tvs, ThetaType
theta, Type
_) <- Type -> ([TcTyVar], ThetaType, Type)
tcSplitSigmaTy Type
ty
      , Bool -> Bool
not (forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs Bool -> Bool -> Bool
&& forall (t :: * -> *) a. Foldable t => t a -> Bool
null ThetaType
theta)
      = do { (HsWrapper
wrap_gen, (HsWrapper
wrap_res, a
result)) <- forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcTopSkolemise UserTypeCtxt
ctx Type
ty forall a b. (a -> b) -> a -> b
$ \Type
ty' ->
                                               [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
go [Scaled ExpRhoType]
acc_arg_tys Int
n Type
ty'
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap_gen HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap_res, a
result) }

    -- No more args; do this /before/ tcView, so
    -- that we do not unnecessarily unwrap synonyms
    go [Scaled ExpRhoType]
acc_arg_tys Int
0 Type
rho_ty
      = do { a
result <- [Scaled ExpRhoType] -> ExpRhoType -> TcM a
thing_inside (forall a. [a] -> [a]
reverse [Scaled ExpRhoType]
acc_arg_tys) (Type -> ExpRhoType
mkCheckExpType Type
rho_ty)
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, a
result) }

    go [Scaled ExpRhoType]
acc_arg_tys Int
n Type
ty
      | Just Type
ty' <- Type -> Maybe Type
tcView Type
ty = [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
go [Scaled ExpRhoType]
acc_arg_tys Int
n Type
ty'

    go [Scaled ExpRhoType]
acc_arg_tys Int
n (FunTy { ft_mult :: Type -> Type
ft_mult = Type
mult, ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
af, ft_arg :: Type -> Type
ft_arg = Type
arg_ty, ft_res :: Type -> Type
ft_res = Type
res_ty })
      = ASSERT( af == VisArg )
        do { (HsWrapper
wrap_res, a
result) <- [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
go ((forall a. Type -> a -> Scaled a
Scaled Type
mult forall a b. (a -> b) -> a -> b
$ Type -> ExpRhoType
mkCheckExpType Type
arg_ty) forall a. a -> [a] -> [a]
: [Scaled ExpRhoType]
acc_arg_tys)
                                      (Int
nforall a. Num a => a -> a -> a
-Int
1) Type
res_ty
           ; let fun_wrap :: HsWrapper
fun_wrap = HsWrapper -> HsWrapper -> Scaled Type -> Type -> SDoc -> HsWrapper
mkWpFun HsWrapper
idHsWrapper HsWrapper
wrap_res (forall a. Type -> a -> Scaled a
Scaled Type
mult Type
arg_ty) Type
res_ty SDoc
doc
           ; forall (m :: * -> *) a. Monad m => a -> m a
return ( HsWrapper
fun_wrap, a
result ) }
      where
        doc :: SDoc
doc = String -> SDoc
text String
"When inferring the argument type of a function with type" SDoc -> SDoc -> SDoc
<+>
              SDoc -> SDoc
quotes (forall a. Outputable a => a -> SDoc
ppr ExpRhoType
orig_ty)

    go [Scaled ExpRhoType]
acc_arg_tys Int
n ty :: Type
ty@(TyVarTy TcTyVar
tv)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
      = do { MetaDetails
cts <- TcTyVar -> TcM MetaDetails
readMetaTyVar TcTyVar
tv
           ; case MetaDetails
cts of
               Indirect Type
ty' -> [Scaled ExpRhoType] -> Int -> Type -> TcM (HsWrapper, a)
go [Scaled ExpRhoType]
acc_arg_tys Int
n Type
ty'
               MetaDetails
Flexi        -> [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
defer [Scaled ExpRhoType]
acc_arg_tys Int
n (Type -> ExpRhoType
mkCheckExpType Type
ty) }

       -- In all other cases we bale out into ordinary unification
       -- However unlike the meta-tyvar case, we are sure that the
       -- number of arguments doesn't match arity of the original
       -- type, so we can add a bit more context to the error message
       -- (cf #7869).
       --
       -- It is not always an error, because specialized type may have
       -- different arity, for example:
       --
       -- > f1 = f2 'a'
       -- > f2 :: Monad m => m Bool
       -- > f2 = undefined
       --
       -- But in that case we add specialized type into error context
       -- anyway, because it may be useful. See also #9605.
    go [Scaled ExpRhoType]
acc_arg_tys Int
n Type
ty = forall a. (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
addErrCtxtM ([Scaled ExpRhoType] -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
mk_ctxt [Scaled ExpRhoType]
acc_arg_tys Type
ty) forall a b. (a -> b) -> a -> b
$
                          [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
defer [Scaled ExpRhoType]
acc_arg_tys Int
n (Type -> ExpRhoType
mkCheckExpType Type
ty)

    ------------
    defer :: [Scaled ExpSigmaType] -> Arity -> ExpRhoType -> TcM (HsWrapper, a)
    defer :: [Scaled ExpRhoType] -> Int -> ExpRhoType -> TcM (HsWrapper, a)
defer [Scaled ExpRhoType]
acc_arg_tys Int
n ExpRhoType
fun_ty
      = do { [Scaled ExpRhoType]
more_arg_tys <- forall (m :: * -> *) a. Applicative m => Int -> m a -> m [a]
replicateM Int
n (forall a. Type -> a -> Scaled a
mkScaled forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Type -> TcM Type
newFlexiTyVarTy Type
multiplicityTy forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> TcM ExpRhoType
newInferExpType)
           ; ExpRhoType
res_ty       <- TcM ExpRhoType
newInferExpType
           ; a
result       <- [Scaled ExpRhoType] -> ExpRhoType -> TcM a
thing_inside (forall a. [a] -> [a]
reverse [Scaled ExpRhoType]
acc_arg_tys forall a. [a] -> [a] -> [a]
++ [Scaled ExpRhoType]
more_arg_tys) ExpRhoType
res_ty
           ; [Scaled Type]
more_arg_tys <- forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (\(Scaled Type
m ExpRhoType
t) -> forall a. Type -> a -> Scaled a
Scaled Type
m forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> ExpRhoType -> TcM Type
readExpType ExpRhoType
t) [Scaled ExpRhoType]
more_arg_tys
           ; Type
res_ty       <- ExpRhoType -> TcM Type
readExpType ExpRhoType
res_ty
           ; let unif_fun_ty :: Type
unif_fun_ty = [Scaled Type] -> Type -> Type
mkVisFunTys [Scaled Type]
more_arg_tys Type
res_ty
           ; HsWrapper
wrap <- CtOrigin -> UserTypeCtxt -> Type -> ExpRhoType -> TcM HsWrapper
tcSubType CtOrigin
AppOrigin UserTypeCtxt
ctx Type
unif_fun_ty ExpRhoType
fun_ty
                         -- Not a good origin at all :-(
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap, a
result) }

    ------------
    mk_ctxt :: [Scaled ExpSigmaType] -> TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
    mk_ctxt :: [Scaled ExpRhoType] -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
mk_ctxt [Scaled ExpRhoType]
arg_tys Type
res_ty TidyEnv
env
      = TidyEnv
-> SDoc -> [Scaled Type] -> Type -> Int -> TcM (TidyEnv, SDoc)
mkFunTysMsg TidyEnv
env SDoc
herald [Scaled Type]
arg_tys' Type
res_ty Int
arity
      where
        arg_tys' :: [Scaled Type]
arg_tys' = forall a b. (a -> b) -> [a] -> [b]
map (\(Scaled Type
u ExpRhoType
v) -> forall a. Type -> a -> Scaled a
Scaled Type
u (String -> ExpRhoType -> Type
checkingExpType String
"matchExpectedFunTys" ExpRhoType
v)) forall a b. (a -> b) -> a -> b
$
                   forall a. [a] -> [a]
reverse [Scaled ExpRhoType]
arg_tys
            -- this is safe b/c we're called from "go"

mkFunTysMsg :: TidyEnv -> SDoc -> [Scaled TcType] -> TcType -> Arity
            -> TcM (TidyEnv, SDoc)
mkFunTysMsg :: TidyEnv
-> SDoc -> [Scaled Type] -> Type -> Int -> TcM (TidyEnv, SDoc)
mkFunTysMsg TidyEnv
env SDoc
herald [Scaled Type]
arg_tys Type
res_ty Int
n_val_args_in_call
  = do { (TidyEnv
env', Type
fun_rho) <- TidyEnv -> Type -> TcM (TidyEnv, Type)
zonkTidyTcType TidyEnv
env forall a b. (a -> b) -> a -> b
$
                            [Scaled Type] -> Type -> Type
mkVisFunTys [Scaled Type]
arg_tys Type
res_ty

       ; let ([Scaled Type]
all_arg_tys, Type
_) = Type -> ([Scaled Type], Type)
splitFunTys Type
fun_rho
             n_fun_args :: Int
n_fun_args = forall (t :: * -> *) a. Foldable t => t a -> Int
length [Scaled Type]
all_arg_tys

             msg :: SDoc
msg | Int
n_val_args_in_call forall a. Ord a => a -> a -> Bool
<= Int
n_fun_args  -- Enough args, in the end
                 = String -> SDoc
text String
"In the result of a function call"
                 | Bool
otherwise
                 = SDoc -> Int -> SDoc -> SDoc
hang (SDoc
full_herald SDoc -> SDoc -> SDoc
<> SDoc
comma)
                      Int
2 ([SDoc] -> SDoc
sep [ String -> SDoc
text String
"but its type" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
quotes (Type -> SDoc
pprType Type
fun_rho)
                             , if Int
n_fun_args forall a. Eq a => a -> a -> Bool
== Int
0 then String -> SDoc
text String
"has none"
                               else String -> SDoc
text String
"has only" SDoc -> SDoc -> SDoc
<+> Int -> SDoc
speakN Int
n_fun_args])

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
env', SDoc
msg) }
 where
  full_herald :: SDoc
full_herald = SDoc
herald SDoc -> SDoc -> SDoc
<+> Int -> SDoc -> SDoc
speakNOf Int
n_val_args_in_call (String -> SDoc
text String
"value argument")

----------------------
matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
-- Special case for lists
matchExpectedListTy :: Type -> TcM (Coercion, Type)
matchExpectedListTy Type
exp_ty
 = do { (Coercion
co, [Type
elt_ty]) <- TyCon -> Type -> TcM (Coercion, ThetaType)
matchExpectedTyConApp TyCon
listTyCon Type
exp_ty
      ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion
co, Type
elt_ty) }

---------------------
matchExpectedTyConApp :: TyCon                -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> *
                      -> TcRhoType            -- orig_ty
                      -> TcM (TcCoercionN,    -- T k1 k2 k3 a b c ~N orig_ty
                              [TcSigmaType])  -- Element types, k1 k2 k3 a b c

-- It's used for wired-in tycons, so we call checkWiredInTyCon
-- Precondition: never called with FunTyCon
-- Precondition: input type :: *
-- Postcondition: (T k1 k2 k3 a b c) is well-kinded

matchExpectedTyConApp :: TyCon -> Type -> TcM (Coercion, ThetaType)
matchExpectedTyConApp TyCon
tc Type
orig_ty
  = ASSERT(not $ isFunTyCon tc) go orig_ty
  where
    go :: Type -> TcM (Coercion, ThetaType)
go Type
ty
       | Just Type
ty' <- Type -> Maybe Type
tcView Type
ty
       = Type -> TcM (Coercion, ThetaType)
go Type
ty'

    go ty :: Type
ty@(TyConApp TyCon
tycon ThetaType
args)
       | TyCon
tc forall a. Eq a => a -> a -> Bool
== TyCon
tycon  -- Common case
       = forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Coercion
mkTcNomReflCo Type
ty, ThetaType
args)

    go (TyVarTy TcTyVar
tv)
       | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
       = do { MetaDetails
cts <- TcTyVar -> TcM MetaDetails
readMetaTyVar TcTyVar
tv
            ; case MetaDetails
cts of
                Indirect Type
ty -> Type -> TcM (Coercion, ThetaType)
go Type
ty
                MetaDetails
Flexi       -> TcM (Coercion, ThetaType)
defer }

    go Type
_ = TcM (Coercion, ThetaType)
defer

    -- If the common case does not occur, instantiate a template
    -- T k1 .. kn t1 .. tm, and unify with the original type
    -- Doing it this way ensures that the types we return are
    -- kind-compatible with T.  For example, suppose we have
    --       matchExpectedTyConApp T (f Maybe)
    -- where data T a = MkT a
    -- Then we don't want to instantiate T's data constructors with
    --    (a::*) ~ Maybe
    -- because that'll make types that are utterly ill-kinded.
    -- This happened in #7368
    defer :: TcM (Coercion, ThetaType)
defer
      = do { (TCvSubst
_, [TcTyVar]
arg_tvs) <- [TcTyVar] -> TcM (TCvSubst, [TcTyVar])
newMetaTyVars (TyCon -> [TcTyVar]
tyConTyVars TyCon
tc)
           ; String -> SDoc -> TcRn ()
traceTc String
"matchExpectedTyConApp" (forall a. Outputable a => a -> SDoc
ppr TyCon
tc SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr (TyCon -> [TcTyVar]
tyConTyVars TyCon
tc) SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [TcTyVar]
arg_tvs)
           ; let args :: ThetaType
args = [TcTyVar] -> ThetaType
mkTyVarTys [TcTyVar]
arg_tvs
                 tc_template :: Type
tc_template = TyCon -> ThetaType -> Type
mkTyConApp TyCon
tc ThetaType
args
           ; Coercion
co <- Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType forall a. Maybe a
Nothing Type
tc_template Type
orig_ty
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion
co, ThetaType
args) }

----------------------
matchExpectedAppTy :: TcRhoType                         -- orig_ty
                   -> TcM (TcCoercion,                   -- m a ~N orig_ty
                           (TcSigmaType, TcSigmaType))  -- Returns m, a
-- If the incoming type is a mutable type variable of kind k, then
-- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.

matchExpectedAppTy :: Type -> TcM (Coercion, (Type, Type))
matchExpectedAppTy Type
orig_ty
  = Type -> TcM (Coercion, (Type, Type))
go Type
orig_ty
  where
    go :: Type -> TcM (Coercion, (Type, Type))
go Type
ty
      | Just Type
ty' <- Type -> Maybe Type
tcView Type
ty = Type -> TcM (Coercion, (Type, Type))
go Type
ty'

      | Just (Type
fun_ty, Type
arg_ty) <- Type -> Maybe (Type, Type)
tcSplitAppTy_maybe Type
ty
      = forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Coercion
mkTcNomReflCo Type
orig_ty, (Type
fun_ty, Type
arg_ty))

    go (TyVarTy TcTyVar
tv)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
      = do { MetaDetails
cts <- TcTyVar -> TcM MetaDetails
readMetaTyVar TcTyVar
tv
           ; case MetaDetails
cts of
               Indirect Type
ty -> Type -> TcM (Coercion, (Type, Type))
go Type
ty
               MetaDetails
Flexi       -> TcM (Coercion, (Type, Type))
defer }

    go Type
_ = TcM (Coercion, (Type, Type))
defer

    -- Defer splitting by generating an equality constraint
    defer :: TcM (Coercion, (Type, Type))
defer
      = do { Type
ty1 <- Type -> TcM Type
newFlexiTyVarTy Type
kind1
           ; Type
ty2 <- Type -> TcM Type
newFlexiTyVarTy Type
kind2
           ; Coercion
co <- Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType forall a. Maybe a
Nothing (Type -> Type -> Type
mkAppTy Type
ty1 Type
ty2) Type
orig_ty
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion
co, (Type
ty1, Type
ty2)) }

    orig_kind :: Type
orig_kind = HasDebugCallStack => Type -> Type
tcTypeKind Type
orig_ty
    kind1 :: Type
kind1 = Type -> Type -> Type
mkVisFunTyMany Type
liftedTypeKind Type
orig_kind
    kind2 :: Type
kind2 = Type
liftedTypeKind    -- m :: * -> k
                              -- arg type :: *

{-
************************************************************************
*                                                                      *
                Subsumption checking
*                                                                      *
************************************************************************

Note [Subsumption checking: tcSubType]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All the tcSubType calls have the form
                tcSubType actual_ty expected_ty
which checks
                actual_ty <= expected_ty

That is, that a value of type actual_ty is acceptable in
a place expecting a value of type expected_ty.  I.e. that

    actual ty   is more polymorphic than   expected_ty

It returns a wrapper function
        co_fn :: actual_ty ~ expected_ty
which takes an HsExpr of type actual_ty into one of type
expected_ty.

Note [Ambiguity check and deep subsumption]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f :: (forall b. Eq b => a -> a) -> Int

Does `f` have an ambiguous type?   The ambiguity check usually checks
that this definition of f' would typecheck, where f' has the exact same
type as f:
   f' :: (forall b. Eq b => a -> a) -> Intp
   f' = f

This will be /rejected/ with DeepSubsumption but /accepted/ with
ShallowSubsumption.  On the other hand, this eta-expanded version f''
would be rejected both ways:
   f'' :: (forall b. Eq b => a -> a) -> Intp
   f'' x = f x

This is squishy in the same way as other examples in GHC.Tc.Validity
Note [The squishiness of the ambiguity check]

The situation in June 2022.  Since we have SimpleSubsumption at the moment,
we don't want introduce new breakage if you add -XDeepSubsumption, by
rejecting types as ambiguous that weren't ambiguous before.  So, as a
holding decision, we /always/ use SimpleSubsumption for the ambiguity check
(erring on the side accepting more programs). Hence tcSubTypeAmbiguity.
-}



-----------------
-- tcWrapResult needs both un-type-checked (for origins and error messages)
-- and type-checked (for wrapping) expressions
tcWrapResult :: HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
             -> TcM (HsExpr GhcTc)
tcWrapResult :: HsExpr GhcRn
-> HsExpr GhcTc -> Type -> ExpRhoType -> TcM (HsExpr GhcTc)
tcWrapResult HsExpr GhcRn
rn_expr = CtOrigin
-> HsExpr GhcRn
-> HsExpr GhcTc
-> Type
-> ExpRhoType
-> TcM (HsExpr GhcTc)
tcWrapResultO (HsExpr GhcRn -> CtOrigin
exprCtOrigin HsExpr GhcRn
rn_expr) HsExpr GhcRn
rn_expr

tcWrapResultO :: CtOrigin -> HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
               -> TcM (HsExpr GhcTc)
tcWrapResultO :: CtOrigin
-> HsExpr GhcRn
-> HsExpr GhcTc
-> Type
-> ExpRhoType
-> TcM (HsExpr GhcTc)
tcWrapResultO CtOrigin
orig HsExpr GhcRn
rn_expr HsExpr GhcTc
expr Type
actual_ty ExpRhoType
res_ty
  = do { String -> SDoc -> TcRn ()
traceTc String
"tcWrapResult" ([SDoc] -> SDoc
vcat [ String -> SDoc
text String
"Actual:  " SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
actual_ty
                                      , String -> SDoc
text String
"Expected:" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr ExpRhoType
res_ty ])
       ; HsWrapper
wrap <- CtOrigin
-> UserTypeCtxt
-> Maybe SDoc
-> Type
-> ExpRhoType
-> TcM HsWrapper
tcSubTypeNC CtOrigin
orig UserTypeCtxt
GenSigCtxt (forall a. a -> Maybe a
Just (forall a. Outputable a => a -> SDoc
ppr HsExpr GhcRn
rn_expr)) Type
actual_ty ExpRhoType
res_ty
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper -> HsExpr GhcTc -> HsExpr GhcTc
mkHsWrap HsWrapper
wrap HsExpr GhcTc
expr) }

tcWrapResultMono :: HsExpr GhcRn -> HsExpr GhcTc
                 -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                 -> ExpRhoType  -- Expected
                 -> TcM (HsExpr GhcTc)
-- A version of tcWrapResult to use when the actual type is a
-- rho-type, so nothing to instantiate; just go straight to unify.
-- It means we don't need to pass in a CtOrigin
tcWrapResultMono :: HsExpr GhcRn
-> HsExpr GhcTc -> Type -> ExpRhoType -> TcM (HsExpr GhcTc)
tcWrapResultMono HsExpr GhcRn
rn_expr HsExpr GhcTc
expr Type
act_ty ExpRhoType
res_ty
  = ASSERT2( isRhoTy act_ty, ppr act_ty $$ ppr rn_expr )
    do { Coercion
co <- HsExpr GhcRn -> Type -> ExpRhoType -> TcM Coercion
unifyExpectedType HsExpr GhcRn
rn_expr Type
act_ty ExpRhoType
res_ty
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsExpr GhcTc -> HsExpr GhcTc
mkHsWrapCo Coercion
co HsExpr GhcTc
expr) }

unifyExpectedType :: HsExpr GhcRn
                  -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                  -> ExpRhoType  -- Expected
                  -> TcM TcCoercionN
unifyExpectedType :: HsExpr GhcRn -> Type -> ExpRhoType -> TcM Coercion
unifyExpectedType HsExpr GhcRn
rn_expr Type
act_ty ExpRhoType
exp_ty
  = case ExpRhoType
exp_ty of
      Infer InferResult
inf_res -> Type -> InferResult -> TcM Coercion
fillInferResult Type
act_ty InferResult
inf_res
      Check Type
exp_ty  -> Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType (forall a. a -> Maybe a
Just (forall a. Outputable a => a -> SDoc
ppr HsExpr GhcRn
rn_expr)) Type
act_ty Type
exp_ty

------------------------
tcSubTypePat :: CtOrigin -> UserTypeCtxt
            -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
-- Used in patterns; polarity is backwards compared
--   to tcSubType
-- If wrap = tc_sub_type_et t1 t2
--    => wrap :: t1 ~> t2
tcSubTypePat :: CtOrigin -> UserTypeCtxt -> ExpRhoType -> Type -> TcM HsWrapper
tcSubTypePat CtOrigin
inst_orig UserTypeCtxt
ctxt (Check Type
ty_actual) Type
ty_expected
  = do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type DynFlags
dflags Type -> Type -> TcM Coercion
unifyTypeET CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected }

tcSubTypePat CtOrigin
_ UserTypeCtxt
_ (Infer InferResult
inf_res) Type
ty_expected
  = do { Coercion
co <- Type -> InferResult -> TcM Coercion
fillInferResult Type
ty_expected InferResult
inf_res
               -- In patterns we do not instantatiate

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsWrapper
mkWpCastN (Coercion -> Coercion
mkTcSymCo Coercion
co)) }

---------------
tcSubType :: CtOrigin -> UserTypeCtxt
          -> TcSigmaType  -- Actual
          -> ExpRhoType   -- Expected
          -> TcM HsWrapper
-- Checks that 'actual' is more polymorphic than 'expected'
tcSubType :: CtOrigin -> UserTypeCtxt -> Type -> ExpRhoType -> TcM HsWrapper
tcSubType CtOrigin
orig UserTypeCtxt
ctxt Type
ty_actual ExpRhoType
ty_expected
  = forall a. Type -> ExpRhoType -> TcM a -> TcM a
addSubTypeCtxt Type
ty_actual ExpRhoType
ty_expected forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcRn ()
traceTc String
"tcSubType" ([SDoc] -> SDoc
vcat [UserTypeCtxt -> SDoc
pprUserTypeCtxt UserTypeCtxt
ctxt, forall a. Outputable a => a -> SDoc
ppr Type
ty_actual, forall a. Outputable a => a -> SDoc
ppr ExpRhoType
ty_expected])
       ; CtOrigin
-> UserTypeCtxt
-> Maybe SDoc
-> Type
-> ExpRhoType
-> TcM HsWrapper
tcSubTypeNC CtOrigin
orig UserTypeCtxt
ctxt forall a. Maybe a
Nothing Type
ty_actual ExpRhoType
ty_expected }

---------------
tcSubTypeDS :: HsExpr GhcRn
            -> TcRhoType   -- Actual -- a rho-type not a sigma-type
            -> ExpRhoType  -- Expected
            -> TcM HsWrapper
-- Similar signature to unifyExpectedType; does deep subsumption
-- Only one call site, in GHC.Tc.Gen.App.tcApp
tcSubTypeDS :: HsExpr GhcRn -> Type -> ExpRhoType -> TcM HsWrapper
tcSubTypeDS HsExpr GhcRn
rn_expr Type
act_rho ExpRhoType
res_ty
  = case ExpRhoType
res_ty of
      Check Type
exp_rho -> do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
                          ; DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_deep DynFlags
dflags (Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType Maybe SDoc
m_thing) CtOrigin
orig
                                        UserTypeCtxt
GenSigCtxt Type
act_rho Type
exp_rho
                          }

      Infer InferResult
inf_res -> do { Coercion
co <- Type -> InferResult -> TcM Coercion
fillInferResult Type
act_rho InferResult
inf_res
                          ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsWrapper
mkWpCastN Coercion
co) }
  where
    orig :: CtOrigin
orig    = HsExpr GhcRn -> CtOrigin
exprCtOrigin HsExpr GhcRn
rn_expr
    m_thing :: Maybe SDoc
m_thing = forall a. a -> Maybe a
Just (forall a. Outputable a => a -> SDoc
ppr HsExpr GhcRn
rn_expr)

---------------
tcSubTypeNC :: CtOrigin          -- ^ Used when instantiating
            -> UserTypeCtxt      -- ^ Used when skolemising
            -> Maybe SDoc        -- ^ The expression that has type 'actual' (if known)
            -> TcSigmaType       -- ^ Actual type
            -> ExpRhoType        -- ^ Expected type
            -> TcM HsWrapper
tcSubTypeNC :: CtOrigin
-> UserTypeCtxt
-> Maybe SDoc
-> Type
-> ExpRhoType
-> TcM HsWrapper
tcSubTypeNC CtOrigin
inst_orig UserTypeCtxt
ctxt Maybe SDoc
m_thing Type
ty_actual ExpRhoType
res_ty
  = case ExpRhoType
res_ty of
      Check Type
ty_expected -> do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
                              ; DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type DynFlags
dflags (Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType Maybe SDoc
m_thing) CtOrigin
inst_orig UserTypeCtxt
ctxt
                                            Type
ty_actual Type
ty_expected }

      Infer InferResult
inf_res -> do { (HsWrapper
wrap, Type
rho) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
topInstantiate CtOrigin
inst_orig Type
ty_actual
                                   -- See Note [Instantiation of InferResult]
                          ; Coercion
co <- Type -> InferResult -> TcM Coercion
fillInferResult Type
rho InferResult
inf_res
                          ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsWrapper
mkWpCastN Coercion
co HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap) }

---------------
tcSubTypeSigma :: CtOrigin       -- where did the actual type arise / why are we
                                 -- doing this subtype check?
               -> UserTypeCtxt   -- where did the expected type arise?
               -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
-- External entry point, but no ExpTypes on either side
-- Checks that actual <= expected
-- Returns HsWrapper :: actual ~ expected
tcSubTypeSigma :: CtOrigin -> UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tcSubTypeSigma CtOrigin
orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  = do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type DynFlags
dflags (Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType forall a. Maybe a
Nothing) CtOrigin
orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
       }

---------------
tcSubTypeAmbiguity :: UserTypeCtxt   -- Where did this type arise
                   -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
-- See Note [Ambiguity check and deep subsumption]
tcSubTypeAmbiguity :: UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tcSubTypeAmbiguity UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  = do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_shallow DynFlags
dflags (Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType forall a. Maybe a
Nothing)
                        (UserTypeCtxt -> CtOrigin
AmbiguityCheckOrigin UserTypeCtxt
ctxt)
                        UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
       }

---------------
addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
addSubTypeCtxt :: forall a. Type -> ExpRhoType -> TcM a -> TcM a
addSubTypeCtxt Type
ty_actual ExpRhoType
ty_expected TcM a
thing_inside
 | Type -> Bool
isRhoTy Type
ty_actual        -- If there is no polymorphism involved, the
 , ExpRhoType -> Bool
isRhoExpTy ExpRhoType
ty_expected   -- TypeEqOrigin stuff (added by the _NC functions)
 = TcM a
thing_inside             -- gives enough context by itself
 | Bool
otherwise
 = forall a. (TidyEnv -> TcM (TidyEnv, SDoc)) -> TcM a -> TcM a
addErrCtxtM TidyEnv -> TcM (TidyEnv, SDoc)
mk_msg TcM a
thing_inside
  where
    mk_msg :: TidyEnv -> TcM (TidyEnv, SDoc)
mk_msg TidyEnv
tidy_env
      = do { (TidyEnv
tidy_env, Type
ty_actual)   <- TidyEnv -> Type -> TcM (TidyEnv, Type)
zonkTidyTcType TidyEnv
tidy_env Type
ty_actual
           ; Type
ty_expected             <- ExpRhoType -> TcM Type
readExpType ExpRhoType
ty_expected
                   -- A worry: might not be filled if we're debugging. Ugh.
           ; (TidyEnv
tidy_env, Type
ty_expected) <- TidyEnv -> Type -> TcM (TidyEnv, Type)
zonkTidyTcType TidyEnv
tidy_env Type
ty_expected
           ; let msg :: SDoc
msg = [SDoc] -> SDoc
vcat [ SDoc -> Int -> SDoc -> SDoc
hang (String -> SDoc
text String
"When checking that:")
                                 Int
4 (forall a. Outputable a => a -> SDoc
ppr Type
ty_actual)
                            , Int -> SDoc -> SDoc
nest Int
2 (SDoc -> Int -> SDoc -> SDoc
hang (String -> SDoc
text String
"is more polymorphic than:")
                                         Int
2 (forall a. Outputable a => a -> SDoc
ppr Type
ty_expected)) ]
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (TidyEnv
tidy_env, SDoc
msg) }


{- Note [Instantiation of InferResult]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We now always instantiate before filling in InferResult, so that
the result is a TcRhoType: see #17173 for discussion.

For example:

1. Consider
    f x = (*)
   We want to instantiate the type of (*) before returning, else we
   will infer the type
     f :: forall {a}. a -> forall b. Num b => b -> b -> b
   This is surely confusing for users.

   And worse, the monomorphism restriction won't work properly. The MR is
   dealt with in simplifyInfer, and simplifyInfer has no way of
   instantiating. This could perhaps be worked around, but it may be
   hard to know even when instantiation should happen.

2. Another reason.  Consider
       f :: (?x :: Int) => a -> a
       g y = let ?x = 3::Int in f
   Here want to instantiate f's type so that the ?x::Int constraint
  gets discharged by the enclosing implicit-parameter binding.

3. Suppose one defines plus = (+). If we instantiate lazily, we will
   infer plus :: forall a. Num a => a -> a -> a. However, the monomorphism
   restriction compels us to infer
      plus :: Integer -> Integer -> Integer
   (or similar monotype). Indeed, the only way to know whether to apply
   the monomorphism restriction at all is to instantiate

There is one place where we don't want to instantiate eagerly,
namely in GHC.Tc.Module.tcRnExpr, which implements GHCi's :type
command. See Note [Implementing :type] in GHC.Tc.Module.
-}

---------------
tc_sub_type, tc_sub_type_deep, tc_sub_type_shallow
    :: DynFlags
    -> (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
    -> CtOrigin       -- Used when instantiating
    -> UserTypeCtxt   -- Used when skolemising
    -> TcSigmaType    -- Actual; a sigma-type
    -> TcSigmaType    -- Expected; also a sigma-type
    -> TcM HsWrapper
-- Checks that actual_ty is more polymorphic than expected_ty
-- If wrap = tc_sub_type t1 t2
--    => wrap :: t1 ~> t2
--
-- The "how to unify argument" is always a call to `uType TypeLevel orig`,
-- but with different ways of constructing the CtOrigin `orig` from
-- the argument types and context.

----------------------
tc_sub_type :: DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type DynFlags
dflags Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  = do { Bool
deep_subsumption <- forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
xoptM Extension
LangExt.DeepSubsumption
       ; if Bool
deep_subsumption
         then DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_deep    DynFlags
dflags Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
         else DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_shallow DynFlags
dflags Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  }

----------------------
tc_sub_type_shallow :: DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_shallow DynFlags
dflags Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  | DynFlags -> Type -> Bool
definitely_poly DynFlags
dflags Type
ty_expected   -- See Note [Don't skolemise unnecessarily]
  , Type -> Bool
definitely_mono_shallow Type
ty_actual
  = do { String -> SDoc -> TcRn ()
traceTc String
"tc_sub_type (drop to equality)" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_actual
              , String -> SDoc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_expected ]
       ; Coercion -> HsWrapper
mkWpCastN forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$>
         Type -> Type -> TcM Coercion
unify Type
ty_actual Type
ty_expected }

  | Bool
otherwise   -- This is the general case
  = do { String -> SDoc -> TcRn ()
traceTc String
"tc_sub_type (general case)" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_actual
              , String -> SDoc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_expected ]

       ; (HsWrapper
sk_wrap, HsWrapper
inner_wrap)
           <- forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcTopSkolemise UserTypeCtxt
ctxt Type
ty_expected forall a b. (a -> b) -> a -> b
$ \ Type
sk_rho ->
              do { (HsWrapper
wrap, Type
rho_a) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
topInstantiate CtOrigin
inst_orig Type
ty_actual
                 ; Coercion
cow           <- Type -> Type -> TcM Coercion
unify Type
rho_a Type
sk_rho
                 ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsWrapper
mkWpCastN Coercion
cow HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap) }

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
sk_wrap HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
inner_wrap) }

----------------------
tc_sub_type_deep :: DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_deep DynFlags
dflags Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  | DynFlags -> Type -> Bool
definitely_poly DynFlags
dflags Type
ty_expected      -- See Note [Don't skolemise unnecessarily]
  , Type -> Bool
definitely_mono_deep Type
ty_actual
  = do { String -> SDoc -> TcRn ()
traceTc String
"tc_sub_type_deep (drop to equality)" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_actual
              , String -> SDoc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_expected ]
       ; Coercion -> HsWrapper
mkWpCastN forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$>
         Type -> Type -> TcM Coercion
unify Type
ty_actual Type
ty_expected }

  | Bool
otherwise   -- This is the general case
  = do { String -> SDoc -> TcRn ()
traceTc String
"tc_sub_type_deep (general case)" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_actual
              , String -> SDoc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_expected ]

       ; (HsWrapper
sk_wrap, HsWrapper
inner_wrap)
           <- forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcDeeplySkolemise UserTypeCtxt
ctxt Type
ty_expected forall a b. (a -> b) -> a -> b
$ \ Type
sk_rho ->
              -- See Note [Deep subsumption]
              (Type -> Type -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tc_sub_type_ds Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
sk_rho

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
sk_wrap HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
inner_wrap) }

definitely_mono_shallow :: TcType -> Bool
definitely_mono_shallow :: Type -> Bool
definitely_mono_shallow Type
ty = Type -> Bool
isRhoTy Type
ty
    -- isRhoTy: no top level forall or (=>)

definitely_mono_deep :: TcType -> Bool
definitely_mono_deep :: Type -> Bool
definitely_mono_deep Type
ty
  | Bool -> Bool
not (Type -> Bool
definitely_mono_shallow Type
ty)     = Bool
False
    -- isRhoTy: False means top level forall or (=>)
  | Just (Scaled Type
_, Type
res) <- Type -> Maybe (Scaled Type, Type)
tcSplitFunTy_maybe Type
ty = Type -> Bool
definitely_mono_deep Type
res
    -- Top level (->)
  | Bool
otherwise                              = Bool
True

definitely_poly :: DynFlags -> TcType -> Bool
-- A very conservative test:
-- see Note [Don't skolemise unnecessarily]
definitely_poly :: DynFlags -> Type -> Bool
definitely_poly DynFlags
dflags Type
ty
  | ([TcTyVar]
tvs, ThetaType
theta, Type
tau) <- Type -> ([TcTyVar], ThetaType, Type)
tcSplitSigmaTy Type
ty
  , (TcTyVar
tv:[TcTyVar]
_) <- [TcTyVar]
tvs   -- At least one tyvar
  , forall (t :: * -> *) a. Foldable t => t a -> Bool
null ThetaType
theta      -- No constraints; see (DP1)
  , DynFlags -> TcTyVar -> Type -> CheckTyEqResult
checkTyVarEq DynFlags
dflags TcTyVar
tv Type
tau CheckTyEqResult -> CheckTyEqProblem -> Bool
`cterHasProblem` CheckTyEqProblem
cteInsolubleOccurs
       -- The tyvar actually occurs (DP2),
       --     and occurs in an injective position (DP3).
       -- Fortunately checkTyVarEq, used for the occur check,
       -- is just what we need.
  = Bool
True
  | Bool
otherwise
  = Bool
False

{- Note [Don't skolemise unnecessarily]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we are trying to solve
     ty_actual   <= ty_expected
    (Char->Char) <= (forall a. a->a)
We could skolemise the 'forall a', and then complain
that (Char ~ a) is insoluble; but that's a pretty obscure
error.  It's better to say that
    (Char->Char) ~ (forall a. a->a)
fails.

If we prematurely go to equality we'll reject a program we should
accept (e.g. #13752).  So the test (which is only to improve error
message) is very conservative:

 * ty_actual   is /definitely/ monomorphic: see `definitely_mono`
   This definitely_mono test comes in "shallow" and "deep" variants

 * ty_expected is /definitely/ polymorphic: see `definitely_poly`
   This definitely_poly test is more subtle than you might think.
   Here are three cases where expected_ty looks polymorphic, but
   isn't, and where it would be /wrong/ to switch to equality:

   (DP1)  (Char->Char) <= (forall a. (a~Char) => a -> a)

   (DP2)  (Char->Char) <= (forall a. Char -> Char)

   (DP3)  (Char->Char) <= (forall a. F [a] Char -> Char)
                          where type instance F [x] t = t


Note [Wrapper returned from tcSubMult]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is no notion of multiplicity coercion in Core, therefore the wrapper
returned by tcSubMult (and derived functions such as tcCheckUsage and
checkManyPattern) is quite unlike any other wrapper: it checks whether the
coercion produced by the constraint solver is trivial, producing a type error
is it is not. This is implemented via the WpMultCoercion wrapper, as desugared
by GHC.HsToCore.Binds.dsHsWrapper, which does the reflexivity check.

This wrapper needs to be placed in the term; otherwise, checking of the
eventual coercion won't be triggered during desugaring. But it can be put
anywhere, since it doesn't affect the desugared code.

Why do we check this in the desugarer? It's a convenient place, since it's
right after all the constraints are solved. We need the constraints to be
solved to check whether they are trivial or not. Plus there is precedent for
type errors during desuraging (such as the levity polymorphism
restriction). An alternative would be to have a kind of constraint which can
only produce trivial evidence, then this check would happen in the constraint
solver.
-}

tcSubMult :: CtOrigin -> Mult -> Mult -> TcM HsWrapper
tcSubMult :: CtOrigin -> Type -> Type -> TcM HsWrapper
tcSubMult CtOrigin
origin Type
w_actual Type
w_expected
  | Just (Type
w1, Type
w2) <- Type -> Maybe (Type, Type)
isMultMul Type
w_actual =
  do { HsWrapper
w1 <- CtOrigin -> Type -> Type -> TcM HsWrapper
tcSubMult CtOrigin
origin Type
w1 Type
w_expected
     ; HsWrapper
w2 <- CtOrigin -> Type -> Type -> TcM HsWrapper
tcSubMult CtOrigin
origin Type
w2 Type
w_expected
     ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
w1 HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
w2) }
  -- Currently, we consider p*q and sup p q to be equal.  Therefore, p*q <= r is
  -- equivalent to p <= r and q <= r.  For other cases, we approximate p <= q by p
  -- ~ q.  This is not complete, but it's sound. See also Note [Overapproximating
  -- multiplicities] in Multiplicity.
tcSubMult CtOrigin
origin Type
w_actual Type
w_expected =
  case Type -> Type -> IsSubmult
submult Type
w_actual Type
w_expected of
    IsSubmult
Submult -> forall (m :: * -> *) a. Monad m => a -> m a
return HsWrapper
WpHole
    IsSubmult
Unknown -> CtOrigin -> Type -> Type -> TcM HsWrapper
tcEqMult CtOrigin
origin Type
w_actual Type
w_expected

tcEqMult :: CtOrigin -> Mult -> Mult -> TcM HsWrapper
tcEqMult :: CtOrigin -> Type -> Type -> TcM HsWrapper
tcEqMult CtOrigin
origin Type
w_actual Type
w_expected = do
  {
  -- Note that here we do not call to `submult`, so we check
  -- for strict equality.
  ; Coercion
coercion <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
TypeLevel CtOrigin
origin Type
w_actual Type
w_expected
  ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ if Coercion -> Bool
isReflCo Coercion
coercion then HsWrapper
WpHole else Coercion -> HsWrapper
WpMultCoercion Coercion
coercion }


{- *********************************************************************
*                                                                      *
                    Deep subsumption
*                                                                      *
********************************************************************* -}

{- Note [Deep subsumption]
~~~~~~~~~~~~~~~~~~~~~~~~~~
The DeepSubsumption extension, documented here

    https://github.com/ghc-proposals/ghc-proposals/pull/511.

makes a best-efforts attempt implement deep subsumption as it was
prior to the the Simplify Subsumption proposal:

    https://github.com/ghc-proposals/ghc-proposals/pull/287

The effects are in these main places:

1. In the subsumption check, tcSubType, we must do deep skolemisation:
   see the call to tcDeeplySkolemise in tc_sub_type_deep

2. In tcPolyExpr we must do deep skolemisation:
   see the call to tcDeeplySkolemise in tcSkolemiseExpType

3. for expression type signatures (e :: ty), and functions with type
   signatures (e.g. f :: ty; f = e), we must deeply skolemise the type;
   see the call to tcDeeplySkolemise in tcSkolemiseScoped.

4. In GHC.Tc.Gen.App.tcApp we call tcSubTypeDS to match the result
   type. Without deep subsumption, unifyExpectedType would be sufficent.

In all these cases note that the deep skolemisation must be done /first/.
Consider (1)
     (forall a. Int -> a -> a)  <=  Int -> (forall b. b -> b)
We must skolemise the `forall b` before instantiating the `forall a`.
See also Note [Deep skolemisation].

Note that we /always/ use shallow subsumption in the ambiguity check.
See Note [Ambiguity check and deep subsumption].

Note [Deep skolemisation]
~~~~~~~~~~~~~~~~~~~~~~~~~
deeplySkolemise decomposes and skolemises a type, returning a type
with all its arrows visible (ie not buried under foralls)

Examples:

  deeplySkolemise (Int -> forall a. Ord a => blah)
    =  ( wp, [a], [d:Ord a], Int -> blah )
    where wp = \x:Int. /\a. \(d:Ord a). <hole> x

  deeplySkolemise  (forall a. Ord a => Maybe a -> forall b. Eq b => blah)
    =  ( wp, [a,b], [d1:Ord a,d2:Eq b], Maybe a -> blah )
    where wp = /\a.\(d1:Ord a).\(x:Maybe a)./\b.\(d2:Ord b). <hole> x

In general,
  if      deeplySkolemise ty = (wrap, tvs, evs, rho)
    and   e :: rho
  then    wrap e :: ty
    and   'wrap' binds tvs, evs

ToDo: this eta-abstraction plays fast and loose with termination,
      because it can introduce extra lambdas.  Maybe add a `seq` to
      fix this

Note [Setting the argument context]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider we are doing the ambiguity check for the (bogus)
  f :: (forall a b. C b => a -> a) -> Int

We'll call
   tcSubType ((forall a b. C b => a->a) -> Int )
             ((forall a b. C b => a->a) -> Int )

with a UserTypeCtxt of (FunSigCtxt "f").  Then we'll do the co/contra thing
on the argument type of the (->) -- and at that point we want to switch
to a UserTypeCtxt of GenSigCtxt.  Why?

* Error messages.  If we stick with FunSigCtxt we get errors like
     * Could not deduce: C b
       from the context: C b0
        bound by the type signature for:
            f :: forall a b. C b => a->a
  But of course f does not have that type signature!
  Example tests: T10508, T7220a, Simple14

* Implications. We may decide to build an implication for the whole
  ambiguity check, but we don't need one for each level within it,
  and TcUnify.alwaysBuildImplication checks the UserTypeCtxt.
  See Note [When to build an implication]

Note [Multiplicity in deep subsumption]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   t1 ->{mt} t2  <=   s1 ->{ms} s2

At the moment we /unify/ ms~mt, via tcEqMult.

Arguably we should use `tcSubMult`. But then if mt=m0 (a unification
variable) and ms=Many, `tcSubMult` is a no-op (since anything is a
sub-multiplicty of Many).  But then `m0` may never get unified with
anything.  It is then skolemised by the zonker; see GHC.HsToCore.Binds
Note [Free tyvars on rule LHS].  So we in RULE foldr/app in GHC.Base
we get this

 "foldr/app"     [1] forall ys m1 m2. foldr (\x{m1} \xs{m2}. (:) x xs) ys
                                       = \xs -> xs ++ ys

where we eta-expanded that (:).  But now foldr expects an argument
with ->{Many} and gets an argument with ->{m1} or ->{m2}, and Lint
complains.

The easiest solution was to use tcEqMult in tc_sub_type_ds, and
insist on equality. This is only in the DeepSubsumption code anyway.
-}

tc_sub_type_ds :: (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
               -> CtOrigin       -- Used when instantiating
               -> UserTypeCtxt   -- Used when skolemising
               -> TcSigmaType    -- Actual; a sigma-type
               -> TcRhoType      -- Expected; deeply skolemised
               -> TcM HsWrapper

-- If wrap = tc_sub_type_ds t1 t2
--    => wrap :: t1 ~> t2
-- Here is where the work actually happens!
-- Precondition: ty_expected is deeply skolemised

tc_sub_type_ds :: (Type -> Type -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tc_sub_type_ds Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_actual Type
ty_expected
  = do { String -> SDoc -> TcRn ()
traceTc String
"tc_sub_type_ds" forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ String -> SDoc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_actual
              , String -> SDoc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_expected ]
       ; Type -> Type -> TcM HsWrapper
go Type
ty_actual Type
ty_expected }
  where
    -- NB: 'go' is not recursive, except for doing tcView
    go :: Type -> Type -> TcM HsWrapper
go Type
ty_a Type
ty_e | Just Type
ty_a' <- Type -> Maybe Type
tcView Type
ty_a = Type -> Type -> TcM HsWrapper
go Type
ty_a' Type
ty_e
                 | Just Type
ty_e' <- Type -> Maybe Type
tcView Type
ty_e = Type -> Type -> TcM HsWrapper
go Type
ty_a  Type
ty_e'

    go (TyVarTy TcTyVar
tv_a) Type
ty_e
      = do { Maybe Type
lookup_res <- TcTyVar -> TcM (Maybe Type)
isFilledMetaTyVar_maybe TcTyVar
tv_a
           ; case Maybe Type
lookup_res of
               Just Type
ty_a' ->
                 do { String -> SDoc -> TcRn ()
traceTc String
"tc_sub_type_ds following filled meta-tyvar:"
                        (forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv_a SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"-->" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty_a')
                    ; (Type -> Type -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tc_sub_type_ds Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
ty_a' Type
ty_e }
               Maybe Type
Nothing -> Type -> Type -> TcM HsWrapper
just_unify Type
ty_actual Type
ty_expected }

    go ty_a :: Type
ty_a@(FunTy { ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
VisArg, ft_mult :: Type -> Type
ft_mult = Type
act_mult, ft_arg :: Type -> Type
ft_arg = Type
act_arg, ft_res :: Type -> Type
ft_res = Type
act_res })
       ty_e :: Type
ty_e@(FunTy { ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
VisArg, ft_mult :: Type -> Type
ft_mult = Type
exp_mult, ft_arg :: Type -> Type
ft_arg = Type
exp_arg, ft_res :: Type -> Type
ft_res = Type
exp_res })
      | Type -> Bool
isTauTy Type
ty_a, Type -> Bool
isTauTy Type
ty_e         -- Short cut common case to avoid
      = Type -> Type -> TcM HsWrapper
just_unify Type
ty_actual Type
ty_expected   -- unnecessary eta expansion

      | Bool
otherwise
      = -- This is where we do the co/contra thing, and generate a WpFun, which in turn
        -- causes eta-expansion, which we don't like; hence encouraging NoDeepSubsumption
        do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
           ; HsWrapper
arg_wrap  <- DynFlags
-> (Type -> Type -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> Type
-> Type
-> TcM HsWrapper
tc_sub_type_deep DynFlags
dflags Type -> Type -> TcM Coercion
unify CtOrigin
given_orig UserTypeCtxt
GenSigCtxt Type
exp_arg Type
act_arg
                          -- GenSigCtxt: See Note [Setting the argument context]
           ; HsWrapper
res_wrap  <- (Type -> Type -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tc_sub_type_ds   Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig  UserTypeCtxt
ctxt       Type
act_res Type
exp_res
           ; HsWrapper
mult_wrap <- CtOrigin -> Type -> Type -> TcM HsWrapper
tcEqMult CtOrigin
inst_orig Type
act_mult Type
exp_mult
                          -- See Note [Multiplicity in deep subsumption]
           ; let doc :: SDoc
doc = String -> SDoc
text String
"When checking that" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
quotes (forall a. Outputable a => a -> SDoc
ppr Type
ty_actual)
                   SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"is more polymorphic than" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
quotes (forall a. Outputable a => a -> SDoc
ppr Type
ty_expected)
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
mult_wrap HsWrapper -> HsWrapper -> HsWrapper
<.>
                     HsWrapper -> HsWrapper -> Scaled Type -> Type -> SDoc -> HsWrapper
mkWpFun HsWrapper
arg_wrap HsWrapper
res_wrap (forall a. Type -> a -> Scaled a
Scaled Type
exp_mult Type
exp_arg) Type
exp_res SDoc
doc)}
                     -- arg_wrap :: exp_arg ~> act_arg
                     -- res_wrap :: act-res ~> exp_res
      where
        given_orig :: CtOrigin
given_orig = SkolemInfo -> CtOrigin
GivenOrigin (UserTypeCtxt -> Type -> [(Name, TcTyVar)] -> SkolemInfo
SigSkol UserTypeCtxt
GenSigCtxt Type
exp_arg [])

    go Type
ty_a Type
ty_e
      | let ([TcTyVar]
tvs, ThetaType
theta, Type
_) = Type -> ([TcTyVar], ThetaType, Type)
tcSplitSigmaTy Type
ty_a
      , Bool -> Bool
not (forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs Bool -> Bool -> Bool
&& forall (t :: * -> *) a. Foldable t => t a -> Bool
null ThetaType
theta)
      = do { (HsWrapper
in_wrap, Type
in_rho) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
topInstantiate CtOrigin
inst_orig Type
ty_a
           ; HsWrapper
body_wrap <- (Type -> Type -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> Type -> Type -> TcM HsWrapper
tc_sub_type_ds Type -> Type -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt Type
in_rho Type
ty_e
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
body_wrap HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
in_wrap) }

      | Bool
otherwise   -- Revert to unification
      = do { -- It's still possible that ty_actual has nested foralls. Instantiate
             -- these, as there's no way unification will succeed with them in.
             -- See typecheck/should_compile/T11305 for an example of when this
             -- is important. The problem is that we're checking something like
             --  a -> forall b. b -> b     <=   alpha beta gamma
             -- where we end up with alpha := (->)
             (HsWrapper
inst_wrap, Type
rho_a) <- CtOrigin -> Type -> TcM (HsWrapper, Type)
deeplyInstantiate CtOrigin
inst_orig Type
ty_actual
           ; HsWrapper
unify_wrap         <- Type -> Type -> TcM HsWrapper
just_unify Type
rho_a Type
ty_expected
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
unify_wrap HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
inst_wrap) }

    just_unify :: Type -> Type -> TcM HsWrapper
just_unify Type
ty_a Type
ty_e = do { Coercion
cow <- Type -> Type -> TcM Coercion
unify Type
ty_a Type
ty_e
                              ; forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> HsWrapper
mkWpCastN Coercion
cow) }

tcDeeplySkolemise
    :: UserTypeCtxt -> TcSigmaType
    -> (TcType -> TcM result)
    -> TcM (HsWrapper, result)
        -- ^ The wrapper has type: spec_ty ~> expected_ty
-- Just like tcTopSkolemise, but calls
-- deeplySkolemise instead of topSkolemise
-- See Note [Deep skolemisation]
tcDeeplySkolemise :: forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcDeeplySkolemise UserTypeCtxt
ctxt Type
expected_ty Type -> TcM result
thing_inside
  | Type -> Bool
isTauTy Type
expected_ty  -- Short cut for common case
  = do { result
res <- Type -> TcM result
thing_inside Type
expected_ty
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, result
res) }
  | Bool
otherwise
  = do  { (HsWrapper
wrap, [(Name, TcTyVar)]
tv_prs, [TcTyVar]
given, Type
rho_ty) <- Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
deeplySkolemise Type
expected_ty
        ; let skol_info :: SkolemInfo
skol_info = UserTypeCtxt -> Type -> [(Name, TcTyVar)] -> SkolemInfo
SigSkol UserTypeCtxt
ctxt Type
expected_ty [(Name, TcTyVar)]
tv_prs
        ; String -> SDoc -> TcRn ()
traceTc String
"tcDeeplySkolemise" (forall a. Outputable a => a -> SDoc
ppr Type
expected_ty SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
rho_ty SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [(Name, TcTyVar)]
tv_prs)

        ; let skol_tvs :: [TcTyVar]
skol_tvs  = forall a b. (a -> b) -> [a] -> [b]
map forall a b. (a, b) -> b
snd [(Name, TcTyVar)]
tv_prs
        ; (TcEvBinds
ev_binds, result
result)
              <- forall result.
SkolemInfo
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given forall a b. (a -> b) -> a -> b
$
                 Type -> TcM result
thing_inside Type
rho_ty

        ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap HsWrapper -> HsWrapper -> HsWrapper
<.> TcEvBinds -> HsWrapper
mkWpLet TcEvBinds
ev_binds, result
result) }
          -- The ev_binds returned by checkConstraints is very
          -- often empty, in which case mkWpLet is a no-op

deeplySkolemise :: TcSigmaType
                -> TcM ( HsWrapper
                       , [(Name,TyVar)]     -- All skolemised variables
                       , [EvVar]            -- All "given"s
                       , TcRhoType )
-- See Note [Deep skolemisation]
deeplySkolemise :: Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
deeplySkolemise Type
ty
  = TCvSubst
-> Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
go TCvSubst
init_subst Type
ty
  where
    init_subst :: TCvSubst
init_subst = InScopeSet -> TCvSubst
mkEmptyTCvSubst (VarSet -> InScopeSet
mkInScopeSet (Type -> VarSet
tyCoVarsOfType Type
ty))

    go :: TCvSubst
-> Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
go TCvSubst
subst Type
ty
      | Just ([Scaled Type]
arg_tys, [TcTyVar]
tvs, ThetaType
theta, Type
ty') <- Type -> Maybe ([Scaled Type], [TcTyVar], ThetaType, Type)
tcDeepSplitSigmaTy_maybe Type
ty
      = do { let arg_tys' :: [Scaled Type]
arg_tys' = HasCallStack => TCvSubst -> [Scaled Type] -> [Scaled Type]
substScaledTys TCvSubst
subst [Scaled Type]
arg_tys
           ; [TcTyVar]
ids1           <- forall gbl lcl.
FastString -> [Scaled Type] -> TcRnIf gbl lcl [TcTyVar]
newSysLocalIds (String -> FastString
fsLit String
"dk") [Scaled Type]
arg_tys'
           ; (TCvSubst
subst', [TcTyVar]
tvs1) <- TCvSubst -> [TcTyVar] -> TcM (TCvSubst, [TcTyVar])
tcInstSkolTyVarsX TCvSubst
subst [TcTyVar]
tvs
           ; [TcTyVar]
ev_vars1       <- ThetaType -> TcM [TcTyVar]
newEvVars (HasCallStack => TCvSubst -> ThetaType -> ThetaType
substTheta TCvSubst
subst' ThetaType
theta)
           ; (HsWrapper
wrap, [(Name, TcTyVar)]
tvs_prs2, [TcTyVar]
ev_vars2, Type
rho) <- TCvSubst
-> Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
go TCvSubst
subst' Type
ty'
           ; let tv_prs1 :: [(Name, TcTyVar)]
tv_prs1 = forall a b. (a -> b) -> [a] -> [b]
map TcTyVar -> Name
tyVarName [TcTyVar]
tvs forall a b. [a] -> [b] -> [(a, b)]
`zip` [TcTyVar]
tvs1
           ; forall (m :: * -> *) a. Monad m => a -> m a
return ( [TcTyVar] -> HsWrapper
mkWpLams [TcTyVar]
ids1
                      HsWrapper -> HsWrapper -> HsWrapper
<.> [TcTyVar] -> HsWrapper
mkWpTyLams [TcTyVar]
tvs1
                      HsWrapper -> HsWrapper -> HsWrapper
<.> [TcTyVar] -> HsWrapper
mkWpLams [TcTyVar]
ev_vars1
                      HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap
                      HsWrapper -> HsWrapper -> HsWrapper
<.> [TcTyVar] -> HsWrapper
mkWpEvVarApps [TcTyVar]
ids1
                    , [(Name, TcTyVar)]
tv_prs1  forall a. [a] -> [a] -> [a]
++ [(Name, TcTyVar)]
tvs_prs2
                    , [TcTyVar]
ev_vars1 forall a. [a] -> [a] -> [a]
++ [TcTyVar]
ev_vars2
                    , [Scaled Type] -> Type -> Type
mkVisFunTys [Scaled Type]
arg_tys' Type
rho ) }

      | Bool
otherwise
      = forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, [], [], HasCallStack => TCvSubst -> Type -> Type
substTy TCvSubst
subst Type
ty)
        -- substTy is a quick no-op on an empty substitution

deeplyInstantiate :: CtOrigin -> TcType -> TcM (HsWrapper, Type)
deeplyInstantiate :: CtOrigin -> Type -> TcM (HsWrapper, Type)
deeplyInstantiate CtOrigin
orig Type
ty
  = TCvSubst -> Type -> TcM (HsWrapper, Type)
go TCvSubst
init_subst Type
ty
  where
    init_subst :: TCvSubst
init_subst = InScopeSet -> TCvSubst
mkEmptyTCvSubst (VarSet -> InScopeSet
mkInScopeSet (Type -> VarSet
tyCoVarsOfType Type
ty))

    go :: TCvSubst -> Type -> TcM (HsWrapper, Type)
go TCvSubst
subst Type
ty
      | Just ([Scaled Type]
arg_tys, [TcTyVar]
tvs, ThetaType
theta, Type
rho) <- Type -> Maybe ([Scaled Type], [TcTyVar], ThetaType, Type)
tcDeepSplitSigmaTy_maybe Type
ty
      = do { (TCvSubst
subst', [TcTyVar]
tvs') <- TCvSubst -> [TcTyVar] -> TcM (TCvSubst, [TcTyVar])
newMetaTyVarsX TCvSubst
subst [TcTyVar]
tvs
           ; let arg_tys' :: [Scaled Type]
arg_tys' = HasCallStack => TCvSubst -> [Scaled Type] -> [Scaled Type]
substScaledTys   TCvSubst
subst' [Scaled Type]
arg_tys
                 theta' :: ThetaType
theta'   = HasCallStack => TCvSubst -> ThetaType -> ThetaType
substTheta TCvSubst
subst' ThetaType
theta
           ; [TcTyVar]
ids1  <- forall gbl lcl.
FastString -> [Scaled Type] -> TcRnIf gbl lcl [TcTyVar]
newSysLocalIds (String -> FastString
fsLit String
"di") [Scaled Type]
arg_tys'
           ; HsWrapper
wrap1 <- CtOrigin -> ThetaType -> ThetaType -> TcM HsWrapper
instCall CtOrigin
orig ([TcTyVar] -> ThetaType
mkTyVarTys [TcTyVar]
tvs') ThetaType
theta'
           ; (HsWrapper
wrap2, Type
rho2) <- TCvSubst -> Type -> TcM (HsWrapper, Type)
go TCvSubst
subst' Type
rho
           ; forall (m :: * -> *) a. Monad m => a -> m a
return ([TcTyVar] -> HsWrapper
mkWpLams [TcTyVar]
ids1
                        HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap2
                        HsWrapper -> HsWrapper -> HsWrapper
<.> HsWrapper
wrap1
                        HsWrapper -> HsWrapper -> HsWrapper
<.> [TcTyVar] -> HsWrapper
mkWpEvVarApps [TcTyVar]
ids1,
                     [Scaled Type] -> Type -> Type
mkVisFunTys [Scaled Type]
arg_tys' Type
rho2) }

      | Bool
otherwise
      = do { let ty' :: Type
ty' = HasCallStack => TCvSubst -> Type -> Type
substTy TCvSubst
subst Type
ty
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, Type
ty') }

tcDeepSplitSigmaTy_maybe
  :: TcSigmaType -> Maybe ([Scaled TcType], [TyVar], ThetaType, TcSigmaType)
-- Looks for a *non-trivial* quantified type, under zero or more function arrows
-- By "non-trivial" we mean either tyvars or constraints are non-empty

tcDeepSplitSigmaTy_maybe :: Type -> Maybe ([Scaled Type], [TcTyVar], ThetaType, Type)
tcDeepSplitSigmaTy_maybe Type
ty
  | Just (Scaled Type
arg_ty, Type
res_ty)           <- Type -> Maybe (Scaled Type, Type)
tcSplitFunTy_maybe Type
ty
  , Just ([Scaled Type]
arg_tys, [TcTyVar]
tvs, ThetaType
theta, Type
rho) <- Type -> Maybe ([Scaled Type], [TcTyVar], ThetaType, Type)
tcDeepSplitSigmaTy_maybe Type
res_ty
  = forall a. a -> Maybe a
Just (Scaled Type
arg_tyforall a. a -> [a] -> [a]
:[Scaled Type]
arg_tys, [TcTyVar]
tvs, ThetaType
theta, Type
rho)

  | ([TcTyVar]
tvs, ThetaType
theta, Type
rho) <- Type -> ([TcTyVar], ThetaType, Type)
tcSplitSigmaTy Type
ty
  , Bool -> Bool
not (forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs Bool -> Bool -> Bool
&& forall (t :: * -> *) a. Foldable t => t a -> Bool
null ThetaType
theta)
  = forall a. a -> Maybe a
Just ([], [TcTyVar]
tvs, ThetaType
theta, Type
rho)

  | Bool
otherwise = forall a. Maybe a
Nothing


{- *********************************************************************
*                                                                      *
                    Generalisation
*                                                                      *
********************************************************************* -}

{- Note [Skolemisation]
~~~~~~~~~~~~~~~~~~~~~~~
tcTopSkolemise takes "expected type" and strip off quantifiers to expose the
type underneath, binding the new skolems for the 'thing_inside'
The returned 'HsWrapper' has type (specific_ty -> expected_ty).

Note that for a nested type like
   forall a. Eq a => forall b. Ord b => blah
we still only build one implication constraint
   forall a b. (Eq a, Ord b) => <constraints>
This is just an optimisation, but it's why we use topSkolemise to
build the pieces from all the layers, before making a single call
to checkConstraints.

tcSkolemiseScoped is very similar, but differs in two ways:

* It deals specially with just the outer forall, bringing those type
  variables into lexical scope.  To my surprise, I found that doing
  this unconditionally in tcTopSkolemise (i.e. doing it even if we don't
  need to bring the variables into lexical scope, which is harmless)
  caused a non-trivial (1%-ish) perf hit on the compiler.

* It handles deep subumption, wheres tcTopSkolemise never looks under
  function arrows.

* It always calls checkConstraints, even if there are no skolem
  variables at all.  Reason: there might be nested deferred errors
  that must not be allowed to float to top level.
  See Note [When to build an implication] below.
-}

tcTopSkolemise, tcSkolemiseScoped
    :: UserTypeCtxt -> TcSigmaType
    -> (TcType -> TcM result)
    -> TcM (HsWrapper, result)
        -- ^ The wrapper has type: spec_ty ~> expected_ty
-- See Note [Skolemisation] for the differences between
-- tcSkolemiseScoped and tcTopSkolemise

tcSkolemiseScoped :: forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcSkolemiseScoped UserTypeCtxt
ctxt Type
expected_ty Type -> TcM result
thing_inside
  = do { Bool
deep_subsumption <- forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
xoptM Extension
LangExt.DeepSubsumption
       ; let skolemise :: Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
skolemise | Bool
deep_subsumption = Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
deeplySkolemise
                       | Bool
otherwise        = Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
topSkolemise
       ; (HsWrapper
wrap, [(Name, TcTyVar)]
tv_prs, [TcTyVar]
given, Type
rho_ty) <- Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
skolemise Type
expected_ty

       ; let skol_tvs :: [TcTyVar]
skol_tvs = forall a b. (a -> b) -> [a] -> [b]
map forall a b. (a, b) -> b
snd [(Name, TcTyVar)]
tv_prs
             skol_info :: SkolemInfo
skol_info = UserTypeCtxt -> Type -> [(Name, TcTyVar)] -> SkolemInfo
SigSkol UserTypeCtxt
ctxt Type
expected_ty [(Name, TcTyVar)]
tv_prs
       ; (TcEvBinds
ev_binds, result
res)
             <- forall result.
SkolemInfo
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given forall a b. (a -> b) -> a -> b
$
                forall r. [(Name, TcTyVar)] -> TcM r -> TcM r
tcExtendNameTyVarEnv [(Name, TcTyVar)]
tv_prs               forall a b. (a -> b) -> a -> b
$
                Type -> TcM result
thing_inside Type
rho_ty

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap HsWrapper -> HsWrapper -> HsWrapper
<.> TcEvBinds -> HsWrapper
mkWpLet TcEvBinds
ev_binds, result
res) }

tcTopSkolemise :: forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcTopSkolemise UserTypeCtxt
ctxt Type
expected_ty Type -> TcM result
thing_inside
  | Type -> Bool
isRhoTy Type
expected_ty  -- Short cut for common case
  = do { result
res <- Type -> TcM result
thing_inside Type
expected_ty
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, result
res) }
  | Bool
otherwise
  = do { rec { let skol_info :: SkolemInfo
skol_info = UserTypeCtxt -> Type -> [(Name, TcTyVar)] -> SkolemInfo
SigSkol UserTypeCtxt
ctxt Type
expected_ty [(Name, TcTyVar)]
tv_prs
             ; (HsWrapper
wrap, [(Name, TcTyVar)]
tv_prs, [TcTyVar]
given, Type
rho_ty) <- Type -> TcM (HsWrapper, [(Name, TcTyVar)], [TcTyVar], Type)
topSkolemise Type
expected_ty
             }

       ; let skol_tvs :: [TcTyVar]
skol_tvs = forall a b. (a -> b) -> [a] -> [b]
map forall a b. (a, b) -> b
snd [(Name, TcTyVar)]
tv_prs
       ; (TcEvBinds
ev_binds, result
result)
             <- forall result.
SkolemInfo
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given forall a b. (a -> b) -> a -> b
$
                Type -> TcM result
thing_inside Type
rho_ty

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
wrap HsWrapper -> HsWrapper -> HsWrapper
<.> TcEvBinds -> HsWrapper
mkWpLet TcEvBinds
ev_binds, result
result) }
         -- The ev_binds returned by checkConstraints is very
        -- often empty, in which case mkWpLet is a no-op

-- | Variant of 'tcTopSkolemise' that takes an ExpType
tcSkolemiseExpType :: UserTypeCtxt -> ExpSigmaType
                   -> (ExpRhoType -> TcM result)
                   -> TcM (HsWrapper, result)
tcSkolemiseExpType :: forall result.
UserTypeCtxt
-> ExpRhoType
-> (ExpRhoType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseExpType UserTypeCtxt
_ et :: ExpRhoType
et@(Infer {}) ExpRhoType -> TcM result
thing_inside
  = (HsWrapper
idHsWrapper, ) forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> ExpRhoType -> TcM result
thing_inside ExpRhoType
et
tcSkolemiseExpType UserTypeCtxt
ctxt (Check Type
ty) ExpRhoType -> TcM result
thing_inside
  = do { Bool
deep_subsumption <- forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
xoptM Extension
LangExt.DeepSubsumption
       ; let skolemise :: UserTypeCtxt
-> Type
-> (Type -> TcM result)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, result)
skolemise | Bool
deep_subsumption = forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcDeeplySkolemise
                       | Bool
otherwise        = forall result.
UserTypeCtxt
-> Type -> (Type -> TcM result) -> TcM (HsWrapper, result)
tcTopSkolemise
       ; UserTypeCtxt
-> Type
-> (Type -> TcM result)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, result)
skolemise UserTypeCtxt
ctxt Type
ty forall a b. (a -> b) -> a -> b
$ \Type
rho_ty ->
         ExpRhoType -> TcM result
thing_inside (Type -> ExpRhoType
mkCheckExpType Type
rho_ty) }

checkConstraints :: SkolemInfo
                 -> [TcTyVar]           -- Skolems
                 -> [EvVar]             -- Given
                 -> TcM result
                 -> TcM (TcEvBinds, result)

checkConstraints :: forall result.
SkolemInfo
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given TcM result
thing_inside
  = do { Bool
implication_needed <- SkolemInfo -> [TcTyVar] -> [TcTyVar] -> TcM Bool
implicationNeeded SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given

       ; if Bool
implication_needed
         then do { (TcLevel
tclvl, WantedConstraints
wanted, result
result) <- forall a. TcM a -> TcM (TcLevel, WantedConstraints, a)
pushLevelAndCaptureConstraints TcM result
thing_inside
                 ; (Bag Implication
implics, TcEvBinds
ev_binds) <- TcLevel
-> SkolemInfo
-> [TcTyVar]
-> [TcTyVar]
-> WantedConstraints
-> TcM (Bag Implication, TcEvBinds)
buildImplicationFor TcLevel
tclvl SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given WantedConstraints
wanted
                 ; String -> SDoc -> TcRn ()
traceTc String
"checkConstraints" (forall a. Outputable a => a -> SDoc
ppr TcLevel
tclvl SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr [TcTyVar]
skol_tvs)
                 ; Bag Implication -> TcRn ()
emitImplications Bag Implication
implics
                 ; forall (m :: * -> *) a. Monad m => a -> m a
return (TcEvBinds
ev_binds, result
result) }

         else -- Fast path.  We check every function argument with tcCheckPolyExpr,
              -- which uses tcTopSkolemise and hence checkConstraints.
              -- So this fast path is well-exercised
              do { result
res <- TcM result
thing_inside
                 ; forall (m :: * -> *) a. Monad m => a -> m a
return (TcEvBinds
emptyTcEvBinds, result
res) } }

checkTvConstraints :: SkolemInfo
                   -> [TcTyVar]          -- Skolem tyvars
                   -> TcM result
                   -> TcM result

checkTvConstraints :: forall result. SkolemInfo -> [TcTyVar] -> TcM result -> TcM result
checkTvConstraints SkolemInfo
skol_info [TcTyVar]
skol_tvs TcM result
thing_inside
  = do { (TcLevel
tclvl, WantedConstraints
wanted, result
result) <- forall a. TcM a -> TcM (TcLevel, WantedConstraints, a)
pushLevelAndCaptureConstraints TcM result
thing_inside
       ; SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcRn ()
emitResidualTvConstraint SkolemInfo
skol_info [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
       ; forall (m :: * -> *) a. Monad m => a -> m a
return result
result }

emitResidualTvConstraint :: SkolemInfo -> [TcTyVar]
                         -> TcLevel -> WantedConstraints -> TcM ()
emitResidualTvConstraint :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcRn ()
emitResidualTvConstraint SkolemInfo
skol_info [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
  | Bool -> Bool
not (WantedConstraints -> Bool
isEmptyWC WantedConstraints
wanted) Bool -> Bool -> Bool
||
    SkolemInfo -> Bool
checkTelescopeSkol SkolemInfo
skol_info
  = -- checkTelescopeSkol: in this case, /always/ emit this implication
    -- even if 'wanted' is empty. We need the implication so that we check
    -- for a bad telescope. See Note [Skolem escape and forall-types] in
    -- GHC.Tc.Gen.HsType
    do { Implication
implic <- SkolemInfo
-> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
buildTvImplication SkolemInfo
skol_info [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
       ; Implication -> TcRn ()
emitImplication Implication
implic }

  | Bool
otherwise  -- Empty 'wanted', emit nothing
  = forall (m :: * -> *) a. Monad m => a -> m a
return ()

buildTvImplication :: SkolemInfo -> [TcTyVar]
                   -> TcLevel -> WantedConstraints -> TcM Implication
buildTvImplication :: SkolemInfo
-> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
buildTvImplication SkolemInfo
skol_info [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
  = do { EvBindsVar
ev_binds <- TcM EvBindsVar
newNoTcEvBinds  -- Used for equalities only, so all the constraints
                                     -- are solved by filling in coercion holes, not
                                     -- by creating a value-level evidence binding
       ; Implication
implic   <- TcM Implication
newImplication

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (Implication
implic { ic_tclvl :: TcLevel
ic_tclvl     = TcLevel
tclvl
                        , ic_skols :: [TcTyVar]
ic_skols     = [TcTyVar]
skol_tvs
                        , ic_given_eqs :: HasGivenEqs
ic_given_eqs = HasGivenEqs
NoGivenEqs
                        , ic_wanted :: WantedConstraints
ic_wanted    = WantedConstraints
wanted
                        , ic_binds :: EvBindsVar
ic_binds     = EvBindsVar
ev_binds
                        , ic_info :: SkolemInfo
ic_info      = SkolemInfo
skol_info }) }

implicationNeeded :: SkolemInfo -> [TcTyVar] -> [EvVar] -> TcM Bool
-- See Note [When to build an implication]
implicationNeeded :: SkolemInfo -> [TcTyVar] -> [TcTyVar] -> TcM Bool
implicationNeeded SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given
  | forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
skol_tvs
  , forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
given
  , Bool -> Bool
not (SkolemInfo -> Bool
alwaysBuildImplication SkolemInfo
skol_info)
  = -- Empty skolems and givens
    do { TcLevel
tc_lvl <- TcM TcLevel
getTcLevel
       ; if Bool -> Bool
not (TcLevel -> Bool
isTopTcLevel TcLevel
tc_lvl)  -- No implication needed if we are
         then forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False             -- already inside an implication
         else
    do { DynFlags
dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags       -- If any deferral can happen,
                                     -- we must build an implication
       ; forall (m :: * -> *) a. Monad m => a -> m a
return (GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_DeferTypeErrors DynFlags
dflags Bool -> Bool -> Bool
||
                 GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_DeferTypedHoles DynFlags
dflags Bool -> Bool -> Bool
||
                 GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_DeferOutOfScopeVariables DynFlags
dflags) } }

  | Bool
otherwise     -- Non-empty skolems or givens
  = forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True   -- Definitely need an implication

alwaysBuildImplication :: SkolemInfo -> Bool
-- See Note [When to build an implication]
alwaysBuildImplication :: SkolemInfo -> Bool
alwaysBuildImplication SkolemInfo
_ = Bool
False

{-  Commmented out for now while I figure out about error messages.
    See #14185

alwaysBuildImplication (SigSkol ctxt _ _)
  = case ctxt of
      FunSigCtxt {} -> True  -- RHS of a binding with a signature
      _             -> False
alwaysBuildImplication (RuleSkol {})      = True
alwaysBuildImplication (InstSkol {})      = True
alwaysBuildImplication (FamInstSkol {})   = True
alwaysBuildImplication _                  = False
-}

buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar]
                   -> [EvVar] -> WantedConstraints
                   -> TcM (Bag Implication, TcEvBinds)
buildImplicationFor :: TcLevel
-> SkolemInfo
-> [TcTyVar]
-> [TcTyVar]
-> WantedConstraints
-> TcM (Bag Implication, TcEvBinds)
buildImplicationFor TcLevel
tclvl SkolemInfo
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given WantedConstraints
wanted
  | WantedConstraints -> Bool
isEmptyWC WantedConstraints
wanted Bool -> Bool -> Bool
&& forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
given
             -- Optimisation : if there are no wanteds, and no givens
             -- don't generate an implication at all.
             -- Reason for the (null given): we don't want to lose
             -- the "inaccessible alternative" error check
  = forall (m :: * -> *) a. Monad m => a -> m a
return (forall a. Bag a
emptyBag, TcEvBinds
emptyTcEvBinds)

  | Bool
otherwise
  = ASSERT2( all (isSkolemTyVar <||> isTyVarTyVar) skol_tvs, ppr skol_tvs )
      -- Why allow TyVarTvs? Because implicitly declared kind variables in
      -- non-CUSK type declarations are TyVarTvs, and we need to bring them
      -- into scope as a skolem in an implication. This is OK, though,
      -- because TyVarTvs will always remain tyvars, even after unification.
    do { EvBindsVar
ev_binds_var <- TcM EvBindsVar
newTcEvBinds
       ; Implication
implic <- TcM Implication
newImplication
       ; let implic' :: Implication
implic' = Implication
implic { ic_tclvl :: TcLevel
ic_tclvl  = TcLevel
tclvl
                              , ic_skols :: [TcTyVar]
ic_skols  = [TcTyVar]
skol_tvs
                              , ic_given :: [TcTyVar]
ic_given  = [TcTyVar]
given
                              , ic_wanted :: WantedConstraints
ic_wanted = WantedConstraints
wanted
                              , ic_binds :: EvBindsVar
ic_binds  = EvBindsVar
ev_binds_var
                              , ic_info :: SkolemInfo
ic_info   = SkolemInfo
skol_info }

       ; forall (m :: * -> *) a. Monad m => a -> m a
return (forall a. a -> Bag a
unitBag Implication
implic', EvBindsVar -> TcEvBinds
TcEvBinds EvBindsVar
ev_binds_var) }

{- Note [When to build an implication]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have some 'skolems' and some 'givens', and we are
considering whether to wrap the constraints in their scope into an
implication.  We must /always/ so if either 'skolems' or 'givens' are
non-empty.  But what if both are empty?  You might think we could
always drop the implication.  Other things being equal, the fewer
implications the better.  Less clutter and overhead.  But we must
take care:

* If we have an unsolved [W] g :: a ~# b, and -fdefer-type-errors,
  we'll make a /term-level/ evidence binding for 'g = error "blah"'.
  We must have an EvBindsVar those bindings!, otherwise they end up as
  top-level unlifted bindings, which are verboten. This only matters
  at top level, so we check for that
  See also Note [Deferred errors for coercion holes] in GHC.Tc.Errors.
  cf #14149 for an example of what goes wrong.

* If you have
     f :: Int;  f = f_blah
     g :: Bool; g = g_blah
  If we don't build an implication for f or g (no tyvars, no givens),
  the constraints for f_blah and g_blah are solved together.  And that
  can yield /very/ confusing error messages, because we can get
      [W] C Int b1    -- from f_blah
      [W] C Int b2    -- from g_blan
  and fundpes can yield [D] b1 ~ b2, even though the two functions have
  literally nothing to do with each other.  #14185 is an example.
  Building an implication keeps them separate.
-}

{-
************************************************************************
*                                                                      *
                Boxy unification
*                                                                      *
************************************************************************

The exported functions are all defined as versions of some
non-exported generic functions.
-}

unifyType :: Maybe SDoc  -- ^ If present, the thing that has type ty1
          -> TcTauType -> TcTauType    -- ty1, ty2
          -> TcM TcCoercionN           -- :: ty1 ~# ty2
-- Actual and expected types
-- Returns a coercion : ty1 ~ ty2
unifyType :: Maybe SDoc -> Type -> Type -> TcM Coercion
unifyType Maybe SDoc
thing Type
ty1 Type
ty2
  = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
TypeLevel CtOrigin
origin Type
ty1 Type
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: Type
uo_actual   = Type
ty1
                          , uo_expected :: Type
uo_expected = Type
ty2
                          , uo_thing :: Maybe SDoc
uo_thing    = forall a. Outputable a => a -> SDoc
ppr forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Maybe SDoc
thing
                          , uo_visible :: Bool
uo_visible  = Bool
True }

unifyTypeET :: TcTauType -> TcTauType -> TcM CoercionN
-- Like unifyType, but swap expected and actual in error messages
-- This is used when typechecking patterns
unifyTypeET :: Type -> Type -> TcM Coercion
unifyTypeET Type
ty1 Type
ty2
  = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
TypeLevel CtOrigin
origin Type
ty1 Type
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: Type
uo_actual   = Type
ty2   -- NB swapped
                          , uo_expected :: Type
uo_expected = Type
ty1   -- NB swapped
                          , uo_thing :: Maybe SDoc
uo_thing    = forall a. Maybe a
Nothing
                          , uo_visible :: Bool
uo_visible  = Bool
True }


unifyKind :: Maybe SDoc -> TcKind -> TcKind -> TcM CoercionN
unifyKind :: Maybe SDoc -> Type -> Type -> TcM Coercion
unifyKind Maybe SDoc
mb_thing Type
ty1 Type
ty2
  = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
KindLevel CtOrigin
origin Type
ty1 Type
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: Type
uo_actual   = Type
ty1
                          , uo_expected :: Type
uo_expected = Type
ty2
                          , uo_thing :: Maybe SDoc
uo_thing    = Maybe SDoc
mb_thing
                          , uo_visible :: Bool
uo_visible  = Bool
True }


{-
%************************************************************************
%*                                                                      *
                 uType and friends
%*                                                                      *
%************************************************************************

uType is the heart of the unifier.
-}

uType, uType_defer
  :: TypeOrKind
  -> CtOrigin
  -> TcType    -- ty1 is the *actual* type
  -> TcType    -- ty2 is the *expected* type
  -> TcM CoercionN

--------------
-- It is always safe to defer unification to the main constraint solver
-- See Note [Deferred unification]
uType_defer :: TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType_defer TypeOrKind
t_or_k CtOrigin
origin Type
ty1 Type
ty2
  = do { Coercion
co <- CtOrigin -> TypeOrKind -> Role -> Type -> Type -> TcM Coercion
emitWantedEq CtOrigin
origin TypeOrKind
t_or_k Role
Nominal Type
ty1 Type
ty2

       -- Error trace only
       -- NB. do *not* call mkErrInfo unless tracing is on,
       --     because it is hugely expensive (#5631)
       ; forall gbl lcl. DumpFlag -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
whenDOptM DumpFlag
Opt_D_dump_tc_trace forall a b. (a -> b) -> a -> b
$ do
            { [ErrCtxt]
ctxt <- TcM [ErrCtxt]
getErrCtxt
            ; SDoc
doc <- TidyEnv -> [ErrCtxt] -> TcM SDoc
mkErrInfo TidyEnv
emptyTidyEnv [ErrCtxt]
ctxt
            ; String -> SDoc -> TcRn ()
traceTc String
"utype_defer" ([SDoc] -> SDoc
vcat [ Type -> SDoc
debugPprType Type
ty1
                                          , Type -> SDoc
debugPprType Type
ty2
                                          , CtOrigin -> SDoc
pprCtOrigin CtOrigin
origin
                                          , SDoc
doc])
            ; String -> SDoc -> TcRn ()
traceTc String
"utype_defer2" (forall a. Outputable a => a -> SDoc
ppr Coercion
co)
            }
       ; forall (m :: * -> *) a. Monad m => a -> m a
return Coercion
co }

--------------
uType :: TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
orig_ty1 Type
orig_ty2
  = do { TcLevel
tclvl <- TcM TcLevel
getTcLevel
       ; String -> SDoc -> TcRn ()
traceTc String
"u_tys" forall a b. (a -> b) -> a -> b
$ [SDoc] -> SDoc
vcat
              [ String -> SDoc
text String
"tclvl" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr TcLevel
tclvl
              , [SDoc] -> SDoc
sep [ forall a. Outputable a => a -> SDoc
ppr Type
orig_ty1, String -> SDoc
text String
"~", forall a. Outputable a => a -> SDoc
ppr Type
orig_ty2]
              , CtOrigin -> SDoc
pprCtOrigin CtOrigin
origin]
       ; Coercion
co <- Type -> Type -> TcM Coercion
go Type
orig_ty1 Type
orig_ty2
       ; if Coercion -> Bool
isReflCo Coercion
co
            then String -> SDoc -> TcRn ()
traceTc String
"u_tys yields no coercion" SDoc
Outputable.empty
            else String -> SDoc -> TcRn ()
traceTc String
"u_tys yields coercion:" (forall a. Outputable a => a -> SDoc
ppr Coercion
co)
       ; forall (m :: * -> *) a. Monad m => a -> m a
return Coercion
co }
  where
    go :: TcType -> TcType -> TcM CoercionN
        -- The arguments to 'go' are always semantically identical
        -- to orig_ty{1,2} except for looking through type synonyms

     -- Unwrap casts before looking for variables. This way, we can easily
     -- recognize (t |> co) ~ (t |> co), which is nice. Previously, we
     -- didn't do it this way, and then the unification above was deferred.
    go :: Type -> Type -> TcM Coercion
go (CastTy Type
t1 Coercion
co1) Type
t2
      = do { Coercion
co_tys <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
t1 Type
t2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Type -> Coercion -> Coercion -> Coercion
mkCoherenceLeftCo Role
Nominal Type
t1 Coercion
co1 Coercion
co_tys) }

    go Type
t1 (CastTy Type
t2 Coercion
co2)
      = do { Coercion
co_tys <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
t1 Type
t2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Type -> Coercion -> Coercion -> Coercion
mkCoherenceRightCo Role
Nominal Type
t2 Coercion
co2 Coercion
co_tys) }

        -- Variables; go for uUnfilledVar
        -- Note that we pass in *original* (before synonym expansion),
        -- so that type variables tend to get filled in with
        -- the most informative version of the type
    go (TyVarTy TcTyVar
tv1) Type
ty2
      = do { Maybe Type
lookup_res <- TcTyVar -> TcM (Maybe Type)
isFilledMetaTyVar_maybe TcTyVar
tv1
           ; case Maybe Type
lookup_res of
               Just Type
ty1 -> do { String -> SDoc -> TcRn ()
traceTc String
"found filled tyvar" (forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv1 SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":->" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty1)
                                ; Type -> Type -> TcM Coercion
go Type
ty1 Type
ty2 }
               Maybe Type
Nothing  -> CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar CtOrigin
origin TypeOrKind
t_or_k SwapFlag
NotSwapped TcTyVar
tv1 Type
ty2 }
    go Type
ty1 (TyVarTy TcTyVar
tv2)
      = do { Maybe Type
lookup_res <- TcTyVar -> TcM (Maybe Type)
isFilledMetaTyVar_maybe TcTyVar
tv2
           ; case Maybe Type
lookup_res of
               Just Type
ty2 -> do { String -> SDoc -> TcRn ()
traceTc String
"found filled tyvar" (forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv2 SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
":->" SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr Type
ty2)
                              ; Type -> Type -> TcM Coercion
go Type
ty1 Type
ty2 }
               Maybe Type
Nothing  -> CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar CtOrigin
origin TypeOrKind
t_or_k SwapFlag
IsSwapped TcTyVar
tv2 Type
ty1 }

      -- See Note [Expanding synonyms during unification]
    go ty1 :: Type
ty1@(TyConApp TyCon
tc1 []) (TyConApp TyCon
tc2 [])
      | TyCon
tc1 forall a. Eq a => a -> a -> Bool
== TyCon
tc2
      = forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ Type -> Coercion
mkNomReflCo Type
ty1

        -- See Note [Expanding synonyms during unification]
        --
        -- Also NB that we recurse to 'go' so that we don't push a
        -- new item on the origin stack. As a result if we have
        --   type Foo = Int
        -- and we try to unify  Foo ~ Bool
        -- we'll end up saying "can't match Foo with Bool"
        -- rather than "can't match "Int with Bool".  See #4535.
    go Type
ty1 Type
ty2
      | Just Type
ty1' <- Type -> Maybe Type
tcView Type
ty1 = Type -> Type -> TcM Coercion
go Type
ty1' Type
ty2
      | Just Type
ty2' <- Type -> Maybe Type
tcView Type
ty2 = Type -> Type -> TcM Coercion
go Type
ty1  Type
ty2'

    -- Functions (t1 -> t2) just check the two parts
    -- Do not attempt (c => t); just defer
    go (FunTy { ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
VisArg, ft_mult :: Type -> Type
ft_mult = Type
w1, ft_arg :: Type -> Type
ft_arg = Type
arg1, ft_res :: Type -> Type
ft_res = Type
res1 })
       (FunTy { ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
VisArg, ft_mult :: Type -> Type
ft_mult = Type
w2, ft_arg :: Type -> Type
ft_arg = Type
arg2, ft_res :: Type -> Type
ft_res = Type
res2 })
      = do { Coercion
co_l <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
arg1 Type
arg2
           ; Coercion
co_r <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
res1 Type
res2
           ; Coercion
co_w <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
w1 Type
w2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ Role -> Coercion -> Coercion -> Coercion -> Coercion
mkFunCo Role
Nominal Coercion
co_w Coercion
co_l Coercion
co_r }

        -- Always defer if a type synonym family (type function)
        -- is involved.  (Data families behave rigidly.)
    go ty1 :: Type
ty1@(TyConApp TyCon
tc1 ThetaType
_) Type
ty2
      | TyCon -> Bool
isTypeFamilyTyCon TyCon
tc1 = Type -> Type -> TcM Coercion
defer Type
ty1 Type
ty2
    go Type
ty1 ty2 :: Type
ty2@(TyConApp TyCon
tc2 ThetaType
_)
      | TyCon -> Bool
isTypeFamilyTyCon TyCon
tc2 = Type -> Type -> TcM Coercion
defer Type
ty1 Type
ty2

    go (TyConApp TyCon
tc1 ThetaType
tys1) (TyConApp TyCon
tc2 ThetaType
tys2)
      -- See Note [Mismatched type lists and application decomposition]
      | TyCon
tc1 forall a. Eq a => a -> a -> Bool
== TyCon
tc2, forall a b. [a] -> [b] -> Bool
equalLength ThetaType
tys1 ThetaType
tys2
      = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 )
        do { [Coercion]
cos <- forall (m :: * -> *) a b c d.
Monad m =>
(a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d]
zipWith3M (TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k) [CtOrigin]
origins' ThetaType
tys1 ThetaType
tys2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ HasDebugCallStack => Role -> TyCon -> [Coercion] -> Coercion
mkTyConAppCo Role
Nominal TyCon
tc1 [Coercion]
cos }
      where
        origins' :: [CtOrigin]
origins' = forall a b. (a -> b) -> [a] -> [b]
map (\Bool
is_vis -> if Bool
is_vis then CtOrigin
origin else CtOrigin -> CtOrigin
toInvisibleOrigin CtOrigin
origin)
                       (TyCon -> [Bool]
tcTyConVisibilities TyCon
tc1)

    go (LitTy TyLit
m) ty :: Type
ty@(LitTy TyLit
n)
      | TyLit
m forall a. Eq a => a -> a -> Bool
== TyLit
n
      = forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ Type -> Coercion
mkNomReflCo Type
ty

        -- See Note [Care with type applications]
        -- Do not decompose FunTy against App;
        -- it's often a type error, so leave it for the constraint solver
    go (AppTy Type
s1 Type
t1) (AppTy Type
s2 Type
t2)
      = Bool -> Type -> Type -> Type -> Type -> TcM Coercion
go_app (Type -> Bool
isNextArgVisible Type
s1) Type
s1 Type
t1 Type
s2 Type
t2

    go (AppTy Type
s1 Type
t1) (TyConApp TyCon
tc2 ThetaType
ts2)
      | Just (ThetaType
ts2', Type
t2') <- forall a. [a] -> Maybe ([a], a)
snocView ThetaType
ts2
      = ASSERT( not (mustBeSaturated tc2) )
        Bool -> Type -> Type -> Type -> Type -> TcM Coercion
go_app (TyCon -> ThetaType -> Bool
isNextTyConArgVisible TyCon
tc2 ThetaType
ts2') Type
s1 Type
t1 (TyCon -> ThetaType -> Type
TyConApp TyCon
tc2 ThetaType
ts2') Type
t2'

    go (TyConApp TyCon
tc1 ThetaType
ts1) (AppTy Type
s2 Type
t2)
      | Just (ThetaType
ts1', Type
t1') <- forall a. [a] -> Maybe ([a], a)
snocView ThetaType
ts1
      = ASSERT( not (mustBeSaturated tc1) )
        Bool -> Type -> Type -> Type -> Type -> TcM Coercion
go_app (TyCon -> ThetaType -> Bool
isNextTyConArgVisible TyCon
tc1 ThetaType
ts1') (TyCon -> ThetaType -> Type
TyConApp TyCon
tc1 ThetaType
ts1') Type
t1' Type
s2 Type
t2

    go (CoercionTy Coercion
co1) (CoercionTy Coercion
co2)
      = do { let ty1 :: Type
ty1 = Coercion -> Type
coercionType Coercion
co1
                 ty2 :: Type
ty2 = Coercion -> Type
coercionType Coercion
co2
           ; Coercion
kco <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
KindLevel
                          (Type -> Type -> CtOrigin -> Maybe TypeOrKind -> CtOrigin
KindEqOrigin Type
orig_ty1 Type
orig_ty2 CtOrigin
origin
                                        (forall a. a -> Maybe a
Just TypeOrKind
t_or_k))
                          Type
ty1 Type
ty2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ Role -> Coercion -> Coercion -> Coercion -> Coercion
mkProofIrrelCo Role
Nominal Coercion
kco Coercion
co1 Coercion
co2 }

        -- Anything else fails
        -- E.g. unifying for-all types, which is relative unusual
    go Type
ty1 Type
ty2 = Type -> Type -> TcM Coercion
defer Type
ty1 Type
ty2

    ------------------
    defer :: Type -> Type -> TcM Coercion
defer Type
ty1 Type
ty2   -- See Note [Check for equality before deferring]
      | Type
ty1 HasDebugCallStack => Type -> Type -> Bool
`tcEqType` Type
ty2 = forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Coercion
mkNomReflCo Type
ty1)
      | Bool
otherwise          = TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType_defer TypeOrKind
t_or_k CtOrigin
origin Type
ty1 Type
ty2

    ------------------
    go_app :: Bool -> Type -> Type -> Type -> Type -> TcM Coercion
go_app Bool
vis Type
s1 Type
t1 Type
s2 Type
t2
      = do { Coercion
co_s <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
origin Type
s1 Type
s2
           ; let arg_origin :: CtOrigin
arg_origin
                   | Bool
vis       = CtOrigin
origin
                   | Bool
otherwise = CtOrigin -> CtOrigin
toInvisibleOrigin CtOrigin
origin
           ; Coercion
co_t <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
t_or_k CtOrigin
arg_origin Type
t1 Type
t2
           ; forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$ Coercion -> Coercion -> Coercion
mkAppCo Coercion
co_s Coercion
co_t }

{- Note [Check for equality before deferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Particularly in ambiguity checks we can get equalities like (ty ~ ty).
If ty involves a type function we may defer, which isn't very sensible.
An egregious example of this was in test T9872a, which has a type signature
       Proxy :: Proxy (Solutions Cubes)
Doing the ambiguity check on this signature generates the equality
   Solutions Cubes ~ Solutions Cubes
and currently the constraint solver normalises both sides at vast cost.
This little short-cut in 'defer' helps quite a bit.

Note [Care with type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note: type applications need a bit of care!
They can match FunTy and TyConApp, so use splitAppTy_maybe
NB: we've already dealt with type variables and Notes,
so if one type is an App the other one jolly well better be too

Note [Mismatched type lists and application decomposition]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we find two TyConApps, you might think that the argument lists
are guaranteed equal length.  But they aren't. Consider matching
        w (T x) ~ Foo (T x y)
We do match (w ~ Foo) first, but in some circumstances we simply create
a deferred constraint; and then go ahead and match (T x ~ T x y).
This came up in #3950.

So either
   (a) either we must check for identical argument kinds
       when decomposing applications,

   (b) or we must be prepared for ill-kinded unification sub-problems

Currently we adopt (b) since it seems more robust -- no need to maintain
a global invariant.

Note [Expanding synonyms during unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We expand synonyms during unification, but:
 * We expand *after* the variable case so that we tend to unify
   variables with un-expanded type synonym. This just makes it
   more likely that the inferred types will mention type synonyms
   understandable to the user

 * Similarly, we expand *after* the CastTy case, just in case the
   CastTy wraps a variable.

 * We expand *before* the TyConApp case.  For example, if we have
      type Phantom a = Int
   and are unifying
      Phantom Int ~ Phantom Char
   it is *wrong* to unify Int and Char.

 * The problem case immediately above can happen only with arguments
   to the tycon. So we check for nullary tycons *before* expanding.
   This is particularly helpful when checking (* ~ *), because * is
   now a type synonym.

Note [Deferred unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
and yet its consistency is undetermined. Previously, there was no way to still
make it consistent. So a mismatch error was issued.

Now these unifications are deferred until constraint simplification, where type
family instances and given equations may (or may not) establish the consistency.
Deferred unifications are of the form
                F ... ~ ...
or              x ~ ...
where F is a type function and x is a type variable.
E.g.
        id :: x ~ y => x -> y
        id e = e

involves the unification x = y. It is deferred until we bring into account the
context x ~ y to establish that it holds.

If available, we defer original types (rather than those where closed type
synonyms have already been expanded via tcCoreView).  This is, as usual, to
improve error messages.


************************************************************************
*                                                                      *
                 uUnfilledVar and friends
*                                                                      *
************************************************************************

@uunfilledVar@ is called when at least one of the types being unified is a
variable.  It does {\em not} assume that the variable is a fixed point
of the substitution; rather, notice that @uVar@ (defined below) nips
back into @uTys@ if it turns out that the variable is already bound.
-}

----------
uUnfilledVar :: CtOrigin
             -> TypeOrKind
             -> SwapFlag
             -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                               --    definitely not a /filled/ meta-tyvar
             -> TcTauType      -- Type 2
             -> TcM Coercion
-- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
--            It might be a skolem, or untouchable, or meta

uUnfilledVar :: CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar CtOrigin
origin TypeOrKind
t_or_k SwapFlag
swapped TcTyVar
tv1 Type
ty2
  = do { Type
ty2 <- Type -> TcM Type
zonkTcType Type
ty2
             -- Zonk to expose things to the
             -- occurs check, and so that if ty2
             -- looks like a type variable then it
             -- /is/ a type variable
       ; CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar1 CtOrigin
origin TypeOrKind
t_or_k SwapFlag
swapped TcTyVar
tv1 Type
ty2 }

----------
uUnfilledVar1 :: CtOrigin
              -> TypeOrKind
              -> SwapFlag
              -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                --    definitely not a /filled/ meta-tyvar
              -> TcTauType      -- Type 2, zonked
              -> TcM Coercion
uUnfilledVar1 :: CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar1 CtOrigin
origin TypeOrKind
t_or_k SwapFlag
swapped TcTyVar
tv1 Type
ty2
  | Just TcTyVar
tv2 <- Type -> Maybe TcTyVar
tcGetTyVar_maybe Type
ty2
  = TcTyVar -> TcM Coercion
go TcTyVar
tv2

  | Bool
otherwise
  = CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar2 CtOrigin
origin TypeOrKind
t_or_k SwapFlag
swapped TcTyVar
tv1 Type
ty2

  where
    -- 'go' handles the case where both are
    -- tyvars so we might want to swap
    -- E.g. maybe tv2 is a meta-tyvar and tv1 is not
    go :: TcTyVar -> TcM Coercion
go TcTyVar
tv2 | TcTyVar
tv1 forall a. Eq a => a -> a -> Bool
== TcTyVar
tv2  -- Same type variable => no-op
           = forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Coercion
mkNomReflCo (TcTyVar -> Type
mkTyVarTy TcTyVar
tv1))

           | Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars Bool
False TcTyVar
tv1 TcTyVar
tv2   -- Distinct type variables
               -- Swap meta tyvar to the left if poss
           = do { TcTyVar
tv1 <- TcTyVar -> TcM TcTyVar
zonkTyCoVarKind TcTyVar
tv1
                     -- We must zonk tv1's kind because that might
                     -- not have happened yet, and it's an invariant of
                     -- uUnfilledTyVar2 that ty2 is fully zonked
                     -- Omitting this caused #16902
                ; CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar2 CtOrigin
origin TypeOrKind
t_or_k (SwapFlag -> SwapFlag
flipSwap SwapFlag
swapped)
                           TcTyVar
tv2 (TcTyVar -> Type
mkTyVarTy TcTyVar
tv1) }

           | Bool
otherwise
           = CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar2 CtOrigin
origin TypeOrKind
t_or_k SwapFlag
swapped TcTyVar
tv1 Type
ty2

----------
uUnfilledVar2 :: CtOrigin
              -> TypeOrKind
              -> SwapFlag
              -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                --    definitely not a /filled/ meta-tyvar
              -> TcTauType      -- Type 2, zonked
              -> TcM Coercion
uUnfilledVar2 :: CtOrigin
-> TypeOrKind -> SwapFlag -> TcTyVar -> Type -> TcM Coercion
uUnfilledVar2 CtOrigin
origin TypeOrKind
t_or_k SwapFlag
swapped TcTyVar
tv1 Type
ty2
  = do { DynFlags
dflags  <- forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; TcLevel
cur_lvl <- TcM TcLevel
getTcLevel
       ; DynFlags -> TcLevel -> TcM Coercion
go DynFlags
dflags TcLevel
cur_lvl }
  where
    go :: DynFlags -> TcLevel -> TcM Coercion
go DynFlags
dflags TcLevel
cur_lvl
      | TcLevel -> TcTyVar -> Bool
isTouchableMetaTyVar TcLevel
cur_lvl TcTyVar
tv1
           -- See Note [Unification preconditions], (UNTOUCHABLE) wrinkles
      , MetaInfo -> Type -> Bool
canSolveByUnification (TcTyVar -> MetaInfo
metaTyVarInfo TcTyVar
tv1) Type
ty2
      , CheckTyEqResult -> Bool
cterHasNoProblem (DynFlags -> TcTyVar -> Type -> CheckTyEqResult
checkTyVarEq DynFlags
dflags TcTyVar
tv1 Type
ty2)
           -- See Note [Prevent unification with type families]
      = do { Coercion
co_k <- TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
KindLevel CtOrigin
kind_origin (HasDebugCallStack => Type -> Type
tcTypeKind Type
ty2) (TcTyVar -> Type
tyVarKind TcTyVar
tv1)
           ; String -> SDoc -> TcRn ()
traceTc String
"uUnfilledVar2 ok" forall a b. (a -> b) -> a -> b
$
             [SDoc] -> SDoc
vcat [ forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv1 SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr (TcTyVar -> Type
tyVarKind TcTyVar
tv1)
                  , forall a. Outputable a => a -> SDoc
ppr Type
ty2 SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> forall a. Outputable a => a -> SDoc
ppr (HasDebugCallStack => Type -> Type
tcTypeKind  Type
ty2)
                  , forall a. Outputable a => a -> SDoc
ppr (Coercion -> Bool
isTcReflCo Coercion
co_k), forall a. Outputable a => a -> SDoc
ppr Coercion
co_k ]

           ; if Coercion -> Bool
isTcReflCo Coercion
co_k
               -- Only proceed if the kinds match
               -- NB: tv1 should still be unfilled, despite the kind unification
               --     because tv1 is not free in ty2 (or, hence, in its kind)
             then do { TcTyVar -> Type -> TcRn ()
writeMetaTyVar TcTyVar
tv1 Type
ty2
                     ; forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Coercion
mkTcNomReflCo Type
ty2) }

             else TcM Coercion
defer } -- This cannot be solved now.  See GHC.Tc.Solver.Canonical
                          -- Note [Equalities with incompatible kinds]

      | Bool
otherwise
      = do { String -> SDoc -> TcRn ()
traceTc String
"uUnfilledVar2 not ok" (forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv1 SDoc -> SDoc -> SDoc
$$ forall a. Outputable a => a -> SDoc
ppr Type
ty2)
               -- Occurs check or an untouchable: just defer
               -- NB: occurs check isn't necessarily fatal:
               --     eg tv1 occurred in type family parameter
            ; TcM Coercion
defer }

    ty1 :: Type
ty1 = TcTyVar -> Type
mkTyVarTy TcTyVar
tv1
    kind_origin :: CtOrigin
kind_origin = Type -> Type -> CtOrigin -> Maybe TypeOrKind -> CtOrigin
KindEqOrigin Type
ty1 Type
ty2 CtOrigin
origin (forall a. a -> Maybe a
Just TypeOrKind
t_or_k)

    defer :: TcM Coercion
defer = forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped (TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType_defer TypeOrKind
t_or_k CtOrigin
origin) Type
ty1 Type
ty2

canSolveByUnification :: MetaInfo -> TcType -> Bool
-- See Note [Unification preconditions, (TYVAR-TV)]
canSolveByUnification :: MetaInfo -> Type -> Bool
canSolveByUnification MetaInfo
info Type
xi
  = case MetaInfo
info of
      MetaInfo
CycleBreakerTv -> Bool
False
      MetaInfo
TyVarTv -> case Type -> Maybe TcTyVar
tcGetTyVar_maybe Type
xi of
                   Maybe TcTyVar
Nothing -> Bool
False
                   Just TcTyVar
tv -> case TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv of
                                 MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
info }
                                            -> case MetaInfo
info of
                                                 MetaInfo
TyVarTv -> Bool
True
                                                 MetaInfo
_       -> Bool
False
                                 SkolemTv {} -> Bool
True
                                 TcTyVarDetails
RuntimeUnk  -> Bool
True
      MetaInfo
_ -> Bool
True

swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars Bool
is_given TcTyVar
tv1 TcTyVar
tv2
  -- See Note [Unification variables on the left]
  | Bool -> Bool
not Bool
is_given, Int
pri1 forall a. Eq a => a -> a -> Bool
== Int
0, Int
pri2 forall a. Ord a => a -> a -> Bool
> Int
0 = Bool
True
  | Bool -> Bool
not Bool
is_given, Int
pri2 forall a. Eq a => a -> a -> Bool
== Int
0, Int
pri1 forall a. Ord a => a -> a -> Bool
> Int
0 = Bool
False

  -- Level comparison: see Note [TyVar/TyVar orientation]
  | TcLevel
lvl1 TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lvl2 = Bool
False
  | TcLevel
lvl2 TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lvl1 = Bool
True

  -- Priority: see Note [TyVar/TyVar orientation]
  | Int
pri1 forall a. Ord a => a -> a -> Bool
> Int
pri2 = Bool
False
  | Int
pri2 forall a. Ord a => a -> a -> Bool
> Int
pri1 = Bool
True

  -- Names: see Note [TyVar/TyVar orientation]
  | Name -> Bool
isSystemName Name
tv2_name, Bool -> Bool
not (Name -> Bool
isSystemName Name
tv1_name) = Bool
True

  | Bool
otherwise = Bool
False

  where
    lvl1 :: TcLevel
lvl1 = TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
tv1
    lvl2 :: TcLevel
lvl2 = TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
tv2
    pri1 :: Int
pri1 = TcTyVar -> Int
lhsPriority TcTyVar
tv1
    pri2 :: Int
pri2 = TcTyVar -> Int
lhsPriority TcTyVar
tv2
    tv1_name :: Name
tv1_name = TcTyVar -> Name
Var.varName TcTyVar
tv1
    tv2_name :: Name
tv2_name = TcTyVar -> Name
Var.varName TcTyVar
tv2


lhsPriority :: TcTyVar -> Int
-- Higher => more important to be on the LHS
--        => more likely to be eliminated
-- See Note [TyVar/TyVar orientation]
lhsPriority :: TcTyVar -> Int
lhsPriority TcTyVar
tv
  = ASSERT2( isTyVar tv, ppr tv)
    case TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv of
      TcTyVarDetails
RuntimeUnk  -> Int
0
      SkolemTv {} -> Int
0
      MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
info } -> case MetaInfo
info of
                                     MetaInfo
CycleBreakerTv -> Int
0
                                     MetaInfo
TyVarTv        -> Int
1
                                     MetaInfo
TauTv          -> Int
2
                                     MetaInfo
RuntimeUnkTv   -> Int
3

{- Note [Unification preconditions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Question: given a homogeneous equality (alpha ~# ty), when is it OK to
unify alpha := ty?

This note only applied to /homogeneous/ equalities, in which both
sides have the same kind.

There are three reasons not to unify:

1. (SKOL-ESC) Skolem-escape
   Consider the constraint
        forall[2] a[2]. alpha[1] ~ Maybe a[2]
   If we unify alpha := Maybe a, the skolem 'a' may escape its scope.
   The level alpha[1] says that alpha may be used outside this constraint,
   where 'a' is not in scope at all.  So we must not unify.

   Bottom line: when looking at a constraint alpha[n] := ty, do not unify
   if any free variable of 'ty' has level deeper (greater) than n

2. (UNTOUCHABLE) Untouchable unification variables
   Consider the constraint
        forall[2] a[2]. b[1] ~ Int => alpha[1] ~ Int
   There is no (SKOL-ESC) problem with unifying alpha := Int, but it might
   not be the principal solution. Perhaps the "right" solution is alpha := b.
   We simply can't tell.  See "OutsideIn(X): modular type inference with local
   assumptions", section 2.2.  We say that alpha[1] is "untouchable" inside
   this implication.

   Bottom line: at amibient level 'l', when looking at a constraint
   alpha[n] ~ ty, do not unify alpha := ty if there are any given equalities
   between levels 'n' and 'l'.

   Exactly what is a "given equality" for the purpose of (UNTOUCHABLE)?
   Answer: see Note [Tracking Given equalities] in GHC.Tc.Solver.Monad

3. (TYVAR-TV) Unifying TyVarTvs and CycleBreakerTvs
   This precondition looks at the MetaInfo of the unification variable:

   * TyVarTv: When considering alpha{tyv} ~ ty, if alpha{tyv} is a
     TyVarTv it can only unify with a type variable, not with a
     structured type.  So if 'ty' is a structured type, such as (Maybe x),
     don't unify.

   * CycleBreakerTv: never unified, except by restoreTyVarCycles.


Needless to say, all three have wrinkles:

* (SKOL-ESC) Promotion.  Given alpha[n] ~ ty, what if beta[k] is free
  in 'ty', where beta is a unification variable, and k>n?  'beta'
  stands for a monotype, and since it is part of a level-n type
  (equal to alpha[n]), we must /promote/ beta to level n.  Just make
  up a fresh gamma[n], and unify beta[k] := gamma[n].

* (TYVAR-TV) Unification variables.  Suppose alpha[tyv,n] is a level-n
  TyVarTv (see Note [TyVarTv] in GHC.Tc.Types.TcMType)? Now
  consider alpha[tyv,n] ~ Bool.  We don't want to unify because that
  would break the TyVarTv invariant.

  What about alpha[tyv,n] ~ beta[tau,n], where beta is an ordinary
  TauTv?  Again, don't unify, because beta might later be unified
  with, say Bool.  (If levels permit, we reverse the orientation here;
  see Note [TyVar/TyVar orientation].)

* (UNTOUCHABLE) Untouchability.  When considering (alpha[n] ~ ty), how
  do we know whether there are any given equalities between level n
  and the ambient level?  We answer in two ways:

  * In the eager unifier, we only unify if l=n.  If not, alpha may be
    untouchable, and defer to the constraint solver.  This check is
    made in GHC.Tc.Utils.uUnifilledVar2, in the guard
    isTouchableMetaTyVar.

  * In the constraint solver, we track where Given equalities occur
    and use that to guard unification in GHC.Tc.Solver.Canonical.unifyTest
    More details in Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet

    Historical note: in the olden days (pre 2021) the constraint solver
    also used to unify only if l=n.  Equalities were "floated" out of the
    implication in a separate step, so that they would become touchable.
    But the float/don't-float question turned out to be very delicate,
    as you can see if you look at the long series of Notes associated with
    GHC.Tc.Solver.floatEqualities, around Nov 2020.  It's much easier
    to unify in-place, with no floating.

Note [TyVar/TyVar orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (a ~ b), should we orient the CEqCan as (a~b) or (b~a)?
This is a surprisingly tricky question! This is invariant (TyEq:TV).

The question is answered by swapOverTyVars, which is used
  - in the eager unifier, in GHC.Tc.Utils.Unify.uUnfilledVar1
  - in the constraint solver, in GHC.Tc.Solver.Canonical.canEqCanLHS2

First note: only swap if you have to!
   See Note [Avoid unnecessary swaps]

So we look for a positive reason to swap, using a three-step test:

* Level comparison. If 'a' has deeper level than 'b',
  put 'a' on the left.  See Note [Deeper level on the left]

* Priority.  If the levels are the same, look at what kind of
  type variable it is, using 'lhsPriority'.

  Generally speaking we always try to put a MetaTv on the left
  in preference to SkolemTv or RuntimeUnkTv:
     a) Because the MetaTv may be touchable and can be unified
     b) Even if it's not touchable, GHC.Tc.Solver.floatEqualities
        looks for meta tyvars on the left

  Tie-breaking rules for MetaTvs:
  - CycleBreakerTv: This is essentially a stand-in for another type;
       it's untouchable and should have the same priority as a skolem: 0.

  - TyVarTv: These can unify only with another tyvar, but we can't unify
       a TyVarTv with a TauTv, because then the TyVarTv could (transitively)
       get a non-tyvar type. So give these a low priority: 1.

  - TauTv: This is the common case; we want these on the left so that they
       can be written to: 2.

  - RuntimeUnkTv: These aren't really meta-variables used in type inference,
       but just a convenience in the implementation of the GHCi debugger.
       Eagerly write to these: 3. See Note [RuntimeUnkTv] in
       GHC.Runtime.Heap.Inspect.

* Names. If the level and priority comparisons are all
  equal, try to eliminate a TyVar with a System Name in
  favour of ones with a Name derived from a user type signature

* Age.  At one point in the past we tried to break any remaining
  ties by eliminating the younger type variable, based on their
  Uniques.  See Note [Eliminate younger unification variables]
  (which also explains why we don't do this any more)

Note [Unification variables on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For wanteds, but not givens, swap (skolem ~ meta-tv) regardless of
level, so that the unification variable is on the left.

* We /don't/ want this for Givens because if we ave
    [G] a[2] ~ alpha[1]
    [W] Bool ~ a[2]
  we want to rewrite the wanted to Bool ~ alpha[1],
  so we can float the constraint and solve it.

* But for Wanteds putting the unification variable on
  the left means an easier job when floating, and when
  reporting errors -- just fewer cases to consider.

  In particular, we get better skolem-escape messages:
  see #18114

Note [Deeper level on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The most important thing is that we want to put tyvars with
the deepest level on the left.  The reason to do so differs for
Wanteds and Givens, but either way, deepest wins!  Simple.

* Wanteds.  Putting the deepest variable on the left maximise the
  chances that it's a touchable meta-tyvar which can be solved.

* Givens. Suppose we have something like
     forall a[2]. b[1] ~ a[2] => beta[1] ~ a[2]

  If we orient the Given a[2] on the left, we'll rewrite the Wanted to
  (beta[1] ~ b[1]), and that can float out of the implication.
  Otherwise it can't.  By putting the deepest variable on the left
  we maximise our changes of eliminating skolem capture.

  See also GHC.Tc.Solver.Monad Note [Let-bound skolems] for another reason
  to orient with the deepest skolem on the left.

  IMPORTANT NOTE: this test does a level-number comparison on
  skolems, so it's important that skolems have (accurate) level
  numbers.

See #15009 for an further analysis of why "deepest on the left"
is a good plan.

Note [Avoid unnecessary swaps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we swap without actually improving matters, we can get an infinite loop.
Consider
    work item:  a ~ b
   inert item:  b ~ c
We canonicalise the work-item to (a ~ c).  If we then swap it before
adding to the inert set, we'll add (c ~ a), and therefore kick out the
inert guy, so we get
   new work item:  b ~ c
   inert item:     c ~ a
And now the cycle just repeats

Historical Note [Eliminate younger unification variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given a choice of unifying
     alpha := beta   or   beta := alpha
we try, if possible, to eliminate the "younger" one, as determined
by `ltUnique`.  Reason: the younger one is less likely to appear free in
an existing inert constraint, and hence we are less likely to be forced
into kicking out and rewriting inert constraints.

This is a performance optimisation only.  It turns out to fix
#14723 all by itself, but clearly not reliably so!

It's simple to implement (see nicer_to_update_tv2 in swapOverTyVars).
But, to my surprise, it didn't seem to make any significant difference
to the compiler's performance, so I didn't take it any further.  Still
it seemed too nice to discard altogether, so I'm leaving these
notes.  SLPJ Jan 18.

Note [Prevent unification with type families]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We prevent unification with type families because of an uneasy compromise.
It's perfectly sound to unify with type families, and it even improves the
error messages in the testsuite. It also modestly improves performance, at
least in some cases. But it's disastrous for test case perf/compiler/T3064.
Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
What do we do? Do we reduce F? Or do we use the given? Hard to know what's
best. GHC reduces. This is a disaster for T3064, where the type's size
spirals out of control during reduction. If we prevent
unification with type families, then the solver happens to use the equality
before expanding the type family.

It would be lovely in the future to revisit this problem and remove this
extra, unnecessary check. But we retain it for now as it seems to work
better in practice.

Revisited in Nov '20, along with removing flattening variables. Problem
is still present, and the solution is still the same.

Note [Refactoring hazard: metaTyVarUpdateOK]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I (Richard E.) have a sad story about refactoring this code, retained here
to prevent others (or a future me!) from falling into the same traps.

It all started with #11407, which was caused by the fact that the TyVarTy
case of defer_me didn't look in the kind. But it seemed reasonable to
simply remove the defer_me check instead.

It referred to two Notes (since removed) that were out of date, and the
fast_check code in occurCheckExpand seemed to do just about the same thing as
defer_me. The one piece that defer_me did that wasn't repeated by
occurCheckExpand was the type-family check. (See Note [Prevent unification
with type families].) So I checked the result of occurCheckExpand for any
type family occurrences and deferred if there were any. This was done
in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 .

This approach turned out not to be performant, because the expanded
type was bigger than the original type, and tyConsOfType (needed to
see if there are any type family occurrences) looks through type
synonyms. So it then struck me that we could dispense with the
defer_me check entirely. This simplified the code nicely, and it cut
the allocations in T5030 by half. But, as documented in Note [Prevent
unification with type families], this destroyed performance in
T3064. Regardless, I missed this regression and the change was
committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d .

Bottom lines:
 * defer_me is back, but now fixed w.r.t. #11407.
 * Tread carefully before you start to refactor here. There can be
   lots of hard-to-predict consequences.

Note [Type synonyms and the occur check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Generally speaking we try to update a variable with type synonyms not
expanded, which improves later error messages, unless looking
inside a type synonym may help resolve a spurious occurs check
error. Consider:
          type A a = ()

          f :: (A a -> a -> ()) -> ()
          f = \ _ -> ()

          x :: ()
          x = f (\ x p -> p x)

We will eventually get a constraint of the form t ~ A t. The ok function above will
properly expand the type (A t) to just (), which is ok to be unified with t. If we had
unified with the original type A t, we would lead the type checker into an infinite loop.

Hence, if the occurs check fails for a type synonym application, then (and *only* then),
the ok function expands the synonym to detect opportunities for occurs check success using
the underlying definition of the type synonym.

The same applies later on in the constraint interaction code; see GHC.Tc.Solver.Interact,
function @occ_check_ok@.

Note [Non-TcTyVars in GHC.Tc.Utils.Unify]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Because the same code is now shared between unifying types and unifying
kinds, we sometimes will see proper TyVars floating around the unifier.
Example (from test case polykinds/PolyKinds12):

    type family Apply (f :: k1 -> k2) (x :: k1) :: k2
    type instance Apply g y = g y

When checking the instance declaration, we first *kind-check* the LHS
and RHS, discovering that the instance really should be

    type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y

During this kind-checking, all the tyvars will be TcTyVars. Then, however,
as a second pass, we desugar the RHS (which is done in functions prefixed
with "tc" in GHC.Tc.TyCl"). By this time, all the kind-vars are proper
TyVars, not TcTyVars, get some kind unification must happen.

Thus, we always check if a TyVar is a TcTyVar before asking if it's a
meta-tyvar.

This used to not be necessary for type-checking (that is, before * :: *)
because expressions get desugared via an algorithm separate from
type-checking (with wrappers, etc.). Types get desugared very differently,
causing this wibble in behavior seen here.
-}

-- | Breaks apart a function kind into its pieces.
matchExpectedFunKind
  :: Outputable fun
  => fun             -- ^ type, only for errors
  -> Arity           -- ^ n: number of desired arrows
  -> TcKind          -- ^ fun_ kind
  -> TcM Coercion    -- ^ co :: fun_kind ~ (arg1 -> ... -> argn -> res)

matchExpectedFunKind :: forall fun. Outputable fun => fun -> Int -> Type -> TcM Coercion
matchExpectedFunKind fun
hs_ty Int
n Type
k = Int -> Type -> TcM Coercion
go Int
n Type
k
  where
    go :: Int -> Type -> TcM Coercion
go Int
0 Type
k = forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Coercion
mkNomReflCo Type
k)

    go Int
n Type
k | Just Type
k' <- Type -> Maybe Type
tcView Type
k = Int -> Type -> TcM Coercion
go Int
n Type
k'

    go Int
n k :: Type
k@(TyVarTy TcTyVar
kvar)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
kvar
      = do { MetaDetails
maybe_kind <- TcTyVar -> TcM MetaDetails
readMetaTyVar TcTyVar
kvar
           ; case MetaDetails
maybe_kind of
                Indirect Type
fun_kind -> Int -> Type -> TcM Coercion
go Int
n Type
fun_kind
                MetaDetails
Flexi ->             Int -> Type -> TcM Coercion
defer Int
n Type
k }

    go Int
n (FunTy { ft_mult :: Type -> Type
ft_mult = Type
w, ft_arg :: Type -> Type
ft_arg = Type
arg, ft_res :: Type -> Type
ft_res = Type
res })
      = do { Coercion
co <- Int -> Type -> TcM Coercion
go (Int
nforall a. Num a => a -> a -> a
-Int
1) Type
res
           ; forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Coercion -> Coercion -> Coercion -> Coercion
mkTcFunCo Role
Nominal (Type -> Coercion
mkTcNomReflCo Type
w) (Type -> Coercion
mkTcNomReflCo Type
arg) Coercion
co) }

    go Int
n Type
other
     = Int -> Type -> TcM Coercion
defer Int
n Type
other

    defer :: Int -> Type -> TcM Coercion
defer Int
n Type
k
      = do { ThetaType
arg_kinds <- Int -> TcM ThetaType
newMetaKindVars Int
n
           ; Type
res_kind  <- TcM Type
newMetaKindVar
           ; let new_fun :: Type
new_fun = ThetaType -> Type -> Type
mkVisFunTysMany ThetaType
arg_kinds Type
res_kind
                 origin :: CtOrigin
origin  = TypeEqOrigin { uo_actual :: Type
uo_actual   = Type
k
                                        , uo_expected :: Type
uo_expected = Type
new_fun
                                        , uo_thing :: Maybe SDoc
uo_thing    = forall a. a -> Maybe a
Just (forall a. Outputable a => a -> SDoc
ppr fun
hs_ty)
                                        , uo_visible :: Bool
uo_visible  = Bool
True
                                        }
           ; TypeOrKind -> CtOrigin -> Type -> Type -> TcM Coercion
uType TypeOrKind
KindLevel CtOrigin
origin Type
k Type
new_fun }

{- *********************************************************************
*                                                                      *
                 Equality invariant checking
*                                                                      *
********************************************************************* -}


{-  Note [Checking for foralls]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unless we have -XImpredicativeTypes (which is a totally unsupported
feature), we do not want to unify
    alpha ~ (forall a. a->a) -> Int
So we look for foralls hidden inside the type, and it's convenient
to do that at the same time as the occurs check (which looks for
occurrences of alpha).

However, it's not just a question of looking for foralls /anywhere/!
Consider
   (alpha :: forall k. k->*)  ~  (beta :: forall k. k->*)
This is legal; e.g. dependent/should_compile/T11635.

We don't want to reject it because of the forall in beta's kind, but
(see Note [Occurrence checking: look inside kinds] in GHC.Core.Type)
we do need to look in beta's kind.  So we carry a flag saying if a
'forall' is OK, and switch the flag on when stepping inside a kind.

Why is it OK?  Why does it not count as impredicative polymorphism?
The reason foralls are bad is because we reply on "seeing" foralls
when doing implicit instantiation.  But the forall inside the kind is
fine.  We'll generate a kind equality constraint
  (forall k. k->*) ~ (forall k. k->*)
to check that the kinds of lhs and rhs are compatible.  If alpha's
kind had instead been
  (alpha :: kappa)
then this kind equality would rightly complain about unifying kappa
with (forall k. k->*)

-}

----------------
{-# NOINLINE checkTyVarEq #-}  -- checkTyVarEq becomes big after the `inline` fires
checkTyVarEq :: DynFlags -> TcTyVar -> TcType -> CheckTyEqResult
checkTyVarEq :: DynFlags -> TcTyVar -> Type -> CheckTyEqResult
checkTyVarEq DynFlags
dflags TcTyVar
tv Type
ty
  = forall a. a -> a
inline DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
checkTypeEq DynFlags
dflags (TcTyVar -> CanEqLHS
TyVarLHS TcTyVar
tv) Type
ty
    -- inline checkTypeEq so that the `case`s over the CanEqLHS get blasted away

{-# NOINLINE checkTyFamEq #-}  -- checkTyFamEq becomes big after the `inline` fires
checkTyFamEq :: DynFlags
             -> TyCon     -- type function
             -> [TcType]  -- args, exactly saturated
             -> TcType    -- RHS
             -> CheckTyEqResult   -- always drops cteTypeFamily
checkTyFamEq :: DynFlags -> TyCon -> ThetaType -> Type -> CheckTyEqResult
checkTyFamEq DynFlags
dflags TyCon
fun_tc ThetaType
fun_args Type
ty
  = forall a. a -> a
inline DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
checkTypeEq DynFlags
dflags (TyCon -> ThetaType -> CanEqLHS
TyFamLHS TyCon
fun_tc ThetaType
fun_args) Type
ty
    CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
`cterRemoveProblem` CheckTyEqProblem
cteTypeFamily
    -- inline checkTypeEq so that the `case`s over the CanEqLHS get blasted away

checkTypeEq :: DynFlags -> CanEqLHS -> TcType -> CheckTyEqResult
-- If cteHasNoProblem (checkTypeEq dflags lhs rhs), then lhs ~ rhs
-- is a canonical CEqCan.
--
-- In particular, this looks for:
--   (a) a forall type (forall a. blah)
--   (b) a predicate type (c => ty)
--   (c) a type family; see Note [Prevent unification with type families]
--   (d) a blocking coercion hole
--   (e) an occurrence of the LHS (occurs check)
--
-- Note that an occurs-check does not mean "definite error".  For example
--   type family F a
--   type instance F Int = Int
-- consider
--   b0 ~ F b0
-- This is perfectly reasonable, if we later get b0 ~ Int.  But we
-- certainly can't unify b0 := F b0
--
-- For (a), (b), and (c) we check only the top level of the type, NOT
-- inside the kinds of variables it mentions.  For (d) we look deeply
-- in coercions when the LHS is a tyvar (but skip coercions for type family
-- LHSs), and for (e) see Note [CEqCan occurs check] in GHC.Tc.Types.Constraint.
--
-- checkTypeEq is called from
--    * checkTyFamEq, checkTyVarEq (which inline it to specialise away the
--      case-analysis on 'lhs')
--    * checkEqCanLHSFinish, which does not know the form of 'lhs'
checkTypeEq :: DynFlags -> CanEqLHS -> Type -> CheckTyEqResult
checkTypeEq DynFlags
dflags CanEqLHS
lhs Type
ty
  = Type -> CheckTyEqResult
go Type
ty
  where
    impredicative :: CheckTyEqResult
impredicative    = CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteImpredicative
    type_family :: CheckTyEqResult
type_family      = CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteTypeFamily
    hole_blocker :: CheckTyEqResult
hole_blocker     = CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteHoleBlocker
    insoluble_occurs :: CheckTyEqResult
insoluble_occurs = CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteInsolubleOccurs
    soluble_occurs :: CheckTyEqResult
soluble_occurs   = CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteSolubleOccurs

    -- The GHCi runtime debugger does its type-matching with
    -- unification variables that can unify with a polytype
    -- or a TyCon that would usually be disallowed by bad_tc
    -- See Note [RuntimeUnkTv] in GHC.Runtime.Heap.Inspect
    ghci_tv :: Bool
ghci_tv
      | TyVarLHS TcTyVar
tv <- CanEqLHS
lhs
      , MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
RuntimeUnkTv } <- TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv
      = Bool
True

      | Bool
otherwise
      = Bool
False

    go :: TcType -> CheckTyEqResult
    go :: Type -> CheckTyEqResult
go (TyVarTy TcTyVar
tv')           = TcTyVar -> CheckTyEqResult
go_tv TcTyVar
tv'
    go (TyConApp TyCon
tc ThetaType
tys)       = TyCon -> ThetaType -> CheckTyEqResult
go_tc TyCon
tc ThetaType
tys
    go (LitTy {})              = CheckTyEqResult
cteOK
    go (FunTy {ft_af :: Type -> AnonArgFlag
ft_af = AnonArgFlag
af, ft_mult :: Type -> Type
ft_mult = Type
w, ft_arg :: Type -> Type
ft_arg = Type
a, ft_res :: Type -> Type
ft_res = Type
r})
                               = Type -> CheckTyEqResult
go Type
w forall a. Semigroup a => a -> a -> a
S.<> Type -> CheckTyEqResult
go Type
a forall a. Semigroup a => a -> a -> a
S.<> Type -> CheckTyEqResult
go Type
r forall a. Semigroup a => a -> a -> a
S.<>
                                 if Bool -> Bool
not Bool
ghci_tv Bool -> Bool -> Bool
&& AnonArgFlag
af forall a. Eq a => a -> a -> Bool
== AnonArgFlag
InvisArg
                                   then CheckTyEqResult
impredicative
                                   else CheckTyEqResult
cteOK
    go (AppTy Type
fun Type
arg) = Type -> CheckTyEqResult
go Type
fun forall a. Semigroup a => a -> a -> a
S.<> Type -> CheckTyEqResult
go Type
arg
    go (CastTy Type
ty Coercion
co)  = Type -> CheckTyEqResult
go Type
ty  forall a. Semigroup a => a -> a -> a
S.<> Coercion -> CheckTyEqResult
go_co Coercion
co
    go (CoercionTy Coercion
co) = Coercion -> CheckTyEqResult
go_co Coercion
co
    go (ForAllTy (Bndr TcTyVar
tv' ArgFlag
_) Type
ty) = (case CanEqLHS
lhs of
      TyVarLHS TcTyVar
tv | TcTyVar
tv forall a. Eq a => a -> a -> Bool
== TcTyVar
tv' -> Type -> CheckTyEqResult
go_occ (TcTyVar -> Type
tyVarKind TcTyVar
tv') forall a. Semigroup a => a -> a -> a
S.<> CheckTyEqResult -> CheckTyEqResult
cterClearOccursCheck (Type -> CheckTyEqResult
go Type
ty)
                  | Bool
otherwise -> Type -> CheckTyEqResult
go_occ (TcTyVar -> Type
tyVarKind TcTyVar
tv') forall a. Semigroup a => a -> a -> a
S.<> Type -> CheckTyEqResult
go Type
ty
      CanEqLHS
_                       -> Type -> CheckTyEqResult
go Type
ty)
      forall a. Semigroup a => a -> a -> a
S.<>
      if Bool
ghci_tv then CheckTyEqResult
cteOK else CheckTyEqResult
impredicative

    go_tv :: TcTyVar -> CheckTyEqResult
      -- this slightly peculiar way of defining this means
      -- we don't have to evaluate this `case` at every variable
      -- occurrence
    go_tv :: TcTyVar -> CheckTyEqResult
go_tv = case CanEqLHS
lhs of
      TyVarLHS TcTyVar
tv -> \ TcTyVar
tv' -> Type -> CheckTyEqResult
go_occ (TcTyVar -> Type
tyVarKind TcTyVar
tv') forall a. Semigroup a => a -> a -> a
S.<>
                              if TcTyVar
tv forall a. Eq a => a -> a -> Bool
== TcTyVar
tv' then CheckTyEqResult
insoluble_occurs else CheckTyEqResult
cteOK
      TyFamLHS {} -> \ TcTyVar
_tv' -> CheckTyEqResult
cteOK
           -- See Note [Occurrence checking: look inside kinds] in GHC.Core.Type

     -- For kinds, we only do an occurs check; we do not worry
     -- about type families or foralls
     -- See Note [Checking for foralls]
    go_occ :: Type -> CheckTyEqResult
go_occ Type
k = CheckTyEqResult -> CheckTyEqResult
cterFromKind forall a b. (a -> b) -> a -> b
$ Type -> CheckTyEqResult
go Type
k

    go_tc :: TyCon -> [TcType] -> CheckTyEqResult
      -- this slightly peculiar way of defining this means
      -- we don't have to evaluate this `case` at every tyconapp
    go_tc :: TyCon -> ThetaType -> CheckTyEqResult
go_tc = case CanEqLHS
lhs of
      TyVarLHS {} -> \ TyCon
tc ThetaType
tys -> TyCon -> CheckTyEqResult
check_tc TyCon
tc forall a. Semigroup a => a -> a -> a
S.<> TyCon -> ThetaType -> CheckTyEqResult
go_tc_args TyCon
tc ThetaType
tys
      TyFamLHS TyCon
fam_tc ThetaType
fam_args -> \ TyCon
tc ThetaType
tys ->
        if TyCon -> ThetaType -> TyCon -> ThetaType -> Bool
tcEqTyConApps TyCon
fam_tc ThetaType
fam_args TyCon
tc ThetaType
tys
          then CheckTyEqResult
insoluble_occurs
          else TyCon -> CheckTyEqResult
check_tc TyCon
tc forall a. Semigroup a => a -> a -> a
S.<> TyCon -> ThetaType -> CheckTyEqResult
go_tc_args TyCon
tc ThetaType
tys

      -- just look at arguments, not the tycon itself
    go_tc_args :: TyCon -> [TcType] -> CheckTyEqResult
    go_tc_args :: TyCon -> ThetaType -> CheckTyEqResult
go_tc_args TyCon
tc ThetaType
tys | TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc Role
Nominal = forall (t :: * -> *) m a.
(Foldable t, Monoid m) =>
(a -> m) -> t a -> m
foldMap Type -> CheckTyEqResult
go ThetaType
tys
                      | Bool
otherwise
                      = let (ThetaType
tf_args, ThetaType
non_tf_args) = forall a. Int -> [a] -> ([a], [a])
splitAt (TyCon -> Int
tyConArity TyCon
tc) ThetaType
tys in
                        CheckTyEqResult -> CheckTyEqResult
cterSetOccursCheckSoluble (forall (t :: * -> *) m a.
(Foldable t, Monoid m) =>
(a -> m) -> t a -> m
foldMap Type -> CheckTyEqResult
go ThetaType
tf_args) forall a. Semigroup a => a -> a -> a
S.<> forall (t :: * -> *) m a.
(Foldable t, Monoid m) =>
(a -> m) -> t a -> m
foldMap Type -> CheckTyEqResult
go ThetaType
non_tf_args

     -- no bother about impredicativity in coercions, as they're
     -- inferred
    go_co :: Coercion -> CheckTyEqResult
go_co Coercion
co | TyVarLHS TcTyVar
tv <- CanEqLHS
lhs
             , TcTyVar
tv TcTyVar -> VarSet -> Bool
`elemVarSet` Coercion -> VarSet
tyCoVarsOfCo Coercion
co
             = CheckTyEqResult
soluble_occurs forall a. Semigroup a => a -> a -> a
S.<> CheckTyEqResult
maybe_hole_blocker

        -- Don't check coercions for type families; see commentary at top of function
             | Bool
otherwise
             = CheckTyEqResult
maybe_hole_blocker
      where
        -- See GHC.Tc.Solver.Canonical Note [Equalities with incompatible kinds]
        -- Wrinkle (2) about this case in general, Wrinkle (4b) about the check for
        -- deferred type errors
        maybe_hole_blocker :: CheckTyEqResult
maybe_hole_blocker | Bool -> Bool
not (GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_DeferTypeErrors DynFlags
dflags)
                           , Coercion -> Bool
hasCoercionHoleCo Coercion
co
                           = CheckTyEqResult
hole_blocker

                           | Bool
otherwise
                           = CheckTyEqResult
cteOK

    check_tc :: TyCon -> CheckTyEqResult
    check_tc :: TyCon -> CheckTyEqResult
check_tc
      | Bool
ghci_tv   = \ TyCon
_tc -> CheckTyEqResult
cteOK
      | Bool
otherwise = \ TyCon
tc  -> (if TyCon -> Bool
isTauTyCon TyCon
tc then CheckTyEqResult
cteOK else CheckTyEqResult
impredicative) forall a. Semigroup a => a -> a -> a
S.<>
                             (if TyCon -> Bool
isFamFreeTyCon TyCon
tc then CheckTyEqResult
cteOK else CheckTyEqResult
type_family)