[GHC] #7503: Bug with PolyKinds, type synonyms & GADTs

[GHC]

#7503: Bug with PolyKinds, type synonyms & GADTs
-------------------------------------+-------------------------------------
        Reporter:  Ashley Yakeley    |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.6.1
  checker)                           |             Keywords:  GADTs,
      Resolution:                    |  TypeInType
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  GHC rejects       |  (amd64)
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14451            |  Differential Rev(s):
       Wiki Page:                    |
  https://ghc.haskell.org/trac/ghc/wiki/GhcKinds/KindInference|
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Continuing my journey started in
 https://ghc.haskell.org/trac/ghc/ticket/14451#comment:7, it appears that
 something changed between GHC 8.4 and 8.6 that affects the programs in
 this ticket. Amazingly, the original program:

 {{{#!hs
 {-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds,
 KindSignatures, GADTs #-}
 module TestConstraintKinds where
     import GHC.Exts hiding (Any)

     data WrappedType = forall a. WrapType a

     data A :: WrappedType -> * where
         MkA :: forall (a :: *). AW a -> A (WrapType a)

     type AW  (a :: k) = A (WrapType a)
     type AW' (a :: k) = A (WrapType a)

     class C (a :: k) where
         aw :: AW a -- workaround: AW'

     instance C [] where
         aw = aw
 }}}

 Now typechecks with GHC 8.6.2 and HEAD!

 Unfortunately, it might be too early to declare victory here, since the
 program in comment:1:

 {{{#!hs
 {-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds,
 KindSignatures, GADTs #-}
 module TestConstraintKinds where

 data Wrap a --  Wrap :: forall k. k -> *

 data A a = MkA a (AW a)

 type AW  a = A (Wrap a)
 type AW2 a = A (Wrap a)

 class C (a :: k) where
     aw :: AW a
 }}}

 Still refuses to typecheck, even on GHC 8.6.2 and HEAD.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7503#comment:10>
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