[GHC] #14203: GHC-inferred type signature doesn't actually typecheck

[GHC]

#14203: GHC-inferred type signature doesn't actually typecheck
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Here's a slightly simpler example that doesn't use `singletons`. This
 compiles:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind

 data TyFun :: * -> * -> *
 type a ~> b = TyFun a b -> *
 infixr 0 ~>
 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2

 data Foo (z :: Apply (p :: a ~> *) (x :: a))

 data Bar where
   MkBar :: Foo z -> Bar
 }}}

 And if you ask GHCi for the type of `MkBar`, you get:

 {{{
 λ> :type +v MkBar
 MkBar
   :: forall a (p :: TyFun a * -> *) (x :: a) (z :: Apply p x).
      Foo z -> Bar
 }}}

 But if you try using that:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind

 data TyFun :: * -> * -> *
 type a ~> b = TyFun a b -> *
 infixr 0 ~>
 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2

 data Foo (z :: Apply (p :: a ~> *) (x :: a))

 data Bar where
   MkBar :: forall a (p :: TyFun a * -> *) (x :: a) (z :: Apply p x).
            Foo z -> Bar
 }}}

 It fails:

 {{{
 GHCi, version 8.3.20170907: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:19:16: error:
     • Expected kind ‘Apply p0 x0’, but ‘z’ has kind ‘Apply p x’
     • In the first argument of ‘Foo’, namely ‘z’
       In the type ‘Foo z’
       In the definition of data constructor ‘MkBar’
    |
 19 |            Foo z -> Bar
    |                ^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14203#comment:1>
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