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

[GHC]

#14203: GHC-inferred type signature doesn't actually typecheck
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        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 goldfire):

 I don't see how you want GHC to accept `bar`. Let's take this slowly.

 ```
 type family F a
 ```

 So far, so good.

 ```
 data Foo (z :: F a)
 ```

 As Simon points out, `Foo` has an ambiguous kind `forall (a :: Type). F a
 -> Type`. There's no way to infer the choice of `a` from a usage of `Foo`.
 GHC does not check for ambiguous kinds (it should, I agree), so this
 definition is accepted with or without `AllowAmbiguousTypes`. Regardless,
 you've specified `AllowAmbiguousTypes`, so accepting this conforms to your
 stated desires.

 ```
 bar :: forall a (z :: F a). Foo z -> Int
 ```

 Now we're in deep trouble. This type mentions `Foo z`. Accordingly, GHC
 must infer the invisible kind parameter to `Foo`. (You might say "Well,
 obviously it should be `a`!", but that would be assuming `F` is
 injective.) So GHC rejects this type signature, as it doesn't know what
 the invisible kind parameter to `Foo` should be. Note that this is ''not''
 the normal ambiguity check that `AllowAmbiguousTypes` disables. This type
 has an ambiguous type variable; the type itself is not ambiguous.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14203#comment:11>
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