[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 simonpj):

 In comment:6 we have
 {{{
 Foo :: forall a. F a -> *
 }}}
 That's an ambiguous kind!  Just like for a function
 {{{
 foo :: forall k. F k -> Int
 }}}
 is ambiguous.  So all uses of `Foo` will give rise to ambiguity.  But alas
 we do not check for ambiguous kinds.

 Instead we see a "call" of `Foo` in the type signature
 {{{
 bar ::  forall a (z :: F a). Foo z -> Int
 }}}
 At the use of `Foo` we intantiate `k` to `kappa` and check that `F kappa`
 is equal o the kind of `z` namely `F a`.  So for the type to be well-
 kinded we need `F kappa ~ F a`.  But we have no way to prove that (unless
 `F` is injective).  And that's the error.

 TL;DR: it's `Foo` that should be rejected as having an ambiguous kind.

 (I have not studied the oroginal Description; just assuming that comment:6
 embodies the essence.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14203#comment:7>
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