[GHC] #14419: Check kinds for ambiguity

[GHC]

#14419: Check kinds for ambiguity
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Actually, the situation in comment:4 in which a visible, dependent kind is
 erroneously flagged as ambiguous can occur //today//, even without that
 patch. Take this program, for example:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Data.Kind

 type KindOf (a :: k) = k
 type family F a
 data A (a :: Type) :: F a -> Type

 data Ex (x :: KindOf A)
 }}}
 {{{
 $ /opt/ghc/8.6.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:13:1: error:
     • Couldn't match type ‘F a’ with ‘F a0’
       Expected type: F a -> *
         Actual type: F a0 -> *
       NB: ‘F’ is a non-injective type family
       The type variable ‘a0’ is ambiguous
     • In the ambiguity check for the kind annotation on the type variable
 ‘x’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
    |
 13 | data Ex (x :: KindOf A)
    | ^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I believe this should be accepted since `KindOf A` reduces to `forall a ->
 F a -> Type`, and the `forall a ->` //should// uniquely determine the `a`
 in `F a`. Alas, GHC doesn't currently recognize this.

 Thankfully, the workaround for the time being is simple: just enable
 `AllowAmbiguousTypes`. But I'll go ahead and post this here to make it
 clear that this is a problem in the present, and not just a problem for a
 future GHC.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14419#comment:9>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler