[GHC] #13192: Ambiguity Caused By PolyKind and Not Helpful Error Messages

[GHC]

#13192: Ambiguity Caused By PolyKind and Not Helpful Error Messages
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        Reporter:  Shayan-Najd       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 The type instance in @rwbarton's case expands to `F k (I k a) = a`. This
 instance requires the two `k`s to be the same. In `identity`'s type
 signature, we get `F (TYPE r) (I k a) -> F (TYPE r') (I k a)`. GHC
 (rightly) doesn't know that `k` and `TYPE r` are the same. (The fact that
 it's `TYPE r` and not `Type` is ''not'' the issue.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13192#comment:5>
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