[GHC] #9063: Default associated type instances are too general

[GHC]

#9063: Default associated type instances are too general
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 Here is an example where this problem bites in instances:

 {{{
 {-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}

 module Bug where

 import Data.Proxy

 class kproxy ~ 'KProxy => PEq (kproxy :: KProxy a) where
   type (:==) (x :: a) (y :: a) :: Bool

 instance PEq ('KProxy :: KProxy Bool) where
   type False :== False = True
   type False :== True  = False
   type True  :== False = False
   type True  :== True  = True

 instance PEq ('KProxy :: KProxy ()) where
   type x :== y = True
 }}}

 I get

 {{{
     Type indexes must match class instance head
     Found ‘k’ but expected ‘()’
     In the type instance declaration for ‘:==’
     In the instance declaration for ‘PEq (KProxy :: KProxy ())’
 }}}

 That's not an overlapping instance error -- I was wrong in my post above.
 I guess I didn't look closely at the error. But, GHC should be able to
 figure out what I mean here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9063#comment:3>
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