[GHC] #8999: Type synonym not normalized in instance head

15 Apr 2014

      #8999: Type synonym not normalized in instance head
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       Reporter:  edsko             |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       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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 Consider

 {{{
 {-# LANGUAGE ConstraintKinds, KindSignatures #-}

 type Foo (m :: * -> *) = Monad m

 data T a = T (Maybe a)

 instance Foo T where
   return = T Nothing
 }}}

 this gives the bogus error

 {{{
 T.hs:8:3: ‘return’ is not a (visible) method of class ‘Foo’
 }}}

 incidentally, it works okay when giving no where clause.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8999
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