[GHC] #8978: Type synonyms are not *exactly* synonyms

[GHC]

#8978: Type synonyms are not *exactly* synonyms
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       Reporter:  joelteon  |             Owner:
           Type:  bug       |            Status:  new
       Priority:  normal    |         Milestone:
      Component:  Compiler  |           Version:  7.8.1
       Keywords:            |  Operating System:  Unknown/Multiple
   Architecture:            |   Type of failure:  GHC rejects valid program
  Unknown/Multiple          |         Test Case:
     Difficulty:  Unknown   |          Blocking:
     Blocked By:            |
Related Tickets:            |
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 {{{
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE TypeFamilies #-}

 type Syn a = Associated a

 class Eq (Associated a) => Foo a where
     type Associated a :: *
     foo :: a -> Syn a -> Bool

 instance Foo () where
     type Associated () = Int
     foo _ x = x == x
 }}}

 In 7.6.3 and 7.8.1-rc2, this file compiles. In 7.8.1 release, GHC produces
 this error:

 {{{
     No instance for (Eq (Syn ())) arising from a use of ‘==’
     In the expression: x == x
     In an equation for ‘foo’: foo _ x = x == x
     In the instance declaration for ‘Foo ()’
 }}}

 even though there is an Eq instance for Associated (), and Syn is just a
 type synonym.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8978>
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