[GHC] #11977: ghc doesn't agree with its own inferred pattern type

[GHC]

#11977: ghc doesn't agree with its own inferred pattern type
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Just fooling around

 {{{#!hs
 pattern Mempty :: Monoid a => b -> forall x. a -> b
 pattern Mempty b <- (($ mempty) -> b)
   where Mempty b _ = b
 }}}

 {{{#!hs
 pattern Mempty :: b -> forall a. (Eq a, Monoid a) => a -> b
 pattern Mempty b <- (($ mempty @String) -> b)
   where Mempty b _ = b

 foo :: (forall a. (Eq a, Monoid a) => a -> b) -> b
 foo (Mempty x) = x
 }}}

 {{{
 ghci> foo (== mempty)
 True
 ghci> :t Mempty 'a'
 Mempty 'a' :: forall {a}. (Monoid a, Eq a) => a -> Char
 ghci> :t foo (Mempty 'a')
 foo (Mempty 'a') :: Char
 ghci> foo (Mempty 'a')
 'a'
 }}}

 or

 {{{#!hs
 type MEMPTY b = forall a. (Eq a, Monoid a) => a -> b

 pattern Mempty :: b -> MEMPTY b
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11977#comment:8>
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