[GHC] #12820: Regression around pattern synonyms and higher-rank types

10 Nov 2016

      #12820: Regression around pattern synonyms and higher-rank types
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 GHC 8.0.1 accepts, but HEAD rejects:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, RankNTypes, ViewPatterns #-}

 module Bug where

 pattern P :: (forall a. a -> a) -> String
 pattern P x <- (\ _ -> id -> x)
 }}}

 (Sidenote: kudos to the parser on figuring out my view pattern.)

 HEAD gives this error:

 {{{
 Bug.hs:6:30: error:
     • Couldn't match expected type ‘forall a. a -> a’
                   with actual type ‘a0 -> a0’
     • In the declaration for pattern synonym ‘P’
     • Relevant bindings include x :: a0 -> a0 (bound at Bug.hs:6:30)
 }}}

 The code looks correct to me.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12820
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