Re: [GHC] #13018: TH-spliced pattern synonym declaration fails to typecheck

21 Dec 2016

      #13018: TH-spliced pattern synonym declaration fails to typecheck
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
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:                    |
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Changes (by RyanGlScott):

 * version:  8.1 => 8.0.1

Comment:

 It turns out this is a regression, since this program:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module M where

 data T a where
   MkT :: Eq b => b -> T a

 pattern P :: forall b a. b -> T a
 pattern P x <- MkT x
 }}}

 compiles with GHC 7.10.3, but not with GHC 8.0.1, 8.0.2, or HEAD.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13018#comment:4
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