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

[GHC]

#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:  8.1
  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):

 * component:  Template Haskell => Compiler (Type checker)


Comment:

 It turns out this has nothing whatsoever to do with Template Haskell. The
 real culprit is `forall`. That is, while this program compiles without
 issue:

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

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

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

 Changing the pattern signature to this:

 {{{#!hs
 pattern P :: forall b a. b -> T a
 }}}

 causes the error to appear:

 {{{
 Bug.hs:10:20: error:
     • Couldn't match expected type ‘b’ with actual type ‘b1’
       ‘b1’ is a rigid type variable bound by
         a pattern with constructor: MkT :: forall a b. Eq b => b -> T a,
         in a pattern synonym declaration
         at Bug.hs:10:16-20
       ‘b’ is a rigid type variable bound by
         the signature for pattern synonym ‘P’ at Bug.hs:9:21
     • In the declaration for pattern synonym ‘P’
     • Relevant bindings include x :: b1 (bound at Bug.hs:10:20)
 }}}

 It turns out that when you quote a pattern synonym declaration with TH
 quotes, it implicitly adds the `forall` behind the scenes, triggering this
 bug when you try to splice it back in.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13018#comment:3>
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