[GHC] #9867: PatternSynonyms + ScopedTypeVariables triggers an internal error

[GHC]

#9867: PatternSynonyms + ScopedTypeVariables triggers an internal error
-------------------------------------+-------------------------------------
       Reporter:  antalsz            |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 When trying to give a type signature with a free type variable to a
 pattern in a pattern synonym, GHC dies with an internal error.  Given the
 file

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, ScopedTypeVariables #-}
 pattern Nil = [] :: [a]
 }}}

 the resulting error is

 {{{
 Example.hs:2:22: GHC internal error: ‘a’ is not in scope during type
 checking, but it passed the renamer …
    tcl_env of environment: []
    In an expression type signature: [a]
    In the expression: [] :: [a]
    In an equation for ‘$WNil’: $WNil = [] :: [a]
 Compilation failed.
 }}}

 The fix is to add `RankNTypes` and an explicit `forall`:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, ScopedTypeVariables, RankNTypes #-}
 pattern Nil = [] :: forall a. [a]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9867>
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