[GHC] #12489: undefined in view pattern inside pattern synonym causes GHC to panic

[GHC]

#12489: undefined in view pattern inside pattern synonym causes GHC to panic
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           Reporter:  pkmx           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following:

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

 pattern P :: a -> b
 pattern P a <- (undefined -> a)
 }}}

 causes GHC HEAD and 8.0.1 to panic:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20160813 for x86_64-unknown-linux):
         StgCmmEnv: variable not found
 }}}

 Removing the pattern signature or moving `undefined` behind another name
 work however:

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

 bottom :: a
 bottom = undefined

 pattern P :: a -> b
 pattern P a <- (bottom -> a) -- OK!

 -- No type signature
 pattern P' a <- (undefined -> a) -- OK!
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12489>
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