undefined in view pattern inside pattern synonym causes GHC to panic

The following:

{-# 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:

{-# 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!

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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