Assertion failed with BuildFlavour = devel2 (one more)

Affected versions: ghc 8.0.1 and ghc git master as of 21 Oct 2016 (c23dc616)

It compiles fine on first time but fails to compile on second attempt. Using -fforce-recomp recompiles both files and bug is not triggered.

% ls
A.hs  B.hs

% cat A.hs

module A where

import B

foo a = case a of
        Foo -> True
        _ -> False

% cat B.hs

{-# LANGUAGE PatternSynonyms, ScopedTypeVariables #-}

module B where

pattern Foo = 0x00000001 :: Int

% ghc -O2 A.hs ; touch A.hs ; ghc -O2 A.hs
[1 of 2] Compiling B                ( B.hs, B.o )
[2 of 2] Compiling A                ( A.hs, A.o )
[2 of 2] Compiling A                ( A.hs, A.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
        ASSERT failed!
  CallStack (from HasCallStack):
  assertPprPanic, called at compiler/iface/BuildTyCl.hs:195:404 in ghc:BuildTyCl
  [] ~ []
  [] ~ []
  Int ~ Int
  [] ~ []
  [] ~ []
  [] ~ [Void#]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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