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I've tried to simplify this as much as possible. In particular, the bug goes away if -O1 is removed.

module Bug1 where

data A a where
  MkA :: A ()

class C w where
  f :: forall a . w a -> Maybe a

instance C A where
  f MkA  = Just ()

module Bug2 where
import Bug1

f' :: A a -> Maybe a
f' = f

Run with this:

{{{ #!/bin/sh rm -f *.hi *.o ghc --version ghc -c -XGADTs -XRankNTypes -O1 Bug1.hs ghc -c -XGADTs -XRankNTypes -O1 Bug2.hs }}}

This is the result I get:

The Glorious Glasgow Haskell Compilation System, version 6.8.2
Bug1.hi
Declaration for a:
  Iface type variable out of scope:  a
Cannot continue after interface file error

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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