UNPACK without optimisation leads to panic

Here's a two-module progam

module Foo where
  import Bar
  blah :: S -> T
  blah (MkS x _) = x

module Bar( S(..), T ) where
  data T = MkT Int Int
  data S = MkS {-# UNPACK #-}!T Int

Now with ghc 7.0.3 we get

bash-3.1$ ghc -c Bar.hs
bash-3.1$ ghc -c Foo.hs
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 7.0.3 for i386-unknown-mingw32):
	reboxProduct: not a product main:Foo1.T{tc r2}

The problem is that

• We are compiling with -O so GHC tries to put as little as possible into the interface file Bar.hi. And it does not put in T's constructors

  data S
      RecFlag NonRecursive
      Generics: no
      = MkS :: Foo1.T -> GHC.Types.Int -> S
        HasWrapper
            Stricts: {-# UNPACK #-} ! _
43edb8535d0555fb50e9f93a9c3203bf
  data T
      RecFlag NonRecursive
      Generics: no
      {- abstract -}

• However the pattern match in Foo requires that GHC can see the full representation for T, becuase it UNPACK's the argument.
• A workaround is to export MkT from Bar.

The solution I am implementing is to ignore UNPACK pragmas when OmitInterfacePragmas is on. This flag is the one that causes trimming of the exposed constructors.

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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