panic! mkWWcpr: not a product

I encountered the following bug:

ghc-6.4: panic! (the `impossible' happened, GHC version
6.4):
        mkWWcpr: not a product SPIRziSPIR.SPIR{tc r3jI}

Please report it as a compiler bug to
glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.

This was in conjunction with the use of .hs-boot files
for recursive modules, though I don't know for sure if that
actually caused the problem.

Unfortunate I can't provide a simple reproducible test case
at this point, but I thought I'd let you know about the
bug anyway.

Fergus Henderson <fjh-mailbox-30@galois.com>