Template Haskell panic with class names

File Bug.hs:
module Bug where
foo = ''Show

Compile using:
ghc -c -fth Bug.hs

Result:
ghc-6.4.1: panic! (the `impossible' happened, GHC version 6.4.1):
        lookupDeprec GHCziShow.Show{tc 2h}

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

The panic does not happen if the type class or one of its method is 
actually used in the same module.

Trac metadata

Trac field	Value
Version	6.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	ResolvedFixed
Component	Template Haskell
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Template Haskell panic with class names