GHC loops on compiling with optimizations

When compiling the snippet below with optimizations enabled (i.e. ghc -O test.hs), GHC does not seem to terminate. This happens with at least 7.0.3 and 7.0.4. Without optimizations, it builds fine.

data A = A Int

class C a where 
    fromHsk :: C a => a -> Int

instance C Int where 
    fromHsk = fromHsk

instance C A where 
    fromHsk (A s) = fromHsk s

main = undefined

When building this example with GHC 6.12.4, it fails with a complaint about the bogus type constraints in the declaration of fromHsk:

test.hs:3:0:
    All of the type variables in the constraint `C a'
    are already in scope (at least one must be universally quantified here)
        (Use -XFlexibleContexts to lift this restriction)
    When checking the class method: fromHsk :: (C a) => a -> Int
    In the class declaration for `C'

After removing these unneeded constraint (i.e. changing the declaration of fromHsk to fromHsk :: a -> Int, the snippet above builds even with GHC 7.0.4 and optimizations.

Trac metadata

Trac field	Value
Version	7.0.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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