id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
5400,GHC loops on compiling with optimizations,noschinl,,"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.",bug,new,normal,_|_,Compiler,7.0.4,,,,Linux,x86_64 (amd64),Compile-time crash,,,,,