Panic due 'instance Foo a => Bar a => Baz a' style declarations

[ksf]

class Foo a

instance Show a => Num a => Foo a  where
    foo = undefined

results in

ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.2 for x86_64-unknown-linux):
        compiler/rename/RnSource.lhs:430:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,
                                                        _,
                                                        SrcLoc.L _ cls,
                                                        _)

It doesn't matter whether foo is declared in the class definition or not.

Things that work:

1. tuple-style context

2. not defining foo in the instance: Not crashing, but complaining about a malformed head, which might or might not be the right thing to do, considering

3.

foo :: Show a => Num a => a
foo = undefined

[guest]

seems to be a duplicate of #5951

[simonpj]

Yes it is a dup, and indeed fixed:

T7039.hs:5:10: Malformed instance: Show a => Num a => Foo a