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

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

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

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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