Class methods must always mention the class variable

As mentioned in ticket:7854#comment:96716, GHC wrongfully accepts the following program. It should reject it, because op doesn't mention any type variables of Foo (the method context doesn't count).

module ShouldFail where
class Foo a where
  op :: Eq a => Int

I'm working on it.

Trac metadata

Trac field	Value
Version	7.10.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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