Using an inferred type as a type signature fails

See the thread starting here http://www.haskell.org/pipermail/glasgow-haskell-users/2006-December/011714.html. Here's a short example:

class C a b where
   op :: a -> a

-- f :: C a b => a -> a
f x = op x

It doesn't get much simpler than that! With the type sig, GHC can't see that the (C a b) provided can satisfy the (C a b1) which arises from the call to op. However, without the constraint, GHC simply abstracts over the constrains arising in the RHS, namely (C a b1), and hence infers the type

        f :: C a b1 => a -> a

It is extremely undesirable that the inferred type does not work as a type signature, but I don't see how to fix it easily. It doesn't affect many programs, I think; hence low priority

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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CC
Operating system	Unknown
Architecture	Unknown