Incorrect behavior with empty functional dependencies

This is a corner case, but it would appear that GHC doesn't do proper improvements for classes with "empty" functional dependencies. Consider the following example:

class C a | -> a where
   m :: a -> ()

instance C Int

f x = m x

The inferred type for f is C a => a -> (), however I was expecting it to infer Int -> ().

The reasoning is as follows: the use of m generates a C a constraint (a is a unification variable). This should have interacted with the C Int instance (in a rather odd vacuous sort of way) to generate a derived constraint a ~ Int, which should have then instantiated a to Int.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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