Functional dependency Coverage Condition is lifted, and should not be

Hello, GHC 6.6 (with flags -fglasgow-exts -fallow-undecidable-instances) is too liberal when accepting instances. Here is an example:

class F a b | a -> b where f :: (a,b)
instance F Int b

The instance violates the functional dependency constraint of F because (in theory) it can be used to solve both F Int Bool and F Int Char. In practice, it seems that some free variable gets bound upon the first use of f with a concrete type, leading to some rather confusing behavior. Example:

x :: (Int,a)
x = f
 
y :: (Int,Bool)
y = f
 
z :: (Int,Char)
z = x -- works, but 'z = f' does not

-Iavor