mistake in class inheritance system

class can't use function of parents class. Code:

class A a where

toInt	a -\> Int
func	a -\> a -\> Int

class (A a) => B a where

toInteger :: a -> Integer

f :: (A a) => (B b) => a -> b -> Int

f a b = func a b

We must be able to use (func a b) because (func :: a -> a -> Int) and b is member of A class.

error:

Could not deduce (b ~ a)

from the context (A a, B b)

bound by the type signature for f	(A a, B b) =\> a -\> b -\> Int
	at bug.hs:9:1-16
	`b' is a rigid type variable bound by the type signature for f :: (A a, B b) => a -> b -> Int at bug.hs:9:1 `a' is a rigid type variable bound by
the type signature for f	(A a, B b) =\> a -\> b -\> Int
	at bug.hs:9:1
	In the second argument of `func', namely `b'
	In the expression: func a b
	In an equation for \`f': f a b = func a b

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