It seems to be that a missing associated type definition should be an error, by default, rather than a warning. The current behavior under those circumstances strikes me as very strange, particularly for data families and particularly in the presence of overlapping.

{-# LANGUAGE TypeFamilies #-}

class Foo a where
data Assoc a
foo :: proxy a -> Assoc a

instance {-# OVERLAPPABLE #-} Foo a where
data Assoc a = AssocGeneral
foo _ = AssocGeneral

instance {-# OVERLAPS #-} Foo Char where
foo _ = AssocGeneral

blah :: Assoc Char
blah = foo (Proxy :: Proxy Char)

This compiles with just a warning because Assoc Char *falls through* to the general case. WAT? This breaks all my intuition about what associated types are supposed to be about.