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.