Missing definitions of associated types

[David Feuer]

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.
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