An idea for a warning

[Sven Panne]

I've just once again fixed a situation in the library which looked as follows:

   module Foo(Bar(..)) where
   data Bar = Baz Blah
   type Blah = String

Although Bar is usable, I very much doubt that it is intended that Blah is not 
exported from Foo. Haddock complains about such situations, BTW. Would it 
make sense to add a warning to GHC in such cases?

Cheers,
   S.