class C a where {
  type T a;
--  foo :: a -> List Char;
}

data D a = D a;

instance C a => C (D (T a)) where {
  type T (D (T a)) = T a;
--  foo _ = "ok";  -- ambiguous signature
}

instance C Int where {
  type T Int = Bool;
--  foo _ = "fail";
}

bar :: T (D (T Int)) -> Bool;
bar b = b;

main = bar True;