{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-overlapping-instances #-}

class Foo x
  where
    foo :: x -> ()
    foo _ = ()

instance Foo String
instance Foo [a]

main = print $ foo "bar"

{-

*Main> main
()
*Main> foo "bar"
 
<interactive>:1:0:
    Overlapping instances for Foo [Char]
      arising from use of `foo' at <interactive>:1:0-2
    Matching instances:
      Test.hs:10:0: instance Foo [a]
      Test.hs:9:0: instance Foo String
    In the definition of `it': it = foo "bar"

-}