tcfail118.hs:10:29:
    Overlapping instances for Eq Foo
      arising from the first field of ‘Bar’ (type ‘Foo’)
    Matching instances:
      instance Eq Foo -- Defined at tcfail118.hs:11:25
      instance Eq Foo -- Defined at tcfail118.hs:13:10
    When deriving the instance for (Eq Bar)