T7279.hs:6:10:
    Could not deduce (Show b0)
    from the context: (Eq a, Show b)
      bound by an instance declaration: (Eq a, Show b) => Eq (T a)
      at T7279.hs:6:10-35
    The type variable ‘b0’ is ambiguous
    In the ambiguity check for an instance declaration:
      forall a b. (Eq a, Show b) => Eq (T a)
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the instance declaration for ‘Eq (T a)’