T5287.hs:6:10:
    Could not deduce (A a oops0)
      arising from the ambiguity check for an instance declaration
    from the context (A a oops)
      bound by an instance declaration: A a oops => Read (D a)
      at T5287.hs:6:10-31
    The type variable ‛oops0’ is ambiguous
    In the ambiguity check for: forall a oops. A a oops => Read (D a)
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the instance declaration for ‛Read (D a)’