T5287.hs:6:10: error:
    • Could not deduce (A a oops0)
      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 an instance declaration
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the instance declaration for ‘Read (D a)’