> If SupportsComplex and SupportsBytes are just for "has __complex__/__bytes__ method", they are virtually useless.

I agree that "SupportsComplex" isn't directly useful in user-land. I think its main value is as a building block in things like `Union[SupportsComplex, SupportsFloat, SupportsIndex]`.

For me, the gain from implementing complex.__complex__ is that the test "can be used as a complex number" can now be expressed purely in terms of the protocols offered, without reference to concrete types.