BUG: SupportsComplex doesn't seem to work

[shoyer]

The following snippet triggers a mypy error:

from typing import SupportsComplex

class A(SupportsComplex):
    def __complex__(self) -> complex:
        return 1j

complex(A())
# No overload variant of "complex" matches argument types [test_simple.A]

I believe this is the right way to indicate that a class supports casting to complex values.

I'm using Python 3.6.4 and mypy 0.560. See numpy/numpy-stubs#10 for context.

[gvanrossum]

Oh, this looks like a bug in typeshed. Both the Py2 and Py3 definition of class complex there are lacking several things -- a base class of SupportsComplex and an overload of the constructor to take a SupportsComplex value.

Can you submit a PR or issue there?

[shoyer]

OK, I'm still a little new to the typeshed/mypy, so I opened at issue rather than PR for now: python/typeshed#1904

Closing this since it seems to be a typeshed rather than mypy issue.