@overload with multiple Generic[Any]

[randolf-scholz]

Am I missing something, or is it impossible to annotate op in a way that satisfies the assert_type test-suite in the example below?

from typing import Any, overload, assert_type

class A[T]:  # covariant
    def get(self) -> T: ...

@overload
def op(l: A[None], r: A[None]) -> A[None]: ...
@overload
def op(l: A[None], r: A[Any]) -> A[None]: ...
@overload
def op(l: A[Any], r: A[None]) -> A[None]: ...
@overload
def op(l: A[Any], r: A[Any]) -> A[Any]: ...
def op(l, r):
    # dummy implementation:
    if l.get() is None or r.get() is None:
        return A[None]()
    return A[Any]()

def test(x: A[None], y: A[Any]) -> None:
    assert_type(op(x, x), A[None])
    assert_type(op(x, y), A[None])  # ❌️: (mypy, zuban)
    assert_type(op(y, x), A[None])  # ❌️: (mypy, zuban)
    assert_type(op(y, y), A[Any])   # ❌️: (pyright, ty, pyrefly)

The schema is relevant for type hinting numeric types that interact with math.nan / nullable values.

[randolf-scholz]

The alternative of not preserving None in the mixed case makes mypy and zuban happy, but gives even worse results on the other type checkers:

from typing import Any, overload, assert_type

class A[T]:  # covariant
    def get(self) -> T: ...

@overload
def op(l: A[None], r: A[None]) -> A[None]: ...
@overload
def op(l: A[None], r: A[Any]) -> A[Any]: ...
@overload
def op(l: A[Any], r: A[None]) -> A[Any]: ...
@overload
def op(l: A[Any], r: A[Any]) -> A[Any]: ...
def op(l, r):
    # dummy implementation:
    if l.get() is None or r.get() is None:
        return A[None]()
    return A[Any]()

def test(x: A[None], y: A[Any]) -> None:
    assert_type(op(x, x), A[None])
    assert_type(op(x, y), A[Any])  # ❌️: (pyright, pyrefly, ty)
    assert_type(op(y, x), A[Any])  # ❌️: (pyright, pyrefly, ty)
    assert_type(op(y, y), A[Any])  # ❌️: (pyright, pyrefly, ty)