Starting from version 1.0.0, a union of Generators is (sometimes) treated as if it has no ReturnType

[mwchase]

Bug Report

Starting from version 1.0.0, yield from a union on Generator types generates the error Function does not return a value [func-returns-value], and attempting to use the return reveals that Mypy is, in fact, treating it as if it's None.

There are ways around this; it appears that the inference is a little smarter at function boundaries than in-line, so I've just defined a bunch of helper functions to work around it for now.

To Reproduce

from typing import Generator, TypeVar

T = TypeVar("T")


def my_iter(arg: Generator[int, None, T] | Generator[str, None, T]) -> Generator[int | str, None, T]:
    return (yield from arg)

(https://mypy-play.net/?mypy=0.991&python=3.11&gist=5c3183a0d4d1c02161211b01654d519f)

(The repro is working with YieldType, but in my code it's changing ReturnType. It breaks either way around.)

Actual Behavior

On post-1.0.0 versions:

main.py:7: error: Function does not return a value  [func-returns-value]
main.py:7: error: Incompatible return value type (got "None", expected "T")  [return-value]
Found 2 errors in 1 file (checked 1 source file)

Pre-1.0.0, produces no output.

[hauntsaninja]

I bisected this back to #14224 , which fixed a case where we were basically not type checking coroutines and generators. My guess is you'd need to make a similar change in

mypy/mypy/checkexpr.py

Line 5079 in 6b326d5

def visit_yield_from_expr(self, e: YieldFromExpr, allow_none_return: bool = False) -> Type:

[hauntsaninja]

By the way, did you happen to encounter this regression in an open source project?

[mwchase]

This is in something I'm working on that's technically open source, but not in any convenient way; I don't really want to publicize it anywhere before it's useful.

EDIT (May 3): Also, it uses some custom plugins, so I think currently it can't go in mypy_primer anyway. I forgot about that.

[hauntsaninja]

Thanks! I'd love to include it in mypy_primer so we avoid regressions on that project in the future; if you're ever comfortable with that at some point, feel free to post a link to the project here