"Yield value expected" on generator using send()

Bug Report

mypy doesn't allow empty yield statements in generator functions that accept send() values

To Reproduce

from typing import Any, Generator


def foo() -> Generator[int, Any, None]:
    yield
    while True:
        yield 42

Expected Behavior

I expected the second parameter (Any) in Generator[int, Any, None] to indicate that this generator accepts a send() value and that therefore the naked yield statement is accepting a value but not sending one. As far as I know the above code is idiomatic.

Actual Behavior

error: Yield value expected

mypy complains that a yield value is expected, but this standalone yield is for the purpose of accepting the value of a send() to the generator.

Your Environment

mypy 0.812
python 3.7

Start by running the supplied Python example with mypy and confirming the “Yield value expected” diagnostic. Trace how mypy checks generator yield statements and the Generator send type; done means a bare yield is accepted in this case, with regression coverage for the example.