"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

まず、提供された Python の例を mypy で実行し、「Yield value expected」診断を確認します。mypy がジェネレーターの yield 文と Generator の send 型をどのようにチェックするかを追跡します。この場合に単独の yield が受け入れられ、例に対するリグレッションカバレッジが追加されれば完了です。