Treat Assigning Coroutine to `_` the Same as Unused Coroutine

Feature

Treat assigning a function's coroutine return value to the _ variable the same as an unused coroutine.

Pitch

Mypy attempts to catch the bug of forgetting to await a coroutine by detecting unused coroutines. For example, with mypy 1.19.1:

async def foo() -> int:
    return 42

def bar() -> None:
    foo()  # error: Value of type "Coroutine[Any, Any, int]" must be used  [unused-coroutine]

It's the style convention of some organizations to assign the unused return value of a function to the _ variable. This signals to readers that the return value was intentionally ignored and not a possible bug. However, this then silences the unused-coroutine issue.

async def foo() -> int:
    return 42

def bar() -> None:
    _ = foo()  # no mypy error

I'm not sure if there is precedent of mypy treating _ differently, but I believe that practically this would aim to catch the same category of bugs as unused-coroutine.

まず、未使用のコルーチンに関する mypy の既存の診断と、_ への代入の処理を見つけます。例 _=foo() がどのように解析されるかを追跡し、続いて、対象を絞った型チェックの例を使って、コルーチンを _ に代入した場合に、未使用のままにした場合と同じ診断が生成されることを確認します。