stubgen generates stubs for `@asynccontextmanager` functions that mypy itself rejects

Bug Report

stubgen copies @asynccontextmanager and the async def keyword verbatim into the generated .pyi. Because a stub body has no yield, mypy then classifies the stubbed function as a coroutine function returning AsyncIterator[T] rather than as an async generator function, and rejects the decorator application.

The result is that stubgen emits a stub which mypy — the same version, with default settings — reports an error on. Source that type-checks cleanly produces a stub that does not.

To Reproduce

# cm.py
from collections.abc import AsyncIterator
from contextlib import asynccontextmanager


@asynccontextmanager
async def ctx() -> AsyncIterator[int]:
    yield 1

$ mypy cm.py
Success: no issues found in 1 source file

$ stubgen -o out cm.py
Processed 1 modules
Generated out/cm.pyi

$ cat out/cm.pyi
from collections.abc import AsyncIterator
from contextlib import asynccontextmanager

@asynccontextmanager
async def ctx() -> AsyncIterator[int]: ...

$ mypy out/cm.pyi
out/cm.pyi:4: error: Argument 1 to "asynccontextmanager" has incompatible type "Callable[[], Coroutine[Any, Any, AsyncIterator[int]]]"; expected "Callable[[], AsyncIterator[Never]]"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

The same happens for methods, and with AsyncGenerator[T, None] in place of AsyncIterator[T].

Expected Behavior

stubgen output should type-check under the mypy version that produced it. For an @asynccontextmanager-decorated async generator, that means emitting one of the two spellings the typing docs sanction for stubs — either dropping async:

@asynccontextmanager
def ctx() -> AsyncIterator[int]: ...

or dropping the decorator and declaring the decorated result, as typeshed does:

from contextlib import AbstractAsyncContextManager

def ctx() -> AbstractAsyncContextManager[int]: ...

Both of these are accepted by mypy 2.3.0.

Actual Behavior

out/cm.pyi:4: error: Argument 1 to "asynccontextmanager" has incompatible type "Callable[[], Coroutine[Any, Any, AsyncIterator[int]]]"; expected "Callable[[], AsyncIterator[Never]]"  [arg-type]

Your Environment

• Mypy version used: 2.3.0 (compiled: yes)
• Mypy command-line flags: none (mypy out/cm.pyi); stub produced with stubgen -o out cm.py
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.15

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Additional notes

This is a change in behaviour from 1.15.0. That version's stubgen omitted the decorator entirely:

# stubgen 1.15.0
async def ctx() -> AsyncIterator[int]: ...

which is also lossy, but happens to type-check, so the problem only becomes visible on 2.3.0 once the decorator is preserved.

The synchronous case is unaffected — @contextmanager over def f() -> Iterator[T] round-trips through stubgen and type-checks on 2.3.0, because there is no async keyword to change how the return type is interpreted.

Inizia dal punto di ingresso di stubgen e riproduci il problema usando cm.py, l’esempio @asynccontextmanager riportato. Confronta l’output .pyi generato per le funzioni e i metodi generatori asincroni, quindi esegui mypy sullo stub generato. Il lavoro è completato quando lo stub generato per questo caso supera il controllo dei tipi senza errori, preservando al contempo la tipizzazione prevista del gestore di contesto.