Thanks for picking this up — I filed #21869.

I applied this patch to a v2.3.0 checkout and tested it. It fixes the issue, and the output is not just error-free but type-equivalent to the source. Running the same consumer against the generated stub and against the original module:

So dropping async from the emitted signature preserves the decorated type exactly, which I think settles that this spelling is the right one of the two the typing docs allow.

I also checked the failure mode this design could have — _has_async_contextmanager is instance state, so the risk is it leaking into a following function and stripping a legitimate async. It doesn't. Interleaving the cases:

@asynccontextmanager
async def ctx() -> AsyncIterator[int]: ...   # -> async dropped
async def plain_coro() -> int: ...           # -> async kept
@contextmanager
def sync_ctx() -> Iterator[int]: ...         # -> unchanged
class A:
    @asynccontextmanager
    async def actx(self) -> AsyncGenerator[str, None]: ...  # -> dropped
    async def method_coro(self) -> int: ...                 # -> kept
    @staticmethod
    @asynccontextmanager
    async def sctx() -> AsyncIterator[int]: ...             # -> dropped, both decorators kept

All correct. That holds because the flag is set in process_decorator and cleared in clear_decorators, i.e. exactly the lifecycle _decorators already has.

One suggestion: that non-interference isn't pinned by a test. Since it's the single property this approach could regress on, and the harness compares whole-file output, it costs four lines folded into the case you already added rather than a new one:

 [case testAsyncContextManager]
 from collections.abc import AsyncGenerator, AsyncIterator
 from contextlib import asynccontextmanager

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

+async def coro() -> int:
+    return 1
+
 class A:
     @asynccontextmanager
     async def ctx(self) -> AsyncGenerator[str, None]:
         yield "value"
+
+    async def coro(self) -> int:
+        return 1
 [out]
 from collections.abc import AsyncGenerator, AsyncIterator
 from contextlib import asynccontextmanager

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

 class A:
     @asynccontextmanager
     def ctx(self) -> AsyncGenerator[str, None]: ...
+    async def coro(self) -> int: ...

I generated that [out] block from the patched stubgen rather than writing it by hand, but worth re-running pytest mypy/test/teststubgen.py to confirm it matches under the harness.

Thanks for the thorough validation and suggestion. Added the top-level and method coroutine non-interference cases in 2fe117a, preserving async for both generated outputs.

Validation with the repository's pinned test requirements: python3 -m pytest mypy/test/teststubgen.py (374 passed, 1 skipped, 2 xfailed); git diff --check passed.