> Yury's theory: maybe BEFORE_ASYNC_WITH's error-handling path is forgetting to DECREF the object.

Nope, that doesn't seem to be it. This version prints "refcount: 2" twice, *and* prints a proper "was never awaited" warning:

-----

import sys

async def open_file():
    pass

async def main():
    open_file_coro = open_file()
    print("refcount:", sys.getrefcount(open_file_coro))

    try:
        async with open_file_coro:
            pass
    except:
        pass

    print("refcount:", sys.getrefcount(open_file_coro))

coro = main()
try:
    coro.send(None)
except:
    pass