Failed TypeVar substitution by asynccontextmanager #9922

Bug Report

Return type of contextlib.asynccontextmanager does not substitute typing.TypeVar determined from parameters.

To Reproduce

Pass the following script to mypy, using mypy --config-file= main.py or otherwise.

#!/usr/bin/env python3

import contextlib, typing

_T = typing.TypeVar('_T')


@contextlib.asynccontextmanager
async def identity(element: _T) -> typing.AsyncIterator[_T]:
    """
    A context manager that does nothing
    and returns the given element.
    Type checking should assume the yielded value
    has the same type as the parameter.
    """
    yield element


async def main() -> None:
    """
    Here we give an ``int`` to ``identity``.
    So the yielded ``number`` should be ``int`` as well.
    """
    async with identity(1) as number:
        """Problem: We got ``_T`-1`` as its type instead of ``int``."""
        reveal_type(number)
        number = 2

Expected Behavior

The type of number should be int. In particular, assignment of the integer 2 to it should be compatible, and so mypy should return without errors.

Actual Behavior

The variable number is assigned, what I assume, is the TypeVar. This leads to an incompatible assignment of 2.

main.py:26: note: Revealed type is '_T`-1'
main.py:27: error: Incompatible types in assignment (expression has type "int", variable has type "_T")
Found 1 error in 1 file (checked 1 source file)

As a side note, a non-async counterpart of this behaves as expected, and does not produce any errors.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: --config-file=
• Mypy configuration options from mypy.ini (and other config files): Not applicable.
• Python version used: Python 3.9.1
• Operating system and version: Debian bullseye/sid and Windows 10 version 1909 build 18363.1256