Obvious errors with coroutines are not detected

[WhyNotHugo]

Bug Report

Some obvious type issues with coroutines are not detected. See two examples below.

To Reproduce

import asyncio


async def function_one():
    return 1


async def function_two():
    function_one()[1]
    function_one() + 1


asyncio.run(function_two())

Expected Behavior

mypy should detect two TypeError here:

• 'coroutine' object is not subscriptable
• unsupported operand type(s) for +: 'coroutine' and 'int'

Actual Behavior

Success: no issues found in 1 source file

Your Environment

mypy 0.910
Python 3.9.6

[hauntsaninja]

https://mypy.readthedocs.io/en/stable/common_issues.html#no-errors-reported-for-obviously-wrong-code

[WhyNotHugo]

Odd, a couroutine is never indexable, shouldn't mypy be able to detect this even if it's not type-annotated?

[JelleZijlstra]

It can, yes, but mypy simply never provides errors for functions that aren't type checked.

[JelleZijlstra]

You can use the --check-untyped-defs option to change this behavior.