wrongly inferred type builtins.object

[robsdedude]

Bug Report

In a rather convoluted nesting, I found that a Union was turned into builtins.object.

To Reproduce

import typing as t


_T = t.TypeVar("_T")
_P = t.ParamSpec("_P")


async def works() -> None:
    async def callback(
        cb: t.Callable[_P, t.Union[t.Awaitable[_T], _T]],
        *args: _P.args, **kwargs: _P.kwargs
    ) -> _T:
        raise NotImplementedError()

    def make_t() -> t.Union[
        t.Callable[[], t.Coroutine[t.Any, t.Any, int]],
        t.Callable[[], int]
    ]:
        raise NotImplementedError()

    t = make_t()
    t_cb = await callback(t)
    reveal_type(t_cb)  # note: Revealed type is "builtins.int"


async def fails() -> None:
    async def callback(
        cb: t.Callable[_P, t.Union[t.Awaitable[_T], _T]],
        *args: _P.args, **kwargs: _P.kwargs
    ) -> _T:
        raise NotImplementedError()

    def make_t() -> t.Union[
        t.Callable[[], t.Coroutine[t.Any, t.Any, t.Union[int, str]]],
        t.Callable[[], t.Union[int, str]]
    ]:
        raise NotImplementedError()

    t = make_t()
    t_cb = await callback(t)
    reveal_type(t_cb)  # Revealed type is "builtins.object"

Gist URL: https://gist.github.com/mypy-play/f932b06f9d8638c2b0c711f8e4048348
Playground URL: https://mypy-play.net/?mypy=latest&python=3.11&gist=f932b06f9d8638c2b0c711f8e4048348

In the example you can see two example functions works and fails which are the exact same except for int in works was replaced with Union[int, str] in fails. Both end with a reveal_type.

Expected Behavior

• works should reveal builtins.int
• fails should reveal Union[builtins.int, builtins.str]

Actual Behavior

• works reveals builtins.int (as expected)
• fails reveals builtins.object (unexpected)

(here's the mypy output of the gist)

main.py:23: note: Revealed type is "builtins.int"
main.py:41: note: Revealed type is "builtins.object"
Success: no issues found in 1 source file

Your Environment

$ python -VV
Python 3.11.0 (main, Nov  2 2022, 13:45:57) [GCC 11.3.0]
$ pip freeze | grep 'mypy\|typing'
mypy==1.3.0
mypy-extensions==1.0.0
typing_extensions==4.5.0

Related
I think there's a whole bunch of related issues already, but I wanted to share this example to potentially validate a future fix.

#12009 (comment)

[erictraut]

This appears to be another case of "union vs join". May want to add that tag for completeness.