Fix overloading with a typevar missing

[A5rocks]

Description

This fixes this following program:

import typing

class A:
    pass

T = typing.TypeVar("T", bound=A)

@typing.overload
def f(a: T) -> None:
    ...

@typing.overload
def f(*, copy: bool = False) -> None:
    ...

def f(a: T = ..., *, copy: bool = False) -> None:
    ...

This also fixes #9023.

Test Plan

I haven't added any tests yet, I plan to do that based on mypy primer output (still not sure this works...).

[A5rocks]

oh that broke terribly didn't it.

[sobolevn]

I think that the given example is not ideal. Because typevar should not even be used in this example:

T = typing.TypeVar("T", bound=A)

@typing.overload
def f(a: T) -> None:
    ...

@typing.overload
def f(*, copy: bool = False) -> None:
    ...

def f(a: T = ..., *, copy: bool = False) -> None:
    ...

Should be:

@typing.overload
def f(a: A) -> None:
    ...

@typing.overload
def f(*, copy: bool = False) -> None:
    ...

def f(a: A = ..., *, copy: bool = False) -> None:
    ...

Maybe you should also add an example, where TypeVar is actually needed?

[A5rocks]

The given example was an oversimplification of the original, which can be simplified to:

import typing
import collections.abc as collections

class BaseSlashCommand:
    ...

BaseSlashCommandT = typing.TypeVar("BaseSlashCommandT", bound=BaseSlashCommand)

@typing.overload
def with_slash_command(command: BaseSlashCommandT, /) -> BaseSlashCommandT:
    ...

@typing.overload
def with_slash_command(
    *, copy: bool = False
) -> collections.Callable[[BaseSlashCommandT], BaseSlashCommandT]:
    ...

def with_slash_command(
    command: BaseSlashCommandT = ..., /, *, copy: bool = False
) -> typing.Union[BaseSlashCommandT, collections.Callable[[BaseSlashCommandT], BaseSlashCommandT]]:
    ...

The original can be found at https://github.com/FasterSpeeding/Tanjun/blob/7edec0fa4418718343826f9c6489bd817c4a9b47/tanjun/components.py#L590-L604 (yes I know the / is missing after self in the second overload, I have that fixed locally)

[A5rocks]

Went through some issues that seemed partially related -- it looks like #9023 gets fixed by this too!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/abc.py:3139: error: Overloaded function implementation cannot satisfy signature 2 due to inconsistencies in how they use type variables  [misc]
- tanjun/abc.py:3205: error: Overloaded function implementation cannot satisfy signature 2 due to inconsistencies in how they use type variables  [misc]

[A5rocks]

Remade in #15320