Regression on validation of manual type annotations to functools.partial between stable version and git

[surrealoatmeal]

Bug Report

When validating manual type hints to the return of a call to functools.partial, there has occurred a regression between the latest stable release (1.10.0) and the master branch, in that the git version asserts that the correct annotation should be partial[partial[T]], where T is the type manually annotated for the functools.partial call.

To Reproduce
TL;DR:
master branch: https://mypy-play.net/?mypy=master&python=3.12&gist=e45e31f06514aabe6a039dfc759e9a3b
1.10.0: https://mypy-play.net/?mypy=latest&python=3.12&gist=e45e31f06514aabe6a039dfc759e9a3b

Consider:

from functools import partial
from typing import Type, TypeVar

T = TypeVar("T")


def generic(string: str, integer: int, resulting_type: Type[T]) -> T:
    if isinstance(string, resulting_type):
        return string
    elif isinstance(integer, resulting_type):
        return integer
    raise RuntimeError

Expected Behavior
For the below definition:

p: partial[str] = partial(generic, resulting_type=str)

The stable release does not generate any errors, as expected.

Actual Behavior
However, on the current git build, the following error message is displayed:

error: Incompatible types in assignment (expression has type "partial[partial[str]]", variable has type "partial[str]")  [assignment]
error: Argument 1 to "generic" has incompatible type "type[str]"; expected "type[partial[str]]"  [arg-type]

If the type annotation is changed to something nonsensical:

p: partial[bool] = partial(generic, resulting_type=str)

mypy now generates an error asserting the type of the expression is of type partial[partial[TYPE GIVEN]]:

main.py:13: error: Incompatible types in assignment (expression has type "partial[partial[bool]]", variable has type "partial[bool]")  [assignment]
main.py:13: error: Argument 1 to "generic" has incompatible type "type[str]"; expected "type[partial[bool]]"  [arg-type]

Playground for this: https://mypy-play.net/?mypy=master&python=3.12&gist=8b38f943d002dc6ce084e8771251e78a

Your Environment
N/A, reproducible in mypy Playground environment.

Thank you for your hard work maintaining mypy!

[surrealoatmeal]

Actually, on further inspection, the current stable release probably doesn't generate any errors because the type given to the annotation is not validated at all.
For example, the partial[bool] annotation is also considered correct by version 1.10.0: https://mypy-play.net/?mypy=latest&python=3.12&gist=8b38f943d002dc6ce084e8771251e78a

So, rather than being a regression, it could be the incomplete and/or buggy implementation of a new check introduced to the codebase after 1.10.0.

I am not sure whether to file a separate bug report or even change the title to reflect my hypothesis though, as they could be unrelated bugs, so I'm leaving it up to the discretion of the maintainers.

[hauntsaninja]

Thanks for testing against latest! If your repo is open source, lmk, I can add it to https://github.com/hauntsaninja/mypy_primer