Assigning to intermediate variable changes type checking results

[randolf-scholz]

Bug Report

I was testing this PR (https://github.com/python/typeshed/issues/14283) for typeshed that simplifies list.__add__ from

@overload
def __add__(self, value: list[_T], /) -> list[_T]: ...
@overload
def __add__(self, value: list[_S], /) -> list[_S | _T]: ...

to

def __add__(self, value: list[_S], /) -> list[_S | _T]: ...

This seems to work generally, but there are some weird circumstances when it bugs out. It seems most of them appear when a concatenation is given as an argument to another function.

As an example, this is one of the lines that gets flagged:

https://github.com/python/mypy/blob/5081c59b9c0c7ebe7070c62a4aeaf3d0de203a24/mypyc/crash.py#L29

However, mypy stops complaing if it is changed to

dummy = tb + tb2
for s in traceback.format_list(dummy):

Assigning to an intermediate variable changed the type checking results (!)

I couldn't reproduce this behavior using a custom class, which makes me believe this is probably due to some weird special casing for builtins.

To Reproduce

git clone --branch polymorphic_overload_test https://github.com/randolf-scholz/typeshed.git
git clone https://github.com/python/mypy.git
cd typeshed
uv venv --seed
source .venv/bin/activate
uv pip install -r requirements-tests.txt
mkdir tmp
cp ../mypy/mypyc/crash.py tmp/tmp.py
python -m mypy.stubtest --custom-typeshed-dir=../typeshed tmp

Expected Behavior

Assigning to an intermediate variable shouldn't affect type inference.