Mypy reports a function is "missing return statement" when unused imports are removed

I know this sounds ridiculous, but I'm not sure how else to explain it. In the middle of a large refactor of code, which can be found here in its original, mypy-passing state:

https://github.com/mesonbuild/meson/blob/83d18d137dc50437a8263983e8883198c7bc41ed/mesonbuild/compilers/detect.py#L272-L352

I have a function which looks like this:

def _handle_exceptions(data: StuffICareAbout) -> typing.NoReturn:
    raise mesonlib.EnvironmentException(f'{data} happened and this is bad')

def detect_static_linker(env: 'Environment', compiler: Compiler) -> StaticLinker:
    data = do_stuff()
    if foo_condition:
        return StaticLinkerA()
    elif bar_condition:
        return StaticLinkerB()
    _handle_exceptions(data)

mypy says this is fine. detect_static_linker() doesn't always return, but if it doesn't return, it runs a function that is guaranteed to never return, always raise.

This somehow stops working during the course of this series of changes, which doesn't affect the high-level overview of that function: https://github.com/eli-schwartz/meson/compare/83d18d137dc50437a8263983e8883198c7bc41ed...681609624b687d660b234a9393d446a8f2f6663b

After way too much troubleshooting, and separating out parts of the commit to see what caused it, I produced the above series of commits, and...

... this commit is fingered by git bisect: https://github.com/eli-schwartz/meson/compare/fe42a1cadade94cb39e2761500a1f5849ad7d2e8...b1676e57f60c6a1fb7533096402ae5c8ff713a35

All it does is finally drop the unused imports rendered redundant by the previous commit. I don't understand how this could be possible.

Furthermore, if I run mypy mesonbuild/compilers/detect.py, mypy passes. If I run ./run_mypy.py, a script that contains a list of files to check, and runs mypy "list" "of" "files", it triggers the issue.

Discussion in the meson review can be seen here, in case that's interesting (two people failed to reproduce it locally at first): https://github.com/mesonbuild/meson/pull/10810#discussion_r970164045

Your Environment

• Mypy version used: 0.971
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): https://github.com/mesonbuild/meson/blob/master/.mypy.ini
• Python version used: 3.10
• Operating system and version: Arch Linux

Inizia riproducendo la differenza tra mypy mesonbuild/compilers/detect.py e ./run_mypy.py, usando Python 3.10, mypy 0.971 e .mypy.ini. Confronta le revisioni indicate e analizza mesonbuild/compilers/detect.py insieme a run_mypy.py; il lavoro è concluso quando viene spiegato perché la rimozione degli import inutilizzati modifica il controllo aggregato e viene corretta la diagnostica incoerente.