Declaration-order-dependent possibly-undefined false positive with nested ifs/matches, decorated any functions and NoReturn

Bug Report

Some obtuse code results in a false positive for the possibly-undefined error code.

To Reproduce

Run mypy --enable-error-code possibly-undefined file.py on this file:

decorator = lambda f: f


def unreachable() -> None:
    raise NotImplementedError()


def do_it(x: bool, y: None) -> None:
    if x:
        decorated()
        var = 0
    else:
        match y:
            case None:
                var = 1
            case _:
                unreachable()
    print(var)


@decorator
def decorated() -> None:
    pass

This is as small as I've gotten the reproducer. All these parts seem to be significant:

• the outer if and an inner match
• the decorated call being separate to the match
• the match being a match (replacing the match with the equivalent if y is None: ... else: ... construct works fine)
• the match having exhaustive arms with the unreachable call being unreachable (as far as mypy can tell) (e.g. deleting the case None: entirely works fine)
• the unreachable call existing (inlining case _: raise NotImplementedError() works fine)
• decorator being a lambda (or an class with __call__ method) (using def decorator(f): return f works fine)
• decorated being defined after do_it in the file (changing the order works fine) (NB. it's just decorated, decorator and unreachable can be defined before or after do_it and still reproduce the behaviour)

Expected Behavior

This to pass type checking.

In particular, have same behaviour as any of the very similar ablations mentioned above, e.g. both of these work:

• changing decorator = lambda f: f to def decorator(f): return f
• moving decorated to be before do_it

Actual Behavior

file.py:19: error: Name "var" may be undefined  [possibly-undefined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.11.2
• Mypy command-line flags: --enable-error-code possibly-undefined
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10

Start with the minimal file.py reproducer and run mypy --enable-error-code possibly-undefined to confirm the reported diagnostic. Compare the listed ablations, including the decorator definition, declaration order, and match branches, to isolate the interaction. Done means the reproducer passes type checking without the false positive.