[1.16 regression] narrowing a typevar via comparison to None

[Redoubts]

Bug Report

Consider the following: https://mypy-play.net/?mypy=master&python=3.12&gist=dbad46bbd0f39d316a2e82262ceeb4fa

In 1.16, this code raises an error at the first return after the None check, Incompatible return value type (got "None", expected "T"), though it was fine in 1.15

Your Environment

• Mypy version used: 1.16
• Mypy command-line flags: stock
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.11, 3.12

[hauntsaninja]

mypy_primer -p test.py --bisect --debug --old 'v1.15.0' says this is #18972

[hauntsaninja]

Simpler repro:

from typing import Callable, TypeVar

T = TypeVar("T")


def foo(f: Callable[[], T]) -> T:
    ret = f()
    if ret is None:
        return ret
    return ret

[ilevkivskyi]

I will take a look at this (hopefully during weekend). The best course of action may be to infer a type variable with upper bound None (or anything else that doesn't make a meaningful intersection with given type variable). Essentially we can do something like (very roughly) meet(T(bound=A), B) == T(bound=meet(A, B)).