[regression] Files with mixed strict-optional forget None'able argument (with ParamSpec decorator)

[intgr]

Bug Report

I am in the progress of introducing strict_optional in my project. Some files have strict_optional disabled, others enabled.

Updating from mypy 1.5.1 to 1.6.0 caused some false positives when passing None to x | None arguments that are called across module boundaries.

To Reproduce

module_a.py:

# mypy: no-strict-optional

from typing import Callable, ParamSpec, TypeVar

P = ParamSpec("P")
T = TypeVar("T")


def paramspec_decorator(func: Callable[P, T]) -> Callable[P, T]:
    return func


@paramspec_decorator
def example_func(arg: str | None) -> None:
    print(arg)

module_b.py:

from module_a import example_func

example_func(None)

Expected Behavior

example_func should accept None argument.

Actual Behavior

% mypy --strict module_b.py
module_b.py:3: error: Argument 1 to "example_func" has incompatible type "None"; expected "str"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.6.0
• Mypy command-line flags: --strict
• Python version used: 3.11.5

[ilevkivskyi]

I understand this behavior may be surprising, but this is not a bug actually. This behavior is consistent with how e.g. regular type variables (and other types) always behaved with no-strict-optional, see #15932 where this was originally reported. ParamSpec was accidentally special.

Note this flag doesn't mean "do not report errors related to None in this file" (because it is actually not a well defined thing), but rather "in this file None is considered as subtype of all types".

[intgr]

Fair enough, this can be solved on my end by migrating more files to strict_optional=true.