When type checking these files, there's a false positive in x.py when using a cold cache (rm -rf .mypy_cache first). This is a regression from mypy 1.17 and is a release blocker. The behavior is inconsistent between cached and uncached runs.

typ.py:

from typing import NamedTuple

NT = NamedTuple("NT", [("x", str | None)])

mod.py:

# mypy: no-strict-optional

from typ import NT

def f() -> NT:
    return NT(x='')

x.py:

import mod

# Argument "x" to "NT" has incompatible type "None"; expected "str"
a = mod.NT(x=None)

It looks like the NT parameter type gets simplified to just str from str | None when the type is used in a module with strict optional checking disabled.

This bisects to #19514 (cc @ilevkivskyi). Maybe the caching should be disabled in modules that don't use strict optional checking?