[2.0 regression] Bad result from heuristic non-narrowing of type[T]

Bug Report

(A clear and concise description of what the bug is.)

To Reproduce

from math import isnan
from typing import TypeVar

V = TypeVar("V")


def assert_scalar_equal(s1: V, s2: V, /) -> None:
    assert type(s1) is type(s2)
    if type(s1) is float and type(s2) is float and isnan(s2):
        assert isnan(s1)
    else:
        assert s1 == s2

Expected Behavior

This snippet should type check.

Actual Behavior

mypy.ini: No [mypy] section in config file
t.py:9: error: Argument 1 to "isnan" has incompatible type "V"; expected "SupportsFloat | SupportsIndex"  [arg-type]
t.py:10: error: Argument 1 to "isnan" has incompatible type "V"; expected "SupportsFloat | SupportsIndex"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Even more minimal reprod:

from math import isnan
from typing import TypeVar

V = TypeVar("V")


def foo(s1: V, /) -> None:
    if type(s1) is float:
        assert isnan(s1)

mypy.ini: No [mypy] section in config file
t.py:9: error: Argument 1 to "isnan" has incompatible type "V"; expected "SupportsFloat | SupportsIndex"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 2.0.0 or 2.1.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: tested 3.11.14 and 3.14.6

まず、mypy 2.0.0 または 2.1.0 で最小限の t.py 再現コードを実行し、type(s1) is float の後の型の絞り込みの挙動を確認してください。isnan に対して期待される絞り込みと比較してください。完了条件は、提示された両方のスニペットが報告された arg-type エラーなしで型チェックを通ることです。