[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

Bắt đầu bằng cách chạy bản tái hiện tối thiểu t.py với mypy 2.0.0 hoặc 2.1.0 và kiểm tra hành vi thu hẹp kiểu sau type(s1) is float. So sánh với việc thu hẹp kiểu dự kiến cho isnan; được xem là hoàn tất khi cả hai snippet được cung cấp đều kiểm tra kiểu thành công mà không có các lỗi arg-type đã báo cáo.