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

[neutrinoceros]

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

[hauntsaninja]

Thanks! This relates to #20662 / #21174
A lot of mypy's narrowing logic got rewritten in recent releases. Basically, in the absence of true intersection types, the current heuristic doesn't work well on this case