Comparison operators emit "Unsupported operand types" when using `isinstance` to narrow `TypeVar` to union of numeric types

[bkeryan]

Bug Report

I have a generic class that wraps a bool, float, int, or str. It implements the comparison operator magic methods like __lt__ etc., using isinstance to ensure that callers don't mix bool/float/int with str. With mypy 1.16.1, this does not error, but with mypy 1.17.0, the comparison operators started emitting "error: Unsupported operand types for < (likely involving Union) [operator]"

To Reproduce

Minimal reproduction (using a generic function instead of a generic class): https://mypy-play.net/?mypy=latest&python=3.12&gist=dd4696c3ba71103c49d7a4aaf34b2aef

def f[T: float | int | str](x: T, y: T) -> bool:
    if isinstance(x, (float, int)) and isinstance(y, (float, int)):
        return x < y
    elif isinstance(x, str) and isinstance(y, str):
        return x < y
    else:
        raise TypeError

Expected Behavior

No errors.

Actual Behavior

test3.py:3: error: Unsupported operand types for < (likely involving Union)  [operator]
Found 1 error in 1 file (checked 1 source file)

It does not error if I remove the type variable and use union types directly:
def f(x: float | int | str, y: float | int | str) -> bool:

Your Environment

• Mypy version used: 1.17.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12.9

[sterliakov]

This was exposed by #19183 due to narrowing (which is now correct), and the MRE is

from typing import TypeVar

_T = TypeVar("_T", bound=float | int)

def lt(x: _T, y: _T) -> bool:
    return x < y  # E: Unsupported operand types for < (likely involving Union)  [operator]

https://mypy-play.net/?mypy=master&python=3.12&gist=01ea3e0ed462cc2b1deee8bc40c25ac9