it's not possible to define an 'add' that works with more than one kind of number when using number types directly

Bug Report

I can't figure out how to write generic numeric code.

This is already covered to some extent by #3186, but that's about a higher-level numeric tower issue, rather than the very practical issue of code that can just work with int or float but has to keep the number type straight.

To Reproduce

from typing import TypeVar

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


def add(a: Num, b: Num) -> Num:
    return a + b

I actually want | Decimal | Fraction in there, but that makes the problem worse.

The same issue seems to occur with Generics.

Expected Behavior

No errors.

Actual Behavior

add.py:7: error: Incompatible return value type (got "float", expected "Num")
add.py:7: error: Unsupported operand types for + (likely involving Union)
Found 2 errors in 1 file (checked 1 source file)

I understand how this would not work if I were using division, or if there were some other variance on Num but is there really some way to add an int to an int and get a float?

Your Environment

• Mypy version used: mypy 0.960 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.10.4 from python.org
• Operating system and version:

ProductName:	macOS
ProductVersion:	12.4
BuildVersion:	21F79

まず、報告された mypy のバージョンで add.py の診断を再現し、issue #3186 の関連する議論を確認してください。bound 付きの TypeVars、数値の union、Generic のオペランドがどのように解析されるかを調査します。対応している汎用的な数値加算パターンが報告されたエラーなしで型チェックを通過し、回帰テストでカバーされていれば完了です。