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

Comienza reproduciendo el diagnóstico de add.py con la versión de mypy indicada y revisa la discusión relacionada en el issue #3186. Investiga cómo se analizan los TypeVars acotados, las uniones numéricas y los operandos Generic; se considera terminado cuando un patrón genérico de suma numérica compatible pasa la comprobación de tipos sin los errores indicados y cuenta con cobertura de regresión.