Same variable with a union type is not checked in "lockstep" within an expression

[Hanaasagi]

Test env:

• mypy 0.670
• Python 3.7.2

Test code

from typing import List, Union


def f(x: Union[int, List[int]]) -> None:  # `TypeVar` can fix it
    x + x

Run mypy with default option, will get

a.py:5: error: Unsupported operand types for + ("int" and "List[int]")
a.py:5: error: Unsupported operand types for + ("List[int]" and "int")
a.py:5: note: Both left and right operands are unions

But actually, both left and right operands are same variable. Only int + int or List[int] + List[int] here. It seems like mypy does not consider object identity. And I'm not sure this is a bug, because above code is meaningless.

[ilevkivskyi]

But actually, both left and right operands are same variable.

Mypy doesn't have any cross-expression type restrictions. You should really use a type variable here. We might support this at some point, but this looks like a low priority.

[ilevkivskyi]

An idea about implementing this #7143 (comment)

[hauntsaninja]

mypy is behaving as expected, and I don't think this is worth the complexity given mypy's current implementation and the lack of demand for this.

[ilevkivskyi]

This is quite popular feature request, and it is actually not that hard to implement. It would be (very) similar to how we type-check functions that are generic in type variables with values currently.