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

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.

Start by running the supplied Python 3.7 example with mypy's default options and compare the reported union combinations with the issue's expected behavior. Read the type-checking path for repeated operands in an expression, then establish whether object identity should affect the diagnostic and add regression coverage if that behavior is accepted.