Incorrect type inference with Union[Any, Any]

Bug Report

Mypy infers an overly specific type for code that (for example) adds 1 to an argument of type Union[Any, Any].

To Reproduce

Run mypy against the following code:

from typing import Any, Union

def function(arg: Union[Any, Any]) -> None:
    reveal_type(1 + arg)

Expected Behavior

The revealed type is Any or Union[Any, Any].

Actual Behavior

The revealed type is builtins.int, which is incorrect (e.g. when arg is a float, or a numpy array).

Note that reveal_type(arg + 1) does return Any.

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.7.3
• Operating system and version: Ubuntu 18.04

No source file or test path is named. Start by running mypy on the supplied Python snippet and compare reveal_type for 1 + arg with arg + 1, then trace the type-inference path for Union[Any, Any]. Done means a regression test captures the expected Any result and the relevant type-checker tests pass.