Infer more specific type after equality or membership check

[jackriccomini]

from typing import Union

def foo(bar: Union[str, int]):
	if bar in ["a", "b", "c"]:
                # to a human, bar is clearly a str. mypy still thinks it's a Union[str, int]
		return bar.upper()
	else:
		return bar

$ mypy test.py 
test.py:5: error: Item "int" of "Union[str, int]" has no attribute "upper"

The same thing happens if the condition is, for example, bar == "a".

• Are you reporting a bug, or opening a feature request?

Feature request

• What is the behavior/output you expect?

Mypy to understand the effect of conditions like bar == "a" or bar in ["a"], like it already does with isinstance checks

[ilevkivskyi]

This is not as safe as it looks because of tricky things like:

>>> Decimal(1) in [1, 2, 3]
True

We can probably special-case some builtin types, but this looks like relatively low priority.

[msullivan]

I think we're probably not going to do this