Inconsistency in Union/promotion interactions

[msullivan]

In the following test

from typing import Union

class A:
    x: Union[int, float]

def foo(x: Union[int, float], y: A) -> None:
    reveal_type(x)
    reveal_type(y.x)

we emit:

lol.py:8: error: Revealed type is 'Union[builtins.int, builtins.float]'
lol.py:9: error: Revealed type is 'builtins.float'

That is to say, class attributes have unions simplified using promotions while arguments (and locals) do not.

Probably they ought to have the same behavior?
(And probably the behavior ought to be to not simplify, especially because of #6114)

[ilevkivskyi]

And probably the behavior ought to be to not simplify

Yes, I think we should never use promotions in union simplification. Also this looks like something that may be easy to fix. We just need to use ignore_promotions=True in UnionType.make_simplified_union().

[ilevkivskyi]

Added usability label because this can cause confusion, see #7241.

[hauntsaninja]

These currently have the same type