Type argument lost when union of collections is passed to generic function

[daniil-berg]

Bug Report

Passing a union of collections to a generic function that should preserve the type argument erases it instead. (Or should I say it consolidates it to object?)

To Reproduce

from typing import Union

x: list[int] | list[str]
y: Union[set[int], set[str]]

reveal_type(set(x))
reveal_type(list(y))

Expected Behavior

Revealed type is "Union[builtins.set[builtins.int], builtins.set[builtins.str]]"
Revealed type is "Union[builtins.list[builtins.int], builtins.list[builtins.str]]"

I hope this is reasonable. I mean, the type argument is either one or the other and nothing else.

Actual Behavior

Revealed type is "builtins.set[builtins.object]"
Revealed type is "builtins.list[builtins.object]"

Environment

Python 3.10.8 on Linux; mypy --strict version 0.982

More examples

Clearly mypy is capable of preserving the type argument, when there is no union of iterables passed to set:

reveal_type(set([1, 2]))  # Revealed type is "builtins.set[builtins.int]"

It also does not seem to have anything to do with the wrong typeshed stubs, as the same problem occurs with custom generic functions:

from collections.abc import Iterable
from typing import TypeVar

T = TypeVar("T")

def f(i: Iterable[T]) -> T:
    ...

z: list[int] | list[str]

reveal_type(f(z))  # Revealed type is "builtins.object"

The following gives me an error:

def my_set(a: list[int] | list[str]) -> set[int] | set[str]:
    return set(a)  # Incompatible return value type (got "Set[object]", expected "Union[Set[int], Set[str]]")

And yes, I know this would be better solved with a generic type variable instead of the unions. It is just to provide an additional demonstration of what I consider a bug.

---

My description of the problem may be poorly worded. Please feel free to correct me, so that I can describe it more precisely.

This issue seems vaguely related: #6463

[erictraut]

This is another issue caused by mypy using joins instead of unions. The join of int and str results in object.