Unions of homogeneous collections unnecessarily broadened when used to construct a new collection

[mharding-hpe]

Bug Report

mypy infers an incorrectly broad type in some cases where collections are constructed from a collection whose type is a union. In particular, if the input to a collection constructor is a union of collections with homogeneous element types, the type that mypy infers ignores the fact that the inputs are homogeneous. The type mypy infers is not wrong (in that the actual type will be covered by it), but it is unnecessarily broad (it matches types which are obviously not possible -- i.e., a collection with heterogeneous element types).

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&gist=04f25887cd105eefebfdcbdbd3c03665
https://gist.github.com/mypy-play/04f25887cd105eefebfdcbdbd3c03665

from typing import Generator
from typing_extensions import reveal_type

list_union: list[int] | list[None]
set_union: set[int] | set[None]
tuple_union: tuple[int, ...] | tuple[None, ...]
generator_union: Generator[int, None, None] | Generator[None, None, None]

# Expected: "Union[builtins.list[builtins.int], builtins.list[None]]"
# Actual: "builtins.list[Union[builtins.int, None]]"
reveal_type(list(list_union))
reveal_type(list(set_union))
reveal_type(list(tuple_union))
reveal_type([x for x in generator_union])

# Expected: "Union[builtins.set[builtins.int], builtins.set[None]]"
# Actual: "builtins.set[Union[builtins.int, None]]"
reveal_type(set(list_union))
reveal_type(set(set_union))
reveal_type(set(tuple_union))
reveal_type({x for x in generator_union})

# Expected: "Union[builtins.tuple[builtins.int, ...], builtins.tuple[None, ...]]"
# Actual: "builtins.tuple[Union[builtins.int, None], ...]"
reveal_type(tuple(list_union))
reveal_type(tuple(set_union))
reveal_type(tuple(tuple_union))
reveal_type(tuple(x for x in generator_union))

Expected Behavior

Because mypy knows that the argument to the collection constructor is a collection whose elements are all of the same type, I assumed it would thus realize that it would result in a collection whose elements are all of the same type.

Actual Behavior

mypy incorrectly reports that the resulting collection may have heterogeneous element types.

Your Environment

• Mypy version used: 1.15.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.12