Set and Dict generalizing the type unnecessarily

Bug Report

See these two example functions:

def f(items: list[str] | list[int]) -> None:
    items_as_set: set[str] | set[int] = set(items)

def g(key_to_count: dict[str, float] | dict[int, float]) -> None:
    key_to_count_2: dict[str, float] | dict[int, float] = {k: v for k, v in key_to_count.items()}

Mypy 1.10.0 complains about the assignment in each of them:

test_mypy_dict.py:2: error: Incompatible types in assignment (expression has type "set[object]", variable has type "set[str] | set[int]")  [assignment]
test_mypy_dict.py:5: error: Incompatible types in assignment (expression has type "dict[str | int, float]", variable has type "dict[str, float] | dict[int, float]")  [assignment]

To Reproduce

Playground gist: https://mypy-play.net/?mypy=latest&python=3.12&gist=83bde5f3f11470e94b6a680158649641

Expected Behavior

The set() or dict() operation applied on a variable that is either a list[str] | list[int] is always either a set[str] or set[int]. The same applies to function g using a dict. Thus the functions should not raise a mypy error.

Workarounds

I tried two workarounds:

• Using a mixed type of list[str | int]. However, I get the type list[str] | list[int] by an external function, thus this is not compatible.
• Using a generic. This is not working either.

See the playground gist: https://mypy-play.net/?mypy=latest&python=3.12&gist=264490179628a0a214a2e5bd947b8ccf

Start by running the two Python reproductions from the issue, including the linked mypy-play examples, and compare the inferred types and assignment errors for set() and the dict comprehension. Trace the type-checking path for these constructor and comprehension expressions; done means both functions type-check without errors while preserving the union of homogeneous container types.