False positive list comprehension for union type source iterable

Bug Report

MyPy incorrectly asses the type of list comprehensions built from union types. I can't see a way to make this pass typing checks. This and it even affects TypeVar. My only option is a # type: ignore.

The problem arises where a variable foo has a type of Iterable[Bar] | Iterable[Baz] and this is then used as part of a list comprehension [v for v in foo]. MyPy incorrectly evaluates the list comprehension as list[Bar | Baz] not list[Bar] | list[Baz].

I had thought passing a TypeVar might be a simple work around, but it appears the TypeVar is stripped off and only it's binding is maintained.

To Reproduce

I ran into this with code like this:

from typing import TypeVar

class Foo:
    item_id: str

class Bar:
    item_id: str

class FooWrapper:
    items: list[Foo]

class BarWrapper:
    items: list[Bar]


_T = TypeVar('_T', bound=FooWrapper | BarWrapper)


def filter_this(value: _T, filter_val: str) -> None:
    # Mypy Error on this line 👇
    value.items = [item for item in value.items if item.item_id == filter_val]

With the MyPy error:

error: Incompatible types in assignment (expression has type "list[Foo | Bar]", variable has type "list[Foo] | list[Bar]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

A list comprehension formed from iterable Iterable[Foo] | Iterable[Bar] should have type list[Foo] | list[Bar]

Actual Behavior

A list comprehension formed from iterable Iterable[Foo] | Iterable[Bar] is assessed as having type list[Foo | Bar]

Your Environment

• Mypy version used: mypy 1.11.2 (compiled: yes)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: python 3.11

先使用 mypy 1.11.2 运行提供的 TypeVar 和 union-iterable reproducer，然后跟踪列表推导式的类型推断以及对 union 源的处理。添加回归覆盖，展示推导式保留列表类型的 union（包括 TypeVar 情况），并验证报告的赋值错误已得到解决。