Force List Expression to Infer Union Type of Member Types.

[ay0503]

Infer Union Types for List Expressions with Heterogeneous Elements

Description:

This PR forces the type inference for list expressions in mypy to correctly infer Union types when a list contains elements of different types that have no common supertype other than object. However, currently this forces this for all list expression type inferences and hence breaks everything else. A pretty quirky fix would be to track any list comprehension accesses since this only happens when a union type list is used in the generator portion of a list comprehension.

Background:

Before, when a list literal was constructed with elements of disparate types without a meaningful common superclass, mypy would default to inferring the list's type as List[object]. This behavior led to type-checking issues in scenarios where a more precise type was expected.

For example:

class A:
    pass

class B:
    pass

a = A()
b = B()

l3: list[A | B] = [x for x in [a, b]]  # This would cause a mypy error

However, while this fixes this issue, it also generalizes the entire list expression inference to be forced as well.

[lexik04]

looks very well written

[vsolskyyy]

Well organized, I like your use of comments. Well done.

[ErkeXia]

Looks good and solve the problem. Just wonder when the isinstance(e, ListExpr) branch will be executed in infer_item_type

[vsolskyyy]

Well written.

[KevnKey]

This looks like a good fix to the issue by inferring directly. I can see the union implementation.