Inconsistent errors adding to list of generic values

[plannigan]

Bug Report

When a list contains Union[Foo[Any],Foo[T]], there are inconsistent errors when adding the result to a list of Foo[T].

To Reproduce

from typing import List, Union, Generic, TypeVar

T = TypeVar("T")


class Tree(Generic[T]):
    children: "List[Union[T, Tree[T]]]"

    def __init__(self, children: "List[Union[T, Tree[T]]]") -> None:
        self.children = children

    def all_sub_trees(self) -> None:
        sub_trees: "List[Tree[T]]" = [self]
        reveal_type(sub_trees)
        only_child_trees = [v for v in self.children if isinstance(v, Tree)]
        reveal_type(only_child_trees)
        # 3 equivalent statements (ignoring that the first mutates instead of replacing)
        sub_trees.extend(only_child_trees)
        sub_trees = sub_trees + only_child_trees
        sub_trees += only_child_trees

Expected Behavior

Initially I was expecting the value of only_child_trees to be List[Tree[T`1]]. However, it is possible that T`1 has the type of Tree[str]. This would mean that the true type of only_child_trees is List[Union[Tree[str],Tree[Tree[str]]]]. So the result is accurate.

However, based on that, I would expect that all of the following operations to have consistent behavior.

Actual Behavior

$ mypy typing_example.py 
typing_example.py:15: note: Revealed type is "builtins.list[typing_example.Tree[T`1]]"
typing_example.py:17: note: Revealed type is "builtins.list[Union[typing_example.Tree[Any], typing_example.Tree[T`1]]]"
typing_example.py:21: error: Result type of + incompatible in assignment
Found 1 error in 1 file (checked 1 source file)

mypy doesn't complain about extend() or explicit add then assign. However, it does show an error with +=. It isn't clear to me what the correct behavior is as Any makes things complicated. The fact that += raises an error, but explicit add then assign doesn't seems odd.

I was also able to reproduce this with collections.dequeue, so this is likely general collection issue and not a list specific issue.

Your Environment

• Mypy version used: 0.910
• Python version used: 3.9

[erictraut]

Mypy's behavior here looks correct to me, at least with the latest version (1.5). The revealed types are as expected, and it doesn't emit any errors in the above code. Pyright agrees with the results.

[hauntsaninja]

Fixed in #12590