Spurious error when adding two list

Bug Report

Mypy reports Unsupported operand types for + when concatenating two lists, when the second operand is a subtype of the first.

This was previously mentioned in this comment in #5492

Below is a simple concise example

To Reproduce

class A:
    pass


class B(A):
    pass


def foo(a: list[A]) -> None:
    pass


l1: list[A] = [A(), B()]
l2: list[B] = [B()]
l3 = l1 + l2
foo(l2 + l1)  # OK, no error
foo(l3)  # also OK
foo(
    l1 + l2
)  # ERROR: Unsupported operand types for + ("List[A]" and "List[B]")  [operator]mypy(error)

gist link

Expected Behavior
No error on the last line

Actual Behavior
Reports spurious error

Your Environment

• Mypy version used: mypy 1.2.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): from pyproject.toml

ignore_missing_imports = true
show_column_numbers = true
warn_redundant_casts = true
warn_unused_ignores = true
warn_unused_configs = true
follow_imports = "normal"

plugins = ["pydantic.mypy"]

• Python version used: 3.10

まず、簡潔な Python の例を mypy 1.2.0 で実行し、l1 + l2 と l2 + l1 の診断を比較します。issue にはソースファイルもテストパスも記載されていないため、この再現例からリスト加算の型チェック経路をたどってください。完了条件は、最終的な l1 + l2 式で unsupported-operand エラーが発生せず、既存の有効なケースが引き続き有効であることです。