isinstance check widens type

• 2025-02-14: reproduced using mypy 1.15 and python 3.13 mypy-playground
• 2023-02-14: reproduced using mypy 1.0 and python 3.11 mypy-playground

from collections.abc import Collection, Mapping

def f(x: int | Collection[int] | Mapping[str, int]) -> int:
    r"""Recursively sum up all the values of possibly nested data."""
    if isinstance(x, int):
        return x
    if isinstance(x, Mapping):
        return sum(f(y) for y in x.values())

    reveal_type(x)   # <- Here, mypy thinks this is Collection only!

    if isinstance(x, Collection):
        reveal_type(x)  # <- Suddenly mypy thinks this is Collection | Mapping
        return sum(f(y) for y in x)  # ❌ "f" has incompatible type "int | str"
    raise TypeError(f"unsupported type: {type(x)}")

先使用列出的 mypy 和 Python 版本，在链接的 mypy-playground 示例中运行提供的 reproducer，然后跟踪 isinstance 检查如何在 Mapping 分支之后缩小 x 的类型。完成标准是 Collection 分支能够一致地缩小 x，并且所示的 f 调用无需出现不兼容的 int | str 错误即可通过类型检查。