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)}")

Start by running the provided reproducer in the linked mypy-playground examples with the listed mypy and Python versions, then trace how the isinstance checks narrow x after the Mapping branch. Done means the Collection branch narrows x consistently and the shown call to f passes type checking without the incompatible int | str error.