Some `isinstance(obj, os.PathLike)` widen the type of `obj`

Bug Report

Given an object of type Union[None, str, pathlib.Path] the revealed type before and after a check

if isinstance(obj, os.PathLike):
    # code

widens from the given type before to builtin.object afterwards.

To Reproduce

Unfortunately, I have not been able to make a minimal reproducer, since all my attempts have type checked fine.

We've encounted this in this commit of the PR https://github.com/systemd/mkosi/pull/2373. The codebase is stdlib only and targets Python 3.9 (because it is shipped in RHEL).

One of the two examples can be found in the function built_uki here. The incoming Context object is a dataclass that holds many things, but relevant for this issue is context.config.secure_boot_key, which has type Union[None, str, pathlib.Path]. This type is defined on the dataclass and mypy reveals it at the beginning of the function.

Adding a lot of reveal_type statements I could find that this line building a command line for a program

            reveal_type(context.config.secure_boot_key)
            if isinstance(context.config.secure_boot_key, os.PathLike):
                options += ["--ro-bind", context.config.secure_boot_key, context.config.secure_boot_key]
            reveal_type(context.config.secure_boot_key)

switches the revealed type:

mkosi/__init__.py:1906:25: note: Revealed type is "Union[pathlib.Path, builtins.str]"
mkosi/__init__.py:1909:25: note: Revealed type is "builtins.object"
mkosi/__init__.py:1931:38: error: List item 1 has incompatible type "object"; expected "Union[Path, str]"  [list-item]
                    "--pcr-private-key", context.config.secure_boot_key,
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 42 source files)

Looking further the changes in the commit, not all isinstance(context.config.secure_boot_key, os.PathLike) checks added there suffer from this problem.

The problem has been encountered with

$ mypy --version
mypy 1.8.0 (compiled: no)

but pyright reveals the proper type.

Expected Behavior

The type of the object does not change.

Actual Behavior

The type is widened to builtin.object

mkosi/__init__.py:1906:25: note: Revealed type is "Union[pathlib.Path, builtins.str]"
mkosi/__init__.py:1909:25: note: Revealed type is "builtins.object"
mkosi/__init__.py:1931:38: error: List item 1 has incompatible type "object"; expected "Union[Path, str]"  [list-item]
                    "--pcr-private-key", context.config.secure_boot_key,
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 42 source files)

The problem can be worked around by changing the check

-if isinstance(context.config.secure_boot_key, os.PathLike):
+if context.config.secure_boot_key and not isinstance(context.config.secure_boot_key, str):

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): The following section in pyproject.toml is used to configure mypy:

[tool.mypy]
python_version = 3.9
# belonging to --strict
warn_unused_configs = true
disallow_any_generics = true
disallow_subclassing_any = true
disallow_untyped_calls = true
disallow_untyped_defs = true
disallow_untyped_decorators = true
disallow_incomplete_defs = true
check_untyped_defs = true
no_implicit_optional = true
warn_redundant_casts = true
warn_unused_ignores = false
warn_return_any = true
no_implicit_reexport = true
# extra options not in --strict
pretty = true
show_error_codes = true
show_column_numbers = true
warn_unreachable = true
allow_redefinition = true
strict_equality = true

• Python version used: 3.11.7 on Arch Linux

I'm happy to provide more details

Start by reproducing the reported behavior from the mkosi/init.py built_uki example, using mypy 1.8.0 and the shown Union[None, str, pathlib.Path] value. Trace the isinstance narrowing and add a regression case showing that the type remains usable as Union[Path, str] after the check.