Property annotated as `FunctionType` triggers false unreachable error on `isinstance` check

Bug Report

Mypy incorrectly reports an unreachable error when assert isinstance(..., FunctionType) on a @property annotated as returning FunctionType, even though the constructor enforces the invariant.
This is a false positive: no MethodType is involved, and the assertion is trivially true.

Also, the option that enables this error is warn_unreachable. Is this supposed to be a warning instead of an error?

As an aside, while asserting that a defined function (def func(): ...) is an instance of FunctionType is True, if a parameter of a function/method defines func: FunctionType, mypy will raise a arg-type error at the call site (error: Argument 1 to "FuncWrap" has incompatible type "Callable[[], int]"; expected "FunctionType" [arg-type]). This also seems like a false positive, as raw function definitions are implicitly of type FunctionType. Not what this bug is about, but something I ran into designing my wrapper.

To Reproduce

from types import FunctionType

class FuncWrap:
    def __init__(self, func: object) -> None:
        if not isinstance(func, FunctionType):
            raise TypeError()
        self._func = func

    @property
    def __func__(self) -> FunctionType:
        return self._func

def good() -> int:
    return 1

wrapped = FuncWrap(good)

assert isinstance(wrapped.__func__, FunctionType)

If you replace the assert with assert wrapped.__func__ is good, mypy reports a new comparison-overlap error: error: Non-overlapping identity check (left operand type: "MethodType", right operand type: "FunctionType") [comparison-overlap].

gist
playground

Expected Behavior

No errors/warnings.

Actual Behavior

main.py:18: error: Subclass of "MethodType" and "FunctionType" cannot exist: "MethodType" is final  [unreachable]
main.py:18: error: Subclass of "MethodType" and "FunctionType" cannot exist: "FunctionType" is final  [unreachable]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 2.3.1, 1.20.x, 1.10.x
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files):

  [tool.mypy]
    cache_dir = "$MYPY_CONFIG_FILE_DIR/.mypy_cache"
    check_untyped_defs = true
    color_output = true
    disallow_any_generics = true
    disallow_incomplete_defs = true
    disallow_untyped_decorators = true
    disallow_untyped_defs = true
    exclude = []
    explicit_package_bases = false
    files = [ "src/", "tests/" ]
    implicit_optional = false
    implicit_reexport = false
    incremental = true
    local_partial_types = true
    mypy_path = "$MYPY_CONFIG_FILE_DIR/src"
    namespace_packages = false
    native_parser = true
    num_workers = 2
    plugins = []
    python_version = "3.11"
    show_error_codes = true
    show_error_context = true
    strict = true
    strict_equality = true
    strict_optional = true
    warn_no_return = true
    warn_redundant_casts = true
    warn_return_any = true
    warn_unreachable = true
    warn_unused_configs = true
    warn_unused_ignores = true
  

• Python version used: 3.[11-14]

Commencez par le reproducteur dans l’issue et suivez l’analyse d’accessibilité de mypy pour l’assertion isinstance, en vous concentrant sur son traitement de FunctionType et MethodType. Ajoutez un test de régression pour le cas annoté avec property et vérifiez que warn_unreachable ne produit ni erreur ni avertissement.