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

[chet-manley]

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]

[A5rocks]

--warn-unreachable is that way (even though mypy errors, not warns) because --error-unreachable doesn't quite make sense as a phrase.

I wonder if this is because we treat functions on classes as methods (i.e. pretend they are always descriptors) as a heuristic. My impression is any better heuristics are significantly more complicated, so IDK. Maybe this is an actual bug though.