stubtest false positive with @type_check_only subtype

[jorenham]

Describe the problem

In scipy.special there are a lot of "ufuncs", which are callables numpy.ufunc instances that have a bunch of extra attributes, methods, and automatically convert an vectorize the input.
But the numpy.ufunc annotations are very limited, and not very helpfull when it comes to annotating the ufuncs in scipy.special.
So instead of annotating e.g. beta: np.ufunc, I created a private @type_check_only subtype of np.ufunc, and annotated it to match the type-signatures of scipy.special.beta.
But running stubtest on it, gives the following error:

error: scipy.special.beta variable differs from runtime type numpy.ufunc
Stub: in file /home/joren/Workspace/scipy-stubs/scipy-stubs/special/__init__.pyi:590
scipy.special._ufuncs._UFunc21f['beta', 0]
Runtime:
def (*args, **kwargs)

(scipy.special._ufuncs._UFunc21f is the ufunc subtype)

The error message here isn't very helpful, and I don't see why it shouldn't be allowed. Because such a subtype is basically an intersection of ufunc and the beta-specific annotations (but in a way that pyright and cringe mypy also understand).

See jorenham/scipy-stubs#189 for the gory details.

Maybe I'm missing something, but I tried this a bunch of different ways, and none of them seemed to get rid of this error, so this looks like a false positive to me 🤷🏻.

Gist to reproduce

No response

Severity

annoying but workaround is available

Your Environment

basedmypy 2.7.0 (compiled: yes)
Based on mypy 1.13.0

[jorenham]

some of the relevant code:

basedmypy/mypy/stubtest.py

Lines 1098 to 1122 in 0935784

	runtime_type = get_mypy_type_of_runtime_value(runtime)
	if (
	runtime_type is not None
	and stub.type is not None
	and not is_subtype_helper(runtime_type, stub.type)
	):
	should_error = True
	# Avoid errors when defining enums, since runtime_type is the enum itself, but we'd
	# annotate it with the type of runtime.value
	if isinstance(runtime, enum.Enum):
	runtime_type = get_mypy_type_of_runtime_value(runtime.value)
	if runtime_type is not None and is_subtype_helper(runtime_type, stub.type):
	should_error = False
	# We always allow setting the stub value to ...
	proper_type = mypy.types.get_proper_type(stub.type)
	if (
	isinstance(proper_type, mypy.types.Instance)
	and proper_type.type.fullname == "builtins.ellipsis"
	):
	should_error = False
	if should_error:
	yield Error(
	object_path, f"variable differs from runtime type {runtime_type}", stub, runtime
	)

basedmypy/mypy/stubtest.py

Lines 1564 to 1584 in 0935784

	def is_subtype_helper(left: mypy.types.Type, right: mypy.types.Type) -> bool:
	"""Checks whether ``left`` is a subtype of ``right``."""
	left = mypy.types.get_proper_type(left)
	right = mypy.types.get_proper_type(right)
	if (
	isinstance(left, mypy.types.LiteralType)
	and isinstance(left.value, int)
	and left.value in (0, 1)
	and mypy.types.is_named_instance(right, "builtins.bool")
	):
	# Pretend Literal[0, 1] is a subtype of bool to avoid unhelpful errors.
	return True
	if isinstance(right, mypy.types.TypedDictType) and mypy.types.is_named_instance(
	left, "builtins.dict"
	):
	# Special case checks against TypedDicts
	return True
	with mypy.state.state.strict_optional_set(True):
	return mypy.subtypes.is_subtype(left, right)

Perhaps this mypy.types.get_proper_type function should return the supertype of a @type_check_only type 🤔?
Or maybe it should use the return type of the overridden __class__ property (which I did in this case as well, but that didn't do much) if present?

basedmypy/mypy/types.py

Lines 3498 to 3514 in 0935784

	def get_proper_type(typ: Type | None) -> ProperType | None:
	"""Get the expansion of a type alias type.
	If the type is already a proper type, this is a no-op. Use this function
	wherever a decision is made on a call like e.g. 'if isinstance(typ, UnionType): ...',
	because 'typ' in this case may be an alias to union. Note: if after making the decision
	on the isinstance() call you pass on the original type (and not one of its components)
	it is recommended to *always* pass on the unexpanded alias.
	"""
	if typ is None:
	return None
	if isinstance(typ, TypeGuardedType): # type: ignore[misc]
	typ = typ.type_guard
	while isinstance(typ, TypeAliasType):
	typ = typ._expand_once()
	# TODO: store the name of original type alias on this type, so we can show it in errors.
	return cast(ProperType, typ)