def _check_function_type(function: object) -> None:
    """Check if the input function is a supported type for `validate_call`."""
    if isinstance(function, _generate_schema.VALIDATE_CALL_SUPPORTED_TYPES):
        try:
            inspect.signature(cast(_generate_schema.ValidateCallSupportedTypes, function))
        except ValueError:
            raise PydanticUserError(
                f"Input function `{function}` doesn't have a valid signature", code=_INVALID_TYPE_ERROR_CODE
            )

        if isinstance(function, partial):
            try:
                assert not isinstance(partial.func, partial), 'Partial of partial'
                _check_function_type(function.func)
            except PydanticUserError as e:
                raise PydanticUserError(
                    f'Partial of `{function.func}` is invalid because the type of `{function.func}` is not supported by `validate_call`',
                    code=_INVALID_TYPE_ERROR_CODE,
                ) from e

        return

    if isinstance(function, BuiltinFunctionType):
        raise PydanticUserError(f'Input built-in function `{function}` is not supported', code=_INVALID_TYPE_ERROR_CODE)
    if isinstance(function, (classmethod, staticmethod, property)):
        name = type(function).__name__
        raise PydanticUserError(
            f'The `@{name}` decorator should be applied after `@validate_call` (put `@{name}` on top)',
            code=_INVALID_TYPE_ERROR_CODE,
        )

    if inspect.isclass(function):
        raise PydanticUserError(
            f'Unable to validate {function}: `validate_call` should be applied to functions, not classes (put `@validate_call` on top of `__init__` or `__new__` instead)',
            code=_INVALID_TYPE_ERROR_CODE,
        )
    if callable(function):
        raise PydanticUserError(
            f'Unable to validate {function}: `validate_call` should be applied to functions, not instances or other callables. Use `validate_call` explicitly on `__call__` instead.',
            code=_INVALID_TYPE_ERROR_CODE,
        )

    raise PydanticUserError(
        f'Unable to validate {function}: `validate_call` should be applied to one of the following: function, method, partial, or lambda',
        code=_INVALID_TYPE_ERROR_CODE,
    )
Check if the input function is a supported type for `validate_call`.
pydantic/validate_call_decorator.py