Detect unsupported use of self types in generic classes

[JukkaL]

The last line generates a strange error message:

from typing import TypeVar, Generic, Type

M = TypeVar("M")
T = TypeVar("T")

class Descriptor(Generic[M]):
    pass

class BaseWrapper(Generic[M]):
    @classmethod
    def create_wrapper(cls, metric_descriptor):
        # type: (Type[T], Descriptor[M]) -> T
        raise NotImplementedError
    
class WrapperInstance(BaseWrapper[M]):
    @classmethod
    def create_wrapper(cls, descriptor):
        # type: (Descriptor[M]) -> WrapperInstance[M]
        return WrapperInstance()
    
def build_wrapper(descriptor):
    # type: (Descriptor[M]) -> BaseWrapper[M]
    wrapper = WrapperInstance  # type: Type[BaseWrapper[M]]
    # Argument 1 to "create_wrapper" of "BaseWrapper" has incompatible type "Descriptor[M]"; 
    # expected "Descriptor[BaseWrapper[M]]"
    return wrapper.create_wrapper(descriptor)

The root cause seems to be the signature of BaseWrapper.create_wrapper; it shouldn't use two type variables M and T. It would be better to generate an error about the signature instead of the call site. However, there may be something deeper going on.