GenericModel - Allow subclasses of concrete generics to be generic

[choogeboom]

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Feature Request

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.6.1
            pydantic compiled: False
                 install path: /wayfair/home/ch438l/pydantic/pydantic
               python version: 3.6.8 (default, Aug 10 2019, 06:54:07)  [GCC 4.8.5 20150623 (Red Hat 4.8.5-36)]
                     platform: Linux-3.10.0-862.3.2.el7.x86_64-x86_64-with-centos-7.5.1804-Core
     optional deps. installed: ['typing-extensions', 'email-validator', 'devtools']

I'd like to be able to define a generic model, then create a subclass of that generic model that both fills in the type parameter of the superclass, while introducing new type parameters. Here's a currently failing test case that illustrates the issue:

@skip_36
def test_generic_subclass_of_concrete_generic():
    T = TypeVar("T")
    U = TypeVar("U")

    class GenericBaseModel(GenericModel, Generic[T]):
        data: T

    class GenericSub(GenericBaseModel[int], Generic[U]):
        extra: U

    GenericSub[str]

Currently that test case fails with TypeError: Cannot parameterize a concrete instantiation of a generic model

This happens because GenericBaseModel[int].__concrete__ gets set to True, and GenericSub inherits it.

This can easily be fixed with the following change to generics.py:

        if cls.__concrete__:
            raise TypeError('Cannot parameterize a concrete instantiation of a generic model')

Should become

        if cls.__concrete__ and typing.Generic not in cls.__bases__:
            raise TypeError('Cannot parameterize a concrete instantiation of a generic model')