@strawberry.type
class MyGenericType(Generic[T]):
    my_value: T

@strawberry.type
class MyConcreteType(MyGenericType[int], Generic[K]):
    other_value: K

import strawberry
from strawberry import Schema
from strawberry.extensions import FieldExtension
from strawberry.field import StrawberryField
from typing import Generic, TypeVar

T = TypeVar("T")


class CheckAtRuntime(FieldExtension):
    def apply(self, field: StrawberryField) -> None:
        print('concrete_of when applying extension is', field.type.__strawberry_definition__.concrete_of)

    def resolve(self, next_, source, info, **kwargs):
        return next_(source, info, **kwargs)


@strawberry.type
class MyGenericType(Generic[T]):
    my_value: T


@strawberry.type
class MyConcreteType(MyGenericType[int]):
    pass


@strawberry.type
class Query:
    @strawberry.field(extensions=[CheckAtRuntime()])
    def my_concrete_type(self) -> MyConcreteType:
        return MyConcreteType(my_value=123)


schema = Schema(query=Query)
print('concrete_of when booting is', MyConcreteType.__strawberry_definition__.concrete_of)