Add __class_getitem__ to the declarative Base class

Fixed issue where the :func:`_orm.as_declarative` decorator and similar
functions used to generate the declarative base class would not copy the
``__class_getitem__()`` method from a given superclass, which prevented the
use of pep-484 generics in conjunction with the ``Base`` class. Pull
request courtesy Kai Mueller.

Fixes: #7368
Closes: #7381
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7381
Pull-request-sha: 7db7fd869a6bb617f767fad5b71ddf7cb5f14ff5

Change-Id: I495718c3082ad6dd2c83fdbf6feba7c529e351cb

3 files changed, 36 insertions, 0 deletions

diff --git a/doc/build/changelog/unreleased_14/7368.rst b/doc/build/changelog/unreleased_14/7368.rst
new file mode 100644
index 000000000..d4415ffec
--- /dev/null
+++ b/doc/build/changelog/unreleased_14/7368.rst
@@ -0,0 +1,9 @@
+.. change::
+    :tags: bug, orm, mypy
+    :tickets: 7368
+
+    Fixed issue where the :func:`_orm.as_declarative` decorator and similar
+    functions used to generate the declarative base class would not copy the
+    ``__class_getitem__()`` method from a given superclass, which prevented the
+    use of pep-484 generics in conjunction with the ``Base`` class. Pull
+    request courtesy Kai Mueller.
diff --git a/lib/sqlalchemy/orm/decl_api.py b/lib/sqlalchemy/orm/decl_api.py
index 6bc857094..6951b3546 100644
--- a/lib/sqlalchemy/orm/decl_api.py
+++ b/lib/sqlalchemy/orm/decl_api.py
@@ -807,6 +807,8 @@ class registry:
         class_dict["__abstract__"] = True
         if mapper:
             class_dict["__mapper_cls__"] = mapper
+        if hasattr(cls, "__class_getitem__"):
+            class_dict["__class_getitem__"] = cls.__class_getitem__
 
         return metaclass(name, bases, class_dict)
 
diff --git a/test/orm/declarative/test_typing_py3k.py b/test/orm/declarative/test_typing_py3k.py
new file mode 100644
index 000000000..823fe54f1
--- /dev/null
+++ b/test/orm/declarative/test_typing_py3k.py
@@ -0,0 +1,25 @@
+from typing import Generic
+from typing import Type
+from typing import TypeVar
+
+from sqlalchemy import Column
+from sqlalchemy import Integer
+from sqlalchemy.orm import as_declarative
+from sqlalchemy.testing import fixtures
+
+
+class DeclarativeBaseTest(fixtures.TestBase):
+    def test_class_getitem(self):
+        T = TypeVar("T", bound="CommonBase")  # noqa
+
+        class CommonBase(Generic[T]):
+            @classmethod
+            def boring(cls: Type[T]) -> Type[T]:
+                return cls
+
+        @as_declarative()
+        class Base(CommonBase[T]):
+            pass
+
+        class Tab(Base["Tab"]):
+            a = Column(Integer, primary_key=True)