Added example in the doc for TypeVar in using generic classes

docs/source/common_issues.rst

@@ -595,6 +595,44 @@ string literal types or :py:data:`~typing.TYPE_CHECKING`:
 
    results: 'Queue[int]' = Queue()  # OK
 
+If you use a type variable in the type alias that is used as a base class and
+implement it in above way, it will give error. Note that type arguments are only
+required when using `--disallow-any-generics` (or `--strict`). However, this will
+still be a problem even if these options aren't enabled, since we want to make the
+subclass generic (otherwise it would be pretty pointless to use a type variable),
+and this would result in a runtime error without the suggested trick.
+
+.. code-block:: python
+
+   from queue import Queue
+   from typing import TYPE_CHECKING, TypeVar
+
+   _T = TypeVar("_T")
+   if TYPE_CHECKING:
+       MyQueueBase = Queue[_T]
+   else:
+       MyQueueBase = Queue
+
+   class MyQueue(MyQueueBase): pass  # error: Missing type parameters for generic type
+
+Note here that this error only comes when some configuration options are enabled.
+To avoid this problem, we need to create another class in the hierarchy which makes
+use of :py:data:`~typing.Generic` when :py:data:`~typing.TYPE_CHECKING` is disabled
+and behaves normally when :py:data:`~typing.TYPE_CHECKING` is enabled.
+
+.. code-block:: python
+
+   from queue import Queue
+   from typing import TYPE_CHECKING, TypeVar, Generic
+
+   _T = TypeVar("_T")
+   if TYPE_CHECKING:
+       class MyQueueBase(Queue[_T]): pass
+   else:
+       class MyQueueBase(Generic[_T], Queue): pass
+
+   class MyQueue(MyQueueBase[_T]): pass  # OK
+
 If you are running Python 3.7+ you can use ``from __future__ import annotations``
 as a (nicer) alternative to string quotes, read more in :pep:`563`.  For example: