Fix false-positive assigning-non-slot with typing.Generic base

ChangeLog

@@ -119,6 +119,11 @@ modules are added.
 
   Closes PyCQA/astroid#942
 
+* Fix ``assigning-non-slot`` false-positive with base that inherits from ``typing.Generic``
+
+  Closes #4509
+  Closes PyCQA/astroid#999
+
 
 What's New in Pylint 2.8.2?
 ===========================

pylint/checkers/classes.py

@@ -1417,6 +1417,13 @@ def _check_in_slots(self, node):
         if "__slots__" not in klass.locals or not klass.newstyle:
             return
 
+        # If 'typing.Generic' is a base of bases of klass, the cached version
+        # of 'slots()' might have been evaluated incorrectly, thus deleted cache entry.
+        if any(base.qname() == "typing.Generic" for base in klass.mro()):
+            cache = getattr(klass, "__cache", None)
+            if cache and cache.get(klass.slots) is not None:
+                del cache[klass.slots]
+
         slots = klass.slots()
         if slots is None:
             return

tests/functional/a/assign/assigning_non_slot_4509.py

@@ -0,0 +1,18 @@
+# pylint: disable=invalid-name,missing-docstring,too-few-public-methods
+
+# Slots with base that inherits from 'Generic'
+# https://github.com/PyCQA/pylint/issues/4509
+# https://github.com/PyCQA/astroid/issues/999
+
+from typing import Generic, TypeVar
+T = TypeVar("T")
+
+class Base(Generic[T]):
+    __slots__ = ()
+
+class Foo(Base[T]):
+    __slots__ = ['_value']
+
+    def __init__(self, value: T):
+        self._value = value
+        self._bar = value  # [assigning-non-slot]

tests/functional/a/assign/assigning_non_slot_4509.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.7

tests/functional/a/assign/assigning_non_slot_4509.txt

@@ -0,0 +1 @@
+assigning-non-slot:18:8:Foo.__init__:Assigning to attribute '_bar' not defined in class slots