Fix crash for ``protected-access`` on (outer)-class traversal (#5305)

* Fix crash for ``protected-access`` on (outer)-class traversal

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

5 files changed, 25 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index a4998baf8..4a8b2eb81 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -175,6 +175,8 @@ Release date: TBA
 
   Follow-up in #5259
 
+* Fix crash for ``protected-access`` on (outer) class traversal
+
 * Added new checker ``useless-with-lock`` to find incorrect usage of with statement and threading module locks.
   Emitted when ``with threading.Lock():`` is used instead of ``with lock_instance:``.
 
diff --git a/doc/whatsnew/2.12.rst b/doc/whatsnew/2.12.rst
index b69af3289..97957d7c5 100644
--- a/doc/whatsnew/2.12.rst
+++ b/doc/whatsnew/2.12.rst
@@ -179,6 +179,8 @@ Other Changes
 
   Closes #4580
 
+* Fix crash for ``protected-access`` on (outer) class traversal
+
 * Make yn validator case insensitive, to allow for ``True`` and ``False`` in config files.
 
 * The last version compatible with python '3.6.0' and '3.6.1' is pylint '2.9.3'. We did not
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index cd3e1178c..319a37f73 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -1624,7 +1624,7 @@ a metaclass class method.",
             parents_callee = callee.split(".")
             parents_callee.reverse()
             for callee in parents_callee:
-                if callee != outer_klass.name:
+                if not outer_klass or callee != outer_klass.name:
                     inside_klass = False
                     break
 
diff --git a/tests/functional/r/regression_02/regression_protected_access.py b/tests/functional/r/regression_02/regression_protected_access.py
new file mode 100644
index 000000000..df36f6795
--- /dev/null
+++ b/tests/functional/r/regression_02/regression_protected_access.py
@@ -0,0 +1,19 @@
+"""Test for the regression on (outer)-class traversal for private methods
+MyClass does not have an outerclass which previously crashed the protected-access check
+"""
+# pylint: disable=too-few-public-methods
+
+
+class MyClass:
+    """Test class"""
+
+    @staticmethod
+    def _a_private_method():
+        """Private method that references the class itself"""
+        return MySecondClass.MyClass._a_private_method()  # [protected-access]
+
+
+class MySecondClass:
+    """Class that uses MyClass"""
+
+    MyClass = MyClass
diff --git a/tests/functional/r/regression_02/regression_protected_access.txt b/tests/functional/r/regression_02/regression_protected_access.txt
new file mode 100644
index 000000000..e3577b4b1
--- /dev/null
+++ b/tests/functional/r/regression_02/regression_protected_access.txt
@@ -0,0 +1 @@
+protected-access:13:15:MyClass._a_private_method:Access to a protected member _a_private_method of a client class:HIGH