Add an exception for IndexError inside uninferable_final_decorator

ChangeLog

@@ -307,6 +307,9 @@ What's New in Pylint 2.13.9?
 ============================
 Release date: TBA
 
+* Fix ``IndexError`` crash in ``uninferable_final_decorators`` method.
+
+ Relates to #6531
 
 
 What's New in Pylint 2.13.8?

doc/whatsnew/2.13.rst

@@ -639,3 +639,7 @@ Other Changes
   ``open``
 
   Closes #6414
+
+* Fix ``IndexError`` crash in ``uninferable_final_decorators`` method.
+
+ Relates to #6531

pylint/checkers/utils.py

@@ -847,11 +847,13 @@ def uninferable_final_decorators(
     """
     decorators = []
     for decorator in getattr(node, "nodes", []):
-        if isinstance(decorator, nodes.Attribute):
-            try:
-                import_node = decorator.expr.lookup(decorator.expr.name)[1][0]
-            except AttributeError:
+        if isinstance(decorator, nodes.Attribute) and hasattr(decorator.expr, "lookup"):
+            _, import_nodes = decorator.expr.lookup(decorator.expr.name)
+            # The `final` decorator is expected to be found in the
+            # import_nodes. In case it is not, continue.
+            if not import_nodes:
                 continue
+            import_node = import_nodes[0]
         elif isinstance(decorator, nodes.Name):
             lookup_values = decorator.lookup(decorator.name)
             if lookup_values[1]:

tests/functional/r/regression/regression_6531_crash_index_error.py

@@ -0,0 +1,30 @@
+"""Regression test for https://github.com/PyCQA/pylint/issues/6531."""
+
+# pylint: disable=missing-docstring, redefined-outer-name
+
+import pytest
+
+
+class Wallet:
+    def __init__(self):
+        self.balance = 0
+
+    def add_cash(self, earned):
+        self.balance += earned
+
+    def spend_cash(self, spent):
+        self.balance -= spent
+
+@pytest.fixture
+def my_wallet():
+    '''Returns a Wallet instance with a zero balance'''
+    return Wallet()
+
+@pytest.mark.parametrize("earned,spent,expected", [
+    (30, 10, 20),
+    (20, 2, 18),
+])
+def test_transactions(my_wallet, earned, spent, expected):
+    my_wallet.add_cash(earned)
+    my_wallet.spend_cash(spent)
+    assert my_wallet.balance == expected