Fix crash on uninferable decorators on Python 3.6 and 3.7

ChangeLog

@@ -79,6 +79,8 @@ Release date: TBA
 
   Closes #5323
 
+* Fixed crash on uninferable decorators on Python 3.6 and 3.7
+
 * Add checker ``unnecessary-ellipsis``: Emitted when the ellipsis constant is used unnecessarily.
 
   Closes #5460

doc/whatsnew/2.13.rst

@@ -73,6 +73,8 @@ Other Changes
 
   Closes #5065
 
+* Fixed crash on uninferable decorators on Python 3.6 and 3.7
+
 * Fatal errors now emit a score of 0.0 regardless of whether the linted module
   contained any statements
 

pylint/checkers/utils.py

@@ -876,7 +876,11 @@ def uninferable_final_decorators(
             except AttributeError:
                 continue
         elif isinstance(decorator, nodes.Name):
-            import_node = decorator.lookup(decorator.name)[1][0]
+            lookup_values = decorator.lookup(decorator.name)
+            if lookup_values[1]:
+                import_node = lookup_values[1][0]
+            else:
+                continue  # pragma: no cover # Covered on Python < 3.8
         else:
             continue
 

tests/functional/o/overridden_final_method_regression.py

@@ -0,0 +1,6 @@
+"""Test a crash regression for the overridden-final-method checker on uninferable decorators"""
+
+
+@unknown_decorator  # [undefined-variable]
+def crash_test():
+    """A docstring"""

tests/functional/o/overridden_final_method_regression.txt

@@ -0,0 +1 @@
+undefined-variable:4:1:4:18:crash_test:Undefined variable 'unknown_decorator':UNDEFINED