Raise AssertionError for uncovered fallback case

Co-authored-by: Pierre Sassoulas <[email protected]>

Author: cdce8p

.coveragerc

@@ -20,3 +20,6 @@ exclude_lines =
 
     # Abstract methods are not executed during pytest runs
     raise NotImplementedError()
+
+    # Fallback cases which should never be executed
+    raise AssertionError

pylint/checkers/variables.py

@@ -686,7 +686,9 @@ def _inferred_to_define_name_raise_or_return(
             case nodes.If():
                 pass
             case _:
-                return False
+                # The function is only called for Try, With, For, While, Match and
+                # If nodes. All of which are being handled above.
+                raise AssertionError
 
         # Be permissive if there is a break or a continue
         if any(node.nodes_of_class(nodes.Break, nodes.Continue)):