include_vars - fix including new hash variables when hash_behaviour is set to merge (#84303)

changelogs/fragments/fix-include_vars-merge-hash.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - include_vars - fix including previously undefined hash variables with hash_behaviour merge (https://github.com/ansible/ansible/issues/84295).

lib/ansible/plugins/action/include_vars.py

@@ -142,9 +142,8 @@ class ActionModule(ActionBase):
             result['message'] = err_msg
         elif self.hash_behaviour is not None and self.hash_behaviour != C.DEFAULT_HASH_BEHAVIOUR:
             merge_hashes = self.hash_behaviour == 'merge'
-            for key, value in results.items():
-                old_value = task_vars.get(key, None)
-                results[key] = combine_vars(old_value, value, merge=merge_hashes)
+            existing_variables = {k: v for k, v in task_vars.items() if k in results}
+            results = combine_vars(existing_variables, results, merge=merge_hashes)
 
         result['ansible_included_var_files'] = self.included_files
         result['ansible_facts'] = results

test/integration/targets/include_vars/vars2/hashes/hash2.yml

@@ -3,3 +3,5 @@ config:
   key1: 1
   key2: { b: 22 }
   key3: 3
+previously_undefined:
+  key1: { a: 1 }