variable merging: detect if both vars are really dicts when combining/merging dicts

lib/ansible/utils/__init__.py

@@ -696,10 +696,17 @@ def parse_kv(args):
                 options[k.strip()] = unquote(v.strip())
     return options
 
+def _validate_both_dicts(a, b):
+
+    if not (isinstance(a, dict) and isinstance(b, dict)):
+        raise errors.AnsibleError("Failed to combine two values which are not "
+            "both hashes, got these differing values now:\n\n%s\n and\n%s" % (a, b))
+
 def merge_hash(a, b):
     ''' recursively merges hash b into a
     keys from b take precedence over keys from a '''
 
+    _validate_both_dicts(a, b)
     result = {}
 
     for dicts in a, b:
@@ -1308,6 +1315,8 @@ def listify_lookup_plugin_terms(terms, basedir, inject):
 
 def combine_vars(a, b):
 
+    _validate_both_dicts(a, b)
+
     if C.DEFAULT_HASH_BEHAVIOUR == "merge":
         return merge_hash(a, b)
     else: