Fix assertoin failure for certain kinds of merges

The RoundTripConstructor.flatten_mapping() method made an assumption that
when merging a mapping into another mapping, the actual data would have been
defined already. For example:

  predefined: &contents { a: 1 }
  merged:
    <<: *contents

However, the following is valid as well, and in this case the data is only
defined at the point of merging:

  merged-1:
    <<: &contents  { a: 1 }
  merged-2:
    <<: *contents

This commit fixes an assertion failure for the latter case.

2 files changed, 39 insertions, 4 deletions

diff --git a/py/constructor.py b/py/constructor.py
index 1de8545..5ce8974 100644
--- a/py/constructor.py
+++ b/py/constructor.py
@@ -851,6 +851,18 @@ class RoundTripConstructor(SafeConstructor):
         by inserting keys from the merge dict/list of dicts if not yet
         available in this node
         """
+
+        def constructed(value_node):
+            # If the contents of a merge are defined within the
+            # merge marker, then they won't have been constructed
+            # yet. But if they were already constructed, we need to use
+            # the existing object.
+            if value_node in self.constructed_objects:
+                value = self.constructed_objects[value_node]
+            else:
+                value = self.construct_object(value_node, deep=False)
+            return value
+
         #merge = []
         merge_map_list = []
         index = 0
@@ -859,10 +871,8 @@ class RoundTripConstructor(SafeConstructor):
             if key_node.tag == u'tag:yaml.org,2002:merge':
                 del node.value[index]
                 if isinstance(value_node, MappingNode):
-                    # such an anchor node is already constructed
-                    assert value_node in self.constructed_objects
                     merge_map_list.append(
-                        (index, self.constructed_objects[value_node]))
+                        (index, constructed(value_node)))
                     #self.flatten_mapping(value_node)
                     #merge.extend(value_node.value)
                 elif isinstance(value_node, SequenceNode):
@@ -875,7 +885,7 @@ class RoundTripConstructor(SafeConstructor):
                                 "expected a mapping for merging, but found %s"
                                 % subnode.id, subnode.start_mark)
                         merge_map_list.append(
-                            (index, self.constructed_objects[subnode]))
+                            (index, constructed(subnode)))
                     #    self.flatten_mapping(subnode)
                     #    submerge.append(subnode.value)
                     #submerge.reverse()
diff --git a/test/test_anchor.py b/test/test_anchor.py
index efbcb21..e48398b 100644
--- a/test/test_anchor.py
+++ b/test/test_anchor.py
@@ -179,6 +179,31 @@ class TestAnchorsAliases:
         data = load(self.merge_yaml)
         compare(data, self.merge_yaml)
 
+    def test_merge_nested(self):
+        yaml = '''
+        a:
+          <<: &content
+            1: plugh
+            2: plover
+          0: xyzzy
+        b:
+          <<: *content
+        '''
+        data = round_trip(yaml)
+
+    def test_merge_nested_with_sequence(self):
+        yaml = '''
+        a:
+          <<: &content
+            <<: &y2
+              1: plugh
+            2: plover
+          0: xyzzy
+        b:
+          <<: [*content, *y2]
+        '''
+        data = round_trip(yaml)
+
     def test_add_anchor(self):
         from ruamel.yaml.comments import CommentedMap
         data = CommentedMap()