Fix check for non-map alias merging in v2 (#529)

The problem does not affect v3.

decode.go

@@ -788,8 +788,7 @@ func (d *decoder) merge(n *node, out reflect.Value) {
 	case mappingNode:
 		d.unmarshal(n, out)
 	case aliasNode:
-		an, ok := d.doc.anchors[n.value]
-		if ok && an.kind != mappingNode {
+		if n.alias != nil && n.alias.kind != mappingNode {
 			failWantMap()
 		}
 		d.unmarshal(n, out)
@@ -798,8 +797,7 @@ func (d *decoder) merge(n *node, out reflect.Value) {
 		for i := len(n.children) - 1; i >= 0; i-- {
 			ni := n.children[i]
 			if ni.kind == aliasNode {
-				an, ok := d.doc.anchors[ni.value]
-				if ok && an.kind != mappingNode {
+				if ni.alias != nil && ni.alias.kind != mappingNode {
 					failWantMap()
 				}
 			} else if ni.kind != mappingNode {

decode_test.go

@@ -848,6 +848,7 @@ var unmarshalErrorTests = []struct {
 	{"a:\n- b: *,", "yaml: line 2: did not find expected alphabetic or numeric character"},
 	{"a: *b\n", "yaml: unknown anchor 'b' referenced"},
 	{"a: &a\n  b: *a\n", "yaml: anchor 'a' value contains itself"},
+	{"a: &x null\n<<:\n- *x\nb: &x {}\n", `yaml: map merge requires map or sequence of maps as the value`}, // Issue #529.
 	{"value: -", "yaml: block sequence entries are not allowed in this context"},
 	{"a: !!binary ==", "yaml: !!binary value contains invalid base64 data"},
 	{"{[.]}", `yaml: invalid map key: \[\]interface \{\}\{"\."\}`},
@@ -874,6 +875,13 @@ func (s *S) TestUnmarshalErrors(c *C) {
 		var value interface{}
 		err := yaml.Unmarshal([]byte(item.data), &value)
 		c.Assert(err, ErrorMatches, item.error, Commentf("Partial unmarshal: %#v", value))
+
+		if strings.Contains(item.data, ":") {
+			// Repeat test with typed value.
+			var value map[string]interface{}
+			err := yaml.Unmarshal([]byte(item.data), &value)
+			c.Assert(err, ErrorMatches, item.error, Commentf("Partial unmarshal: %#v", value))
+		}
 	}
 }