Fix operator bool() exception on zombie node

include/yaml-cpp/node/impl.h

@@ -60,8 +60,9 @@ inline void Node::EnsureNodeExists() const {
 }
 
 inline bool Node::IsDefined() const {
-  if (!m_isValid)
-    throw InvalidNode();
+  if (!m_isValid) {
+    return false;
+  }
   return m_pNode ? m_pNode->is_defined() : true;
 }
 

test/integration/load_node_test.cpp

@@ -181,8 +181,8 @@ TEST(LoadNodeTest, DereferenceIteratorError) {
   EXPECT_THROW(node.begin()->first.as<int>(), InvalidNode);
   EXPECT_EQ(true, (*node.begin()).IsMap());
   EXPECT_EQ(true, node.begin()->IsMap());
-  EXPECT_THROW((*node.begin()->begin()).IsDefined(), InvalidNode);
-  EXPECT_THROW(node.begin()->begin()->IsDefined(), InvalidNode);
+  EXPECT_THROW((*node.begin()->begin()).Type(), InvalidNode);
+  EXPECT_THROW(node.begin()->begin()->Type(), InvalidNode);
 }
 
 TEST(NodeTest, EmitEmptyNode) {

test/node/node_test.cpp

@@ -281,6 +281,13 @@ TEST(NodeTest, DefaultNodeStyle) {
   EXPECT_EQ(EmitterStyle::Default, node.Style());
 }
 
+TEST(NodeTest, AccessNonexistentKeyOnConstNode) {
+  YAML::Node node;
+  node["3"] = "4";
+  const YAML::Node& other = node;
+  ASSERT_FALSE(other["5"]);
+}
+
 class NodeEmitterTest : public ::testing::Test {
  protected:
   void ExpectOutput(const std::string& output, const Node& node) {