fix: fix stack underflow of PopIndent

include/yaml-cpp/exceptions.h

@@ -92,6 +92,8 @@ const char* const BAD_FILE = "bad file";
 const char* const UNEXPECTED_TOKEN_AFTER_DOC = "unexpected token after end of document";
 const char* const NON_UNIQUE_MAP_KEY = "map keys must be unique";
 
+const char* const INDENT_STACK_UNDERFLOW = "indentation stack underflow (please report this bug to yaml-cpp)";
+
 template <typename T>
 inline const std::string KEY_NOT_FOUND_WITH_KEY(
     const T&, typename disable_if<is_numeric<T>>::type* = 0) {

src/scanner.cpp

@@ -177,7 +177,15 @@ void Scanner::ScanNextToken() {
     //  scalar
     if (!m_simpleKeys.empty() &&
       m_simpleKeys.top().pKey->status == Token::UNVERIFIED) {
+      // if the top of the indents does not match the unverified simple key,
+      // just invalidate the simple key and do not pop indent to avoid crash.
+      // eg: an unverified key crossing lines, like issue #1475.
+      if (m_simpleKeys.top().pIndent && !m_indents.empty() &&
+          m_indents.top() == m_simpleKeys.top().pIndent) {
         PopIndent();
+      } else {
+        InvalidateSimpleKey();
+      }
     }
     return ScanBlockScalar();
   }
@@ -393,6 +401,9 @@ void Scanner::PopAllIndents() {
 }
 
 void Scanner::PopIndent() {
+  if (m_indents.empty()) {
+    ThrowParserException(ErrorMsg::INDENT_STACK_UNDERFLOW);
+  }
   const IndentMarker& indent = *m_indents.top();
   m_indents.pop();
 

test/integration/handler_test.cpp

@@ -73,6 +73,13 @@ TEST_F(HandlerTest, CommentOnNewlineOfMapValueWithManySpace) {
   Parse("key: value\n    # comment");
 }
 
+// example from issue #1475: a stale simple key (from a plain scalar spanning
+// a line break) must not pop an unrelated indent when parsing a block scalar
+TEST_F(HandlerTest, LiteralScalarWithMultiLineUnverifiedPotentialSimpleKey) {
+  EXPECT_THROW_PARSER_EXCEPTION(IgnoreParse("!\n: |\nb\n>\n|\n  !\n>"),
+                                ErrorMsg::END_OF_MAP);
+}
+
 // examples from issue #1163
 TEST_F(HandlerTest, LiteralScalarWithTagAndLargeIndentation) {
   EXPECT_CALL(handler, OnDocumentStart(_));