Tighten up the yamilizer so it stops eliding empty sequences if the embedded empty sequence is the first key/value in a map which is itself in a sequence.

Patch with help from Nick Kledzik.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188508 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/YAMLTraits.cpp

@@ -334,6 +334,10 @@ void Input::setError(const Twine &Message) {
   this->setError(CurrentNode, Message);
 }
 
+bool Input::canElideEmptySequence() {
+  return false;
+}
+
 Input::MapHNode::~MapHNode() {
   for (MapHNode::NameToNode::iterator i = Mapping.begin(), End = Mapping.end();
                                                                 i != End; ++i) {
@@ -532,6 +536,19 @@ void Output::scalarString(StringRef &S) {
 void Output::setError(const Twine &message) {
 }
 
+bool Output::canElideEmptySequence() {
+  // Normally, with an optional key/value where the value is an empty sequence,
+  // the whole key/value can be not written.  But, that produces wrong yaml
+  // if the key/value is the only thing in the map and the map is used in
+  // a sequence.  This detects if the this sequence is the first key/value
+  // in map that itself is embedded in a sequnce.
+  if (StateStack.size() < 2) 
+    return true;
+  if (StateStack.back() != inMapFirstKey) 
+    return true;
+  return (StateStack[StateStack.size()-2] != inSeq);
+}
+
 void Output::output(StringRef s) {
   Column += s.size();
   Out << s;