Return stricter type from mjCModel::operator+=(mjDef&).

PiperOrigin-RevId: 772871198
Change-Id: I4c840389b38e2d8d6bcf92311fad3242324fa3af

src/user/user_model.cc

@@ -653,7 +653,7 @@ mjCModel& mjCModel::operator-=(const mjCBody& subtree) {
 
 
 // add default tree to this model
-mjCModel_& mjCModel::operator+=(mjCDef& subtree) {
+mjCModel& mjCModel::operator+=(mjCDef& subtree) {
   defaults_.push_back(&subtree);
   def_map[subtree.name] = &subtree;
   subtree.model = this;

src/user/user_model.h

@@ -187,7 +187,7 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCModel& operator=(const mjCModel& other);     // copy other into this, if they are not the same
   mjCModel& operator+=(const mjCModel& other);    // add other into this, even if they are the same
   mjCModel& operator-=(const mjCBody& subtree);   // remove subtree and all references from model
-  mjCModel_& operator+=(mjCDef& subtree);         // add default tree to this model
+  mjCModel& operator+=(mjCDef& subtree);          // add default tree to this model
   mjCModel& operator-=(const mjCDef& subtree);    // remove default tree from this model
 
   mjSpec spec;