Add a pair of overloads to dynamic::setDefault

Summary:
This is a workaround to an (already reported) bug in MSVC that results in it not considering the constructors of `dynamic` when attempting to construct the default value of a parameter if the type of the parameter is a universal reference.
It works by simply adding a pair of template specializations that cause MSVC to properly construct the default value.
This also removes the default value on the universal reference version, as the only things that should be using that overload now are non-dynamic values.

Reviewed By: yfeldblum

Differential Revision: D3704990

fbshipit-source-id: 7b85c4e48a1a1023bc2fe0a76a9632b11c4e9364

folly/dynamic-inl.h

@@ -530,6 +530,17 @@ template<class K, class V> inline dynamic& dynamic::setDefault(K&& k, V&& v) {
                                    std::forward<V>(v))).first->second;
 }
 
+template<class K> inline dynamic& dynamic::setDefault(K&& k, dynamic&& v) {
+  auto& obj = get<ObjectImpl>();
+  return obj.insert(std::make_pair(std::forward<K>(k),
+                                   std::move(v))).first->second;
+}
+
+template<class K> inline dynamic& dynamic::setDefault(K&& k, const dynamic& v) {
+  auto& obj = get<ObjectImpl>();
+  return obj.insert(std::make_pair(std::forward<K>(k), v)).first->second;
+}
+
 inline dynamic* dynamic::get_ptr(dynamic const& idx) & {
   return const_cast<dynamic*>(const_cast<dynamic const*>(this)->get_ptr(idx));
 }

folly/dynamic.h

@@ -429,8 +429,16 @@ public:
   dynamic getDefault(const dynamic& k, dynamic&& v) const&;
   dynamic getDefault(const dynamic& k, const dynamic& v = dynamic::object) &&;
   dynamic getDefault(const dynamic& k, dynamic&& v) &&;
-  template<class K, class V = dynamic>
-  dynamic& setDefault(K&& k, V&& v = dynamic::object);
+  template<class K, class V>
+  dynamic& setDefault(K&& k, V&& v);
+  // MSVC 2015 Update 3 needs these extra overloads because if V were a
+  // defaulted template parameter, it causes MSVC to consider v an rvalue
+  // reference rather than a universal reference, resulting in it not being
+  // able to find the correct overload to construct a dynamic with.
+  template<class K>
+  dynamic& setDefault(K&& k, dynamic&& v);
+  template<class K>
+  dynamic& setDefault(K&& k, const dynamic& v = dynamic::object);
 
   /*
    * Resizes an array so it has at n elements, using the supplied