Construct a scalar member in place; only a throwing value goes through a temporary

Creating the value first and moving it over the member is needed only when
creating it can throw (a string, a binary value or a container). A scalar
or null is again constructed in place, without the extra move and destroy
per member (and, with JSON_DIAGNOSTICS, without the extra set_parents()).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -16946,13 +16946,9 @@ class json_sax_dom_parser : public json_sax_dom_error_state
     {
         if (JSON_HEDLEY_LIKELY(member.is_null()))
         {
-            // a new member is null and holds nothing: move the value over it
-            // instead of destroying and swapping. The value is created first,
-            // because creating a container or a string can throw, and a
-            // constructor that was interrupted must not have touched the
-            // member (#5985); the move constructor is noexcept
-            BasicJsonType value(std::forward<Value>(v));
-            ::new (static_cast<void*>(&member)) BasicJsonType(std::move(value));
+            // a new member is null and holds nothing: construct the value
+            // over it instead of destroying and swapping
+            construct_member(member, std::forward<Value>(v), std::is_nothrow_constructible < BasicJsonType, Value && > {});
         }
         else
         {
@@ -16961,6 +16957,25 @@ class json_sax_dom_parser : public json_sax_dom_error_state
         }
     }
 
+    /// a scalar cannot throw: construct it in place
+    template<typename Value>
+    JSON_HEDLEY_ALWAYS_INLINE
+    static void construct_member(BasicJsonType& member, Value&& v, std::true_type /*nothrow*/)
+    {
+        ::new (static_cast<void*>(&member)) BasicJsonType(std::forward<Value>(v));
+    }
+
+    /// creating a container or a string can throw, and a constructor that was
+    /// interrupted must not have touched the member (#5985): create the value
+    /// first, then move it over the member (the move constructor is noexcept)
+    template<typename Value>
+    JSON_HEDLEY_ALWAYS_INLINE
+    static void construct_member(BasicJsonType& member, Value&& v, std::false_type /*may throw*/)
+    {
+        BasicJsonType value(std::forward<Value>(v));
+        ::new (static_cast<void*>(&member)) BasicJsonType(std::move(value));
+    }
+
     /// a custom base class may care about the lifetime of the value
     template<typename Value>
     static void set_member(BasicJsonType& member, Value&& v, std::false_type /*custom base*/)