forward mapped value in object::insert_or_assign

Author: Ramya-9353

include/boost/json/impl/object.hpp

@@ -398,7 +398,7 @@ insert_or_assign(
         key, key, static_cast<M&&>(m) );
     if( !result.second )
     {
-        value(static_cast<M>(m), sp_).swap(
+        value(static_cast<M&&>(m), sp_).swap(
             result.first->value());
     }
     return result;

test/object.cpp

@@ -1201,6 +1201,17 @@ class object_test
                     { "k3", 3 },
                     { "k4", 2 }}));
             }
+
+            // rvalue is moved, not copied, on the assign branch
+            {
+                object o = {
+                    { "k1", 1 },
+                    { "k2", { 1, 2, 3 } } };
+                value v = { 4, 5, 6 };
+                auto const d = v.as_array().data();
+                o.insert_or_assign("k2", std::move(v));
+                BOOST_TEST(o.at("k2").as_array().data() == d);
+            }
         }
 
         // emplace(key, arg)