Fix bug in reflection based Swap of map fields.

It was swapping the arena pointers too when they differed. In that case a full
copy must be made instead. The instances can't change arenas.

PiperOrigin-RevId: 569166797

src/google/protobuf/map.h

@@ -536,7 +536,7 @@ class PROTOBUF_EXPORT UntypedMapBase {
  public:
   Arena* arena() const { return this->alloc_.arena(); }
 
-  void Swap(UntypedMapBase* other) {
+  void InternalSwap(UntypedMapBase* other) {
     std::swap(num_elements_, other->num_elements_);
     std::swap(num_buckets_, other->num_buckets_);
     std::swap(seed_, other->seed_);
@@ -1501,7 +1501,9 @@ class Map : private internal::KeyMapBase<internal::KeyForBase<Key>> {
     }
   }
 
-  void InternalSwap(Map* other) { this->Swap(other); }
+  void InternalSwap(Map* other) {
+    internal::UntypedMapBase::InternalSwap(other);
+  }
 
   hasher hash_function() const { return {}; }
 

src/google/protobuf/map_field_inl.h

@@ -129,7 +129,7 @@ template <typename Key, typename T>
 void TypeDefinedMapFieldBase<Key, T>::Swap(MapFieldBase* other) {
   MapFieldBase::Swap(other);
   auto* other_field = DownCast<TypeDefinedMapFieldBase*>(other);
-  map_.Swap(&other_field->map_);
+  map_.swap(other_field->map_);
 }
 
 template <typename Key, typename T>

src/google/protobuf/map_test.inc

@@ -1270,7 +1270,6 @@ TEST_F(MapImplTest, SwapArena) {
 
 TEST_F(MapImplTest, SwapFieldArenaReflection) {
   MapReflectionTester reflection_tester(UNITTEST::TestMap::descriptor());
-  Arena arena;
 
   {
     // Tests filled lfs and empty rhs.
@@ -1289,9 +1288,15 @@ TEST_F(MapImplTest, SwapFieldArenaReflection) {
       reflection->SwapFields(lhs, &rhs, fields);
 
       reflection_tester.ExpectClearViaReflection(*lhs);
+
+      // Add an entry to make sure it is using the right arena.
+      (*lhs->mutable_map_int32_int32())[1234] = 1234;
     }
 
     reflection_tester.ExpectMapFieldsSetViaReflection(rhs);
+
+      // Add an entry to make sure it is using the right arena.
+    (*rhs.mutable_map_int32_int32())[1234] = 1234;
   }
 }