Fix a bad cast in put<X>BuilderIfAbsent when already present due to using the result of put() directly (which is null if it currently has no value)

PiperOrigin-RevId: 565425310

java/core/src/test/java/com/google/protobuf/MapTest.java

@@ -553,11 +553,16 @@ public class MapTest {
   public void testPutBuilderIfAbsent() {
     TestMap.Builder builder = TestMap.newBuilder();
     MessageValue.Builder subBuilder = builder.putInt32ToMessageFieldBuilderIfAbsent(1);
+
     assertThat(builder.putInt32ToMessageFieldBuilderIfAbsent(1)).isSameInstanceAs(subBuilder);
+
     subBuilder.setValue(11);
     assertThat(builder.getInt32ToMessageFieldOrThrow(1).getValue()).isEqualTo(11);
     builder.putInt32ToMessageFieldBuilderIfAbsent(1).setValue(22);
     assertThat(builder.getInt32ToMessageFieldOrThrow(1).getValue()).isEqualTo(22);
+
+    builder.putInt32ToMessageField(2, MessageValue.newBuilder().setValue(33).build());
+    assertThat(builder.putInt32ToMessageFieldBuilderIfAbsent(2).getValue()).isEqualTo(33);
   }
 
   @Test

src/google/protobuf/compiler/java/map_field.cc

@@ -877,10 +877,11 @@ void ImmutableMapFieldGenerator::GenerateMessageMapBuilderMembers(
       "    entry = $value_type$.newBuilder();\n"
       "    builderMap.put(key, entry);\n"
       "  }\n"
-      "  if (entry instanceof $value_builder_type$) { return "
-      "($value_builder_type$) entry; }\n"
-      "  return ($value_builder_type$) builderMap.put(key, "
-      "(($value_type$) entry).toBuilder());\n"
+      "  if (entry instanceof $value_type$) {\n"
+      "    entry = (($value_type$) entry).toBuilder();\n"
+      "    builderMap.put(key, entry);\n"
+      "  }\n"
+      "  return ($value_builder_type$) entry;\n"
       "}\n");
   printer->Annotate("{", "}", descriptor_, Semantic::kSet);
 }