Binary compatibility shims for GeneratedMessageV3, SingleFieldBuilderV3, RepeatedFieldBuilderV3, and their nested classes to restore binary compatibility with <=v3.x.x generated code built against v3.x.x prior to v4.26.0 breaking release.

3.x.x descriptor.proto generated code is *not* supported with 4.x.x runtime, since this results in an ODR violation with the descriptor.proto built into the 4.x.x runtime. This is expected to result in undefined behavior / failures.

Tested against //java/core:v25_generated_message_test_jar (binary compatibility) and //java/core:v25_generated_message_test_srcjar (source compatibility)

PiperOrigin-RevId: 664819152

WORKSPACE

@@ -233,10 +233,6 @@ http_archive(
     url = "https://github.com/protocolbuffers/protobuf/releases/download/v25.0/protobuf-25.0.tar.gz",
 )
 
-# Needed as a dependency of @com_google_protobuf_v25.x, which was before
-# utf8_range was merged in.
-http_archive(
-    name = "utf8_range",
-    strip_prefix = "utf8_range-d863bc33e15cba6d873c878dcca9e6fe52b2f8cb",
-    url = "https://github.com/protocolbuffers/utf8_range/archive/d863bc33e15cba6d873c878dcca9e6fe52b2f8cb.zip",
-)
+# Needed as a dependency of @com_google_protobuf_v25.0
+load("@com_google_protobuf_v25.0//:protobuf_deps.bzl", protobuf_v25_deps="protobuf_deps")
+protobuf_v25_deps()

compatibility/BUILD.bazel

@@ -6,6 +6,26 @@
 
 load("//compatibility:runtime_conformance.bzl", "java_runtime_conformance")
 
+java_library(
+    name = "v25_test_protos_srcjar",
+    testonly = True,
+    srcs = glob([
+        "v3.25.0/*.srcjar",
+    ]),
+    visibility = ["//java/core:__pkg__"],
+    deps = ["//java/core"],
+)
+
+java_library(
+    name = "v25_test_protos_jar",
+    testonly = True,
+    srcs = glob([
+        "v3.25.0/*.srcjar",
+    ]),
+    visibility = ["//java/core:__pkg__"],
+    deps = ["@com_google_protobuf_v25.0//java/core"],
+)
+
 # main gencode builds with main runtime as a proof of concept.
 java_runtime_conformance(
     name = "java_conformance_main",

java/core/BUILD.bazel

@@ -581,6 +581,102 @@ junit_tests(
     ],
 )
 
+protobuf_java_library(
+    name = "v25_test_util_srcjar",
+    testonly = True,
+    srcs = [
+        "src/test/java/com/google/protobuf/TestUtil.java",
+        "src/test/java/com/google/protobuf/TestUtilLite.java",
+    ],
+    deps = [
+        ":core",
+        "//compatibility:v25_test_protos_srcjar",
+        "@maven//:com_google_guava_guava",
+        "@maven//:junit_junit",
+    ],
+)
+
+# Tests source compatibility against v25 gencode jar compiled against current runtime
+junit_tests(
+    name = "v25_core_tests_srcjar",
+    size = "small",
+    srcs = glob(
+        ["src/test/java/**/*.java"],
+        exclude = [
+            # Depends on test protos or API changes added in v4.x.x (e.g. editions)
+            "src/test/java/com/google/protobuf/TextFormatTest.java",
+            "src/test/java/com/google/protobuf/DescriptorsTest.java",
+            "src/test/java/com/google/protobuf/DebugFormatTest.java",
+            "src/test/java/com/google/protobuf/CodedOutputStreamTest.java",
+            "src/test/java/com/google/protobuf/ProtobufToStringOutputTest.java",
+            # Excluded in core_tests
+            "src/test/java/com/google/protobuf/DecodeUtf8Test.java",
+            "src/test/java/com/google/protobuf/IsValidUtf8Test.java",
+            "src/test/java/com/google/protobuf/TestUtil.java",
+            "src/test/java/com/google/protobuf/TestUtilLite.java",
+            "src/test/java/com/google/protobuf/RuntimeVersionTest.java",
+        ],
+    ),
+    test_prefix = "v25SrcJar",
+    deps = [
+        ":core",
+        ":v25_test_util_srcjar",
+        "//compatibility:v25_test_protos_srcjar",
+        "@maven//:com_google_guava_guava",
+        "@maven//:com_google_truth_truth",
+        "@maven//:junit_junit",
+        "@maven//:org_mockito_mockito_core",
+    ],
+)
+
+protobuf_java_library(
+    name = "v25_test_util_jar",
+    testonly = True,
+    srcs = [
+        "src/test/java/com/google/protobuf/TestUtil.java",
+        "src/test/java/com/google/protobuf/TestUtilLite.java",
+    ],
+    deps = [
+        ":core",
+        "//compatibility:v25_test_protos_jar",
+        "@maven//:com_google_guava_guava",
+        "@maven//:junit_junit",
+    ],
+)
+
+# Tests binary compatibility against v25 gencode ja compiled against v25 runtime
+junit_tests(
+    name = "v25_core_tests_jar",
+    size = "small",
+    srcs = glob(
+        ["src/test/java/**/*.java"],
+        exclude = [
+            # Depends on test protos or API changes added in v4.x.x (e.g. editions)
+            "src/test/java/com/google/protobuf/TextFormatTest.java",
+            "src/test/java/com/google/protobuf/DescriptorsTest.java",
+            "src/test/java/com/google/protobuf/DebugFormatTest.java",
+            "src/test/java/com/google/protobuf/CodedOutputStreamTest.java",
+            "src/test/java/com/google/protobuf/ProtobufToStringOutputTest.java",
+            # Excluded in core_tests
+            "src/test/java/com/google/protobuf/DecodeUtf8Test.java",
+            "src/test/java/com/google/protobuf/IsValidUtf8Test.java",
+            "src/test/java/com/google/protobuf/TestUtil.java",
+            "src/test/java/com/google/protobuf/TestUtilLite.java",
+            "src/test/java/com/google/protobuf/RuntimeVersionTest.java",
+        ],
+    ),
+    test_prefix = "v25Jar",
+    deps = [
+        ":core",
+        ":v25_test_util_jar",
+        "//compatibility:v25_test_protos_jar",
+        "@maven//:com_google_guava_guava",
+        "@maven//:com_google_truth_truth",
+        "@maven//:junit_junit",
+        "@maven//:org_mockito_mockito_core",
+    ],
+)
+
 pkg_files(
     name = "dist_files",
     srcs = glob([

java/core/src/main/java/com/google/protobuf/GeneratedMessage.java

@@ -886,16 +886,16 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
     Message getDefaultInstanceForType();
 
     /** Check if a singular extension is present. */
-    <T> boolean hasExtension(ExtensionLite<MessageT, T> extension);
+    <T> boolean hasExtension(ExtensionLite<? extends MessageT, T> extension);
 
     /** Get the number of elements in a repeated extension. */
-    <T> int getExtensionCount(ExtensionLite<MessageT, List<T>> extension);
+    <T> int getExtensionCount(ExtensionLite<? extends MessageT, List<T>> extension);
 
     /** Get the value of an extension. */
-    <T> T getExtension(ExtensionLite<MessageT, T> extension);
+    <T> T getExtension(ExtensionLite<? extends MessageT, T> extension);
 
     /** Get one element of a repeated extension. */
-    <T> T getExtension(ExtensionLite<MessageT, List<T>> extension, int index);
+    <T> T getExtension(ExtensionLite<? extends MessageT, List<T>> extension, int index);
   }
 
   /**
@@ -946,7 +946,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
       this.extensions = builder.buildExtensions();
     }
 
-    private void verifyExtensionContainingType(final Extension<MessageT, ?> extension) {
+    private void verifyExtensionContainingType(final Extension<? extends MessageT, ?> extension) {
       if (extension.getDescriptor().getContainingType() != getDescriptorForType()) {
         // This can only happen if someone uses unchecked operations.
         throw new IllegalArgumentException(
@@ -960,7 +960,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Check if a singular extension is present. */
     @Override
-    public final <T> boolean hasExtension(final ExtensionLite<MessageT, T> extensionLite) {
+    public final <T> boolean hasExtension(
+        final ExtensionLite<? extends MessageT, T> extensionLite) {
       Extension<MessageT, T> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -969,7 +970,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Get the number of elements in a repeated extension. */
     @Override
-    public final <T> int getExtensionCount(final ExtensionLite<MessageT, List<T>> extensionLite) {
+    public final <T> int getExtensionCount(
+        final ExtensionLite<? extends MessageT, List<T>> extensionLite) {
       Extension<MessageT, List<T>> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -980,7 +982,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
     /** Get the value of an extension. */
     @Override
     @SuppressWarnings("unchecked")
-    public final <T> T getExtension(final ExtensionLite<MessageT, T> extensionLite) {
+    public final <T> T getExtension(final ExtensionLite<? extends MessageT, T> extensionLite) {
       Extension<MessageT, T> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1003,7 +1005,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
     @Override
     @SuppressWarnings("unchecked")
     public final <T> T getExtension(
-        final ExtensionLite<MessageT, List<T>> extensionLite, final int index) {
+        final ExtensionLite<? extends MessageT, List<T>> extensionLite, final int index) {
       Extension<MessageT, List<T>> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1036,7 +1038,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
       private Map.Entry<FieldDescriptor, Object> next;
       private final boolean messageSetWireFormat;
 
-      private ExtensionWriter(final boolean messageSetWireFormat) {
+      // TODO: Should be marked private in v5.x.x once GeneratedMessageV3 is removed.
+      protected ExtensionWriter(final boolean messageSetWireFormat) {
         if (iter.hasNext()) {
           next = iter.next();
         }
@@ -1254,7 +1257,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Check if a singular extension is present. */
     @Override
-    public final <T> boolean hasExtension(final ExtensionLite<MessageT, T> extensionLite) {
+    public final <T> boolean hasExtension(
+        final ExtensionLite<? extends MessageT, T> extensionLite) {
       Extension<MessageT, T> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1263,7 +1267,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Get the number of elements in a repeated extension. */
     @Override
-    public final <T> int getExtensionCount(final ExtensionLite<MessageT, List<T>> extensionLite) {
+    public final <T> int getExtensionCount(
+        final ExtensionLite<? extends MessageT, List<T>> extensionLite) {
       Extension<MessageT, List<T>> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1273,7 +1278,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Get the value of an extension. */
     @Override
-    public final <T> T getExtension(final ExtensionLite<MessageT, T> extensionLite) {
+    public final <T> T getExtension(final ExtensionLite<? extends MessageT, T> extensionLite) {
       Extension<MessageT, T> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1295,7 +1300,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
     /** Get one element of a repeated extension. */
     @Override
     public final <T> T getExtension(
-        final ExtensionLite<MessageT, List<T>> extensionLite, final int index) {
+        final ExtensionLite<? extends MessageT, List<T>> extensionLite, final int index) {
       Extension<MessageT, List<T>> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1309,7 +1314,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Set the value of an extension. */
     public final <T> BuilderT setExtension(
-        final ExtensionLite<MessageT, T> extensionLite, final T value) {
+        final ExtensionLite<? extends MessageT, T> extensionLite, final T value) {
       Extension<MessageT, T> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1322,7 +1327,9 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Set the value of one element of a repeated extension. */
     public final <T> BuilderT setExtension(
-        final ExtensionLite<MessageT, List<T>> extensionLite, final int index, final T value) {
+        final ExtensionLite<? extends MessageT, List<T>> extensionLite,
+        final int index,
+        final T value) {
       Extension<MessageT, List<T>> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1335,7 +1342,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
 
     /** Append a value to a repeated extension. */
     public final <T> BuilderT addExtension(
-        final ExtensionLite<MessageT, List<T>> extensionLite, final T value) {
+        final ExtensionLite<? extends MessageT, List<T>> extensionLite, final T value) {
       Extension<MessageT, List<T>> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1347,7 +1354,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
     }
 
     /** Clear an extension. */
-    public final <T> BuilderT clearExtension(final ExtensionLite<MessageT, T> extensionLite) {
+    public final <T> BuilderT clearExtension(
+        final ExtensionLite<? extends MessageT, T> extensionLite) {
       Extension<MessageT, T> extension = checkNotLite(extensionLite);
 
       verifyExtensionContainingType(extension);
@@ -1563,7 +1571,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
       }
     }
 
-    protected final void mergeExtensionFields(final ExtendableMessage<?> other) {
+    // TODO: Should be marked final in v5.x.x once GeneratedMessageV3 is removed.
+    protected void mergeExtensionFields(final ExtendableMessage<?> other) {
       if (other.extensions != null) {
         ensureExtensionsIsMutable();
         extensions.mergeFrom(other.extensions);
@@ -1939,7 +1948,8 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
    * Users should ignore this class. This class provides the implementation with access to the
    * fields of a message object using Java reflection.
    */
-  public static final class FieldAccessorTable {
+  // TODO: Should be marked final in v5.x.x once GeneratedMessageV3 is removed.
+  public static class FieldAccessorTable {
 
     /**
      * Construct a FieldAccessorTable for a particular message class. Only one FieldAccessorTable
@@ -3136,7 +3146,7 @@ public abstract class GeneratedMessage extends AbstractMessage implements Serial
    * Checks that the {@link Extension} is non-Lite and returns it as a {@link GeneratedExtension}.
    */
   private static <MessageT extends ExtendableMessage<MessageT>, T>
-      Extension<MessageT, T> checkNotLite(ExtensionLite<MessageT, T> extension) {
+      Extension<MessageT, T> checkNotLite(ExtensionLite<? extends MessageT, T> extension) {
     if (extension.isLite()) {
       throw new IllegalArgumentException("Expected non-lite extension.");
     }

java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java

@@ -7,58 +7,538 @@
 
 package com.google.protobuf;
 
+import com.google.protobuf.Descriptors.Descriptor;
+import com.google.protobuf.Descriptors.FieldDescriptor;
+import com.google.protobuf.Descriptors.OneofDescriptor;
 import java.util.List;
 
 /**
  * Stub for GeneratedMessageV3 wrapping GeneratedMessage for compatibility with older gencode.
  *
+ * <p>Extends GeneratedMessage.ExtendableMessage instead of GeneratedMessage to allow "multiple
+ * inheritance" for GeneratedMessageV3.ExtendableMessage subclass.
+ *
  * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
- *     (5.x in 2025 Q1). Users should update gencode to >= 4.26.x which uses GeneratedMessage
- *     instead of GeneratedMessageV3.
+ *     (5.x). Users should update gencode to >= 4.26.x which uses GeneratedMessage instead.
  */
 @Deprecated
-public abstract class GeneratedMessageV3 extends GeneratedMessage {
+public abstract class GeneratedMessageV3
+    extends GeneratedMessage.ExtendableMessage<GeneratedMessageV3> {
   private static final long serialVersionUID = 1L;
 
+  @Deprecated
   protected GeneratedMessageV3() {
     super();
   }
 
+  @Deprecated
   protected GeneratedMessageV3(Builder<?> builder) {
     super(builder);
   }
 
+  /* Overrides abstract GeneratedMessage.internalGetFieldAccessorTable().
+   *
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * GeneratedMessage.internalGetFieldAccessorTable() instead.
+   */
+  @Deprecated
+  @Override
+  protected FieldAccessorTable internalGetFieldAccessorTable() {
+    throw new UnsupportedOperationException("Should be overridden in gencode.");
+  }
+
+  /**
+   * Stub for GeneratedMessageV3.UnusedPrivateParameter for compatibility with older gencode.
+   *
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   *     (5.x). Users should update gencode to >= 4.26.x which uses
+   *     GeneratedMessage.UnusedPrivateParameter instead.
+   */
+  @Deprecated
+  protected static final class UnusedPrivateParameter {
+    static final UnusedPrivateParameter INSTANCE = new UnusedPrivateParameter();
+
+    private UnusedPrivateParameter() {}
+  }
+
+  /* Stub for method overridden from old generated code
+
+  * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+  *     (5.x). Users should update gencode to >= 4.26.x which overrides
+  *     GeneratedMessage.newInstance() instead.
+  */
+  @Deprecated
+  @SuppressWarnings({"unused"})
+  protected Object newInstance(UnusedPrivateParameter unused) {
+    throw new UnsupportedOperationException("This method must be overridden by the subclass.");
+  }
+
+  @Deprecated
+  protected interface BuilderParent extends AbstractMessage.BuilderParent {}
+
+  @Deprecated
+  protected abstract Message.Builder newBuilderForType(BuilderParent parent);
+
+  /* Removed from GeneratedMessage in
+   * https://github.com/protocolbuffers/protobuf/commit/787447430fc9a69c071393e85a380b664d261ab4
+   *
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which no longer uses this method.
+   */
+  @Deprecated
+  @Override
+  protected Message.Builder newBuilderForType(final AbstractMessage.BuilderParent parent) {
+    return newBuilderForType(
+        new BuilderParent() {
+          @Override
+          public void markDirty() {
+            parent.markDirty();
+          }
+        });
+  }
+
+  /**
+   * Stub for GeneratedMessageV3.Builder wrapping GeneratedMessage.Builder for compatibility with
+   * older gencode.
+   *
+   * <p>Extends GeneratedMessage.ExtendableBuilder instead of GeneratedMessage.Builder to allow
+   * "multiple inheritance" for GeneratedMessageV3.ExtendableBuilder subclass.
+   *
+   * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+   *     (5.x). Users should update gencode to >= 4.26.x which uses GeneratedMessage.Builder
+   *     instead.
+   */
+  @Deprecated
+  public abstract static class Builder<BuilderT extends Builder<BuilderT>>
+      extends GeneratedMessage.ExtendableBuilder<GeneratedMessageV3, BuilderT> {
+
+    private BuilderParentImpl meAsParent;
+
+    @Deprecated
+    protected Builder() {
+      super(null);
+    }
+
+    @Deprecated
+    protected Builder(BuilderParent builderParent) {
+      super(builderParent);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.clone() instead. */
+    @Deprecated
+    @Override
+    public BuilderT clone() {
+      return super.clone();
+    }
+
+    /* Stub for method overridden from old generated code
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.setField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT clear() {
+      return super.clear();
+    }
+
+    /* Overrides abstract GeneratedMessage.Builder.internalGetFieldAccessorTable().
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which overrides
+     * GeneratedMessage.Builder.internalGetFieldAccessorTable() instead.
+     */
+    @Deprecated
+    @Override
+    protected FieldAccessorTable internalGetFieldAccessorTable() {
+      throw new UnsupportedOperationException("Should be overridden in gencode.");
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.setField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT setField(final FieldDescriptor field, final Object value) {
+      return super.setField(field, value);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.clearField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT clearField(final FieldDescriptor field) {
+      return super.clearField(field);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.clearOneof() instead. */
+    @Deprecated
+    @Override
+    public BuilderT clearOneof(final OneofDescriptor oneof) {
+      return super.clearOneof(oneof);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.setRepeatedField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT setRepeatedField(
+        final FieldDescriptor field, final int index, final Object value) {
+      return super.setRepeatedField(field, index, value);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.addRepeatedField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT addRepeatedField(final FieldDescriptor field, final Object value) {
+      return super.addRepeatedField(field, value);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.setUnknownFields() instead. */
+    @Deprecated
+    @Override
+    public BuilderT setUnknownFields(final UnknownFieldSet unknownFields) {
+      return super.setUnknownFields(unknownFields);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.Builder.mergeUnknownFields() instead. */
+    @Deprecated
+    @Override
+    public BuilderT mergeUnknownFields(final UnknownFieldSet unknownFields) {
+      return super.mergeUnknownFields(unknownFields);
+    }
+
+    @Deprecated
+    private class BuilderParentImpl implements BuilderParent {
+      @Override
+      public void markDirty() {
+        onChanged();
+      }
+    }
+
+    /* Returns GeneratedMessageV3.Builder.BuilderParent instead of
+     * GeneratedMessage.Builder.BuilderParent.
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * GeneratedMessage.Builder.getParentForChildren() instead.
+     */
+    @Deprecated
+    @Override
+    protected BuilderParent getParentForChildren() {
+      if (meAsParent == null) {
+        meAsParent = new BuilderParentImpl();
+      }
+      return meAsParent;
+    }
+  }
+
+  /**
+   * Stub for GeneratedMessageV3.ExtendableMessageOrBuilder wrapping
+   * GeneratedMessage.ExtendableMessageOrBuilder for compatibility with older gencode.
+   *
+   * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+   *     (5.x). Users should update gencode to >= 4.26.x which uses
+   *     GeneratedMessage.ExtendableMessageOrBuilder.
+   */
+  @Deprecated
+  public interface ExtendableMessageOrBuilder<MessageT extends ExtendableMessage<MessageT>>
+      extends GeneratedMessage.ExtendableMessageOrBuilder<GeneratedMessageV3> {
+
+    /* Removed from GeneratedMessage.ExtendableMessageOrBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    <T> boolean hasExtension(GeneratedExtension<MessageT, T> extension);
+
+    /* Removed from GeneratedMessage.ExtendableMessageOrBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    <T> int getExtensionCount(GeneratedExtension<MessageT, List<T>> extension);
+
+    /* Removed from GeneratedMessage.ExtendableMessageOrBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    <T> T getExtension(GeneratedExtension<MessageT, T> extension);
+
+    /* Removed from GeneratedMessage.ExtendableMessageOrBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    <T> T getExtension(GeneratedExtension<MessageT, List<T>> extension, int index);
+  }
+
+  /**
+   * Stub for GeneratedMessageV3.ExtendableMessage wrapping GeneratedMessage.ExtendableMessage for
+   * compatibility with older gencode.
+   *
+   * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+   *     (5.x). Users should update gencode to >= 4.26.x which uses
+   *     GeneratedMessage.ExtendableMessage.
+   */
+  @Deprecated
+  public abstract static class ExtendableMessage<MessageT extends ExtendableMessage<MessageT>>
+      extends GeneratedMessageV3 // Extends GeneratedMessage.ExtendableMessage via
+      // GeneratedMessageV3
+      implements ExtendableMessageOrBuilder<MessageT> {
+
+    @Deprecated
+    protected ExtendableMessage() {
+      super();
+    }
+
+    @Deprecated
+    protected ExtendableMessage(ExtendableBuilder<MessageT, ?> builder) {
+      super(builder);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableMessage in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> boolean hasExtension(final GeneratedExtension<MessageT, T> extension) {
+      return hasExtension((ExtensionLite<MessageT, T>) extension);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableMessage in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> int getExtensionCount(final GeneratedExtension<MessageT, List<T>> extension) {
+      return getExtensionCount((ExtensionLite<MessageT, List<T>>) extension);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableMessage in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> T getExtension(final GeneratedExtension<MessageT, T> extension) {
+      return getExtension((ExtensionLite<MessageT, T>) extension);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableMessage in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> T getExtension(
+        final GeneratedExtension<MessageT, List<T>> extension, final int index) {
+      return getExtension((ExtensionLite<MessageT, List<T>>) extension, index);
+    }
+
+    /* Overrides abstract GeneratedMessage.ExtendableMessage.internalGetFieldAccessorTable().
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which overrides
+     * GeneratedMessage.ExtendableMessage.internalGetFieldAccessorTable() instead.
+     */
+    @Deprecated
+    @Override
+    protected FieldAccessorTable internalGetFieldAccessorTable() {
+      throw new UnsupportedOperationException("Should be overridden in gencode.");
+    }
+
+    /**
+     * Stub for GeneratedMessageV3.ExtendableMessage.ExtensionWriter wrapping
+     * GeneratedMessage.ExtendableMessage.ExtensionWriter for compatibility with older gencode.
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which uses
+     *     GeneratedMessage.ExtendableMessage.ExtensionWriter instead.
+     */
+    @Deprecated
+    protected class ExtensionWriter extends GeneratedMessage.ExtendableMessage.ExtensionWriter {
+      private ExtensionWriter(final boolean messageSetWireFormat) {
+        super(messageSetWireFormat);
+      }
+    }
+
+    /* Returns GeneratedMessageV3.ExtendableMessage.ExtensionWriter instead of
+     * GeneratedMessage.ExtendableMessage.ExtensionWriter.
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which uses
+     * GeneratedMessage.ExtendableMessage.newExtensionWriter() instead.
+     */
+    @Deprecated
+    @Override
+    protected ExtensionWriter newExtensionWriter() {
+      return new ExtensionWriter(false);
+    }
+
+    /* Returns GeneratedMessageV3.ExtendableMessage.ExtensionWriter instead of
+     * GeneratedMessage.ExtendableMessage.ExtensionWriter.
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which uses
+     * GeneratedMessage.ExtendableMessage.newMessageSetExtensionWriter() instead.
+     */
+    @Deprecated
+    @Override
+    protected ExtensionWriter newMessageSetExtensionWriter() {
+      return new ExtensionWriter(true);
+    }
+  }
+
   /**
    * Stub for GeneratedMessageV3.ExtendableBuilder wrapping GeneratedMessage.ExtendableBuilder for
    * compatibility with older gencode.
    *
    * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
-   *     (5.x in 2025 Q1). Users should update gencode to >= 4.26.x which uses
-   *     GeneratedMessage.ExtendableBuilder instead of GeneratedMessageV3.ExtendableBuilder.
+   *     (5.x). Users should update gencode to >= 4.26.x which uses
+   *     GeneratedMessage.ExtendableBuilder instead.
    */
   @Deprecated
   public abstract static class ExtendableBuilder<
           MessageT extends ExtendableMessage<MessageT>,
           BuilderT extends ExtendableBuilder<MessageT, BuilderT>>
-      extends GeneratedMessage.ExtendableBuilder<MessageT, BuilderT>
-      implements GeneratedMessage.ExtendableMessageOrBuilder<MessageT> {
+      extends Builder<BuilderT> // Extends GeneratedMessage.ExtendableBuilder via Builder
+      implements ExtendableMessageOrBuilder<MessageT> {
+
+    @Deprecated
     protected ExtendableBuilder() {
       super();
     }
 
+    @Deprecated
     protected ExtendableBuilder(BuilderParent parent) {
       super(parent);
     }
 
-    // Support old gencode override method removed in
-    // https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> boolean hasExtension(final GeneratedExtension<MessageT, T> extension) {
+      return hasExtension((ExtensionLite<MessageT, T>) extension);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> int getExtensionCount(final GeneratedExtension<MessageT, List<T>> extension) {
+      return getExtensionCount((ExtensionLite<MessageT, List<T>>) extension);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> T getExtension(final GeneratedExtension<MessageT, T> extension) {
+      return getExtension((ExtensionLite<MessageT, T>) extension);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
+    @Override
+    public final <T> T getExtension(
+        final GeneratedExtension<MessageT, List<T>> extension, final int index) {
+      return getExtension((ExtensionLite<MessageT, List<T>>) extension, index);
+    }
+
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
     public <T> BuilderT setExtension(
         final GeneratedMessage.GeneratedExtension<MessageT, T> extension, final T value) {
       return setExtension((ExtensionLite<MessageT, T>) extension, value);
     }
 
-    // Support old gencode override method removed in
-    // https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
     public <T> BuilderT setExtension(
         final GeneratedMessage.GeneratedExtension<MessageT, List<T>> extension,
         final int index,
@@ -66,18 +546,141 @@ public abstract class GeneratedMessageV3 extends GeneratedMessage {
       return setExtension((ExtensionLite<MessageT, List<T>>) extension, index, value);
     }
 
-    // Support old gencode override method removed in
-    // https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
     public <T> BuilderT addExtension(
         final GeneratedMessage.GeneratedExtension<MessageT, List<T>> extension, final T value) {
       return addExtension((ExtensionLite<MessageT, List<T>>) extension, value);
     }
 
-    // Support old gencode override method removed in
-    // https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+    /* Removed from GeneratedMessage.ExtendableBuilder in
+     * https://github.com/protocolbuffers/protobuf/commit/94a2a448518403341b8aa71335ab1123fbdcccd8
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which no longer overrides this method.
+     */
+    @Deprecated
     public <T> BuilderT clearExtension(
         final GeneratedMessage.GeneratedExtension<MessageT, T> extension) {
       return clearExtension((ExtensionLite<MessageT, T>) extension);
     }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.ExtendableBuilder.setField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT setField(final FieldDescriptor field, final Object value) {
+      return super.setField(field, value);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.ExtendableBuilder.clearField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT clearField(final FieldDescriptor field) {
+      return super.clearField(field);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.ExtendableBuilder.clearOneof() instead. */
+    @Deprecated
+    @Override
+    public BuilderT clearOneof(final OneofDescriptor oneof) {
+      return super.clearOneof(oneof);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.ExtendableBuilder.setRepeatedField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT setRepeatedField(
+        final FieldDescriptor field, final int index, final Object value) {
+      return super.setRepeatedField(field, index, value);
+    }
+
+    /* Stub for method overridden from old generated code removed in
+     * https://github.com/protocolbuffers/protobuf/commit/7bff169d32710b143951ec6ce2c4ea9a56e2ad24
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which overrides
+     *     GeneratedMessage.ExtendableBuilder.addRepeatedField() instead. */
+    @Deprecated
+    @Override
+    public BuilderT addRepeatedField(final FieldDescriptor field, final Object value) {
+      return super.addRepeatedField(field, value);
+    }
+
+    /* Stub for method called from old generated code.
+     * @Override not allowed despite inheriting from
+     * GeneratedMessage.ExtendableBuilder.mergeExtensionFields().
+     *
+     * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+     *     (5.x). Users should update gencode to >= 4.26.x which uses
+     *     GeneratedMessage.ExtendableBuilder.mergeExtensionFields() instead. */
+    @Deprecated
+    protected final void mergeExtensionFields(final ExtendableMessage<?> other) {
+      super.mergeExtensionFields(other);
+    }
+  }
+
+  /**
+   * Stub for GeneratedMessageV3.FieldAccessorTable wrapping GeneratedMessage.FieldAccessorTable for
+   * compatibility with older gencode.
+   *
+   * @deprecated This class is deprecated, and slated for removal in the next Java breaking change
+   *     (5.x). Users should update gencode to >= 4.26.x which uses
+   *     GeneratedMessage.FieldAccessorTable instead.
+   */
+  @Deprecated
+  public static final class FieldAccessorTable extends GeneratedMessage.FieldAccessorTable {
+
+    @Deprecated
+    public FieldAccessorTable(
+        final Descriptor descriptor,
+        final String[] camelCaseNames,
+        final Class<? extends GeneratedMessageV3> messageClass,
+        final Class<? extends Builder<?>> builderClass) {
+      super(descriptor, camelCaseNames, messageClass, builderClass);
+    }
+
+    @Deprecated
+    public FieldAccessorTable(final Descriptor descriptor, final String[] camelCaseNames) {
+      super(descriptor, camelCaseNames);
+    }
+
+    /* Returns GeneratedMessageV3.FieldAccessorTable instead of GeneratedMessage.FieldAccessorTable.
+     *
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking
+     * change (5.x). Users should update gencode to >= 4.26.x which uses
+     * GeneratedMessage.ensureFieldAccessorsInitialized() instead.
+     */
+    @Deprecated
+    @Override
+    public FieldAccessorTable ensureFieldAccessorsInitialized(
+        Class<? extends GeneratedMessage> messageClass,
+        Class<? extends GeneratedMessage.Builder<?>> builderClass) {
+      return (FieldAccessorTable) super.ensureFieldAccessorsInitialized(messageClass, builderClass);
+    }
   }
 }

java/core/src/main/java/com/google/protobuf/RepeatedFieldBuilderV3.java

@@ -7,28 +7,579 @@
 
 package com.google.protobuf;
 
+import static com.google.protobuf.Internal.checkNotNull;
+
+import java.util.AbstractList;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
+import java.util.RandomAccess;
 
 /**
- * Stub for RepeatedFieldBuilderV3 wrapping RepeatedFieldBuilder for compatibility with older
- * gencode.
+ * Stub for RepeatedFieldBuilderV3 matching RepeatedFieldBuilder for compatibility with older
+ * gencode. This cannot wrap RepeatedFieldBuilder directly due to RepeatedFieldBuilder having more
+ * restrictive extends GeneratedMessage for MType and BType.
  *
  * @deprecated This class is deprecated, and slated for removal in the next breaking change. Users
  *     should update gencode to >= 4.26.x which replaces RepeatedFieldBuilderV3 with
  *     RepeatedFieldBuilder.
  */
-@Deprecated
 public class RepeatedFieldBuilderV3<
-        MType extends GeneratedMessage,
-        BType extends GeneratedMessage.Builder,
+        MType extends AbstractMessage,
+        BType extends AbstractMessage.Builder,
         IType extends MessageOrBuilder>
-    extends RepeatedFieldBuilder<MType, BType, IType> {
+    implements AbstractMessage.BuilderParent {
+
+  private AbstractMessage.BuilderParent parent;
+
+  private List<MType> messages;
+
+  private boolean isMessagesListMutable;
+
+  private List<SingleFieldBuilderV3<MType, BType, IType>> builders;
+
+  private boolean isClean;
 
+  private MessageExternalList<MType, BType, IType> externalMessageList;
+
+  private BuilderExternalList<MType, BType, IType> externalBuilderList;
+
+  private MessageOrBuilderExternalList<MType, BType, IType> externalMessageOrBuilderList;
+
+  @Deprecated
   public RepeatedFieldBuilderV3(
       List<MType> messages,
       boolean isMessagesListMutable,
-      GeneratedMessage.BuilderParent parent,
+      AbstractMessage.BuilderParent parent,
       boolean isClean) {
-    super(messages, isMessagesListMutable, parent, isClean);
+    this.messages = messages;
+    this.isMessagesListMutable = isMessagesListMutable;
+    this.parent = parent;
+    this.isClean = isClean;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.dispose() instead.
+   */
+  @Deprecated
+  public void dispose() {
+    parent = null;
+  }
+
+  private void ensureMutableMessageList() {
+    if (!isMessagesListMutable) {
+      messages = new ArrayList<MType>(messages);
+      isMessagesListMutable = true;
+    }
+  }
+
+  private void ensureBuilders() {
+    if (this.builders == null) {
+      this.builders = new ArrayList<SingleFieldBuilderV3<MType, BType, IType>>(messages.size());
+      for (int i = 0; i < messages.size(); i++) {
+        builders.add(null);
+      }
+    }
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getCount() instead.
+   */
+  @Deprecated
+  public int getCount() {
+    return messages.size();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.isEmpty() instead.
+   */
+  @Deprecated
+  public boolean isEmpty() {
+    return messages.isEmpty();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getMessage() instead.
+   */
+  @Deprecated
+  public MType getMessage(int index) {
+    return getMessage(index, false);
+  }
+
+  private MType getMessage(int index, boolean forBuild) {
+    if (this.builders == null) {
+      return messages.get(index);
+    }
+
+    SingleFieldBuilderV3<MType, BType, IType> builder = builders.get(index);
+    if (builder == null) {
+      return messages.get(index);
+
+    } else {
+      return forBuild ? builder.build() : builder.getMessage();
+    }
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getBuilder() instead.
+   */
+  @Deprecated
+  public BType getBuilder(int index) {
+    ensureBuilders();
+    SingleFieldBuilderV3<MType, BType, IType> builder = builders.get(index);
+    if (builder == null) {
+      MType message = messages.get(index);
+      builder = new SingleFieldBuilderV3<MType, BType, IType>(message, this, isClean);
+      builders.set(index, builder);
+    }
+    return builder.getBuilder();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getMessageOrBuilder() instead.
+   */
+  @Deprecated
+  @SuppressWarnings("unchecked")
+  public IType getMessageOrBuilder(int index) {
+    if (this.builders == null) {
+      return (IType) messages.get(index);
+    }
+
+    SingleFieldBuilderV3<MType, BType, IType> builder = builders.get(index);
+    if (builder == null) {
+      return (IType) messages.get(index);
+
+    } else {
+      return builder.getMessageOrBuilder();
+    }
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.setMessage() instead.
+   */
+  @Deprecated
+  @CanIgnoreReturnValue
+  public RepeatedFieldBuilderV3<MType, BType, IType> setMessage(int index, MType message) {
+    checkNotNull(message);
+    ensureMutableMessageList();
+    messages.set(index, message);
+    if (builders != null) {
+      SingleFieldBuilderV3<MType, BType, IType> entry = builders.set(index, null);
+      if (entry != null) {
+        entry.dispose();
+      }
+    }
+    onChanged();
+    incrementModCounts();
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.addMessage() instead.
+   */
+  @Deprecated
+  @CanIgnoreReturnValue
+  public RepeatedFieldBuilderV3<MType, BType, IType> addMessage(MType message) {
+    checkNotNull(message);
+    ensureMutableMessageList();
+    messages.add(message);
+    if (builders != null) {
+      builders.add(null);
+    }
+    onChanged();
+    incrementModCounts();
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.addMessage() instead.
+   */
+  @Deprecated
+  @CanIgnoreReturnValue
+  public RepeatedFieldBuilderV3<MType, BType, IType> addMessage(int index, MType message) {
+    checkNotNull(message);
+    ensureMutableMessageList();
+    messages.add(index, message);
+    if (builders != null) {
+      builders.add(index, null);
+    }
+    onChanged();
+    incrementModCounts();
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.addAllMessages() instead.
+   */
+  @Deprecated
+  @CanIgnoreReturnValue
+  public RepeatedFieldBuilderV3<MType, BType, IType> addAllMessages(
+      Iterable<? extends MType> values) {
+    for (final MType value : values) {
+      checkNotNull(value);
+    }
+
+    int size = -1;
+    if (values instanceof Collection) {
+      final Collection<?> collection = (Collection<?>) values;
+      if (collection.isEmpty()) {
+        return this;
+      }
+      size = collection.size();
+    }
+    ensureMutableMessageList();
+
+    if (size >= 0 && messages instanceof ArrayList) {
+      ((ArrayList<MType>) messages).ensureCapacity(messages.size() + size);
+    }
+
+    for (MType value : values) {
+      addMessage(value);
+    }
+
+    onChanged();
+    incrementModCounts();
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.addBuilder() instead.
+   */
+  @Deprecated
+  public BType addBuilder(MType message) {
+    ensureMutableMessageList();
+    ensureBuilders();
+    SingleFieldBuilderV3<MType, BType, IType> builder =
+        new SingleFieldBuilderV3<MType, BType, IType>(message, this, isClean);
+    messages.add(null);
+    builders.add(builder);
+    onChanged();
+    incrementModCounts();
+    return builder.getBuilder();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.addBuilder() instead.
+   */
+  @Deprecated
+  public BType addBuilder(int index, MType message) {
+    ensureMutableMessageList();
+    ensureBuilders();
+    SingleFieldBuilderV3<MType, BType, IType> builder =
+        new SingleFieldBuilderV3<MType, BType, IType>(message, this, isClean);
+    messages.add(index, null);
+    builders.add(index, builder);
+    onChanged();
+    incrementModCounts();
+    return builder.getBuilder();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.remove() instead.
+   */
+  @Deprecated
+  public void remove(int index) {
+    ensureMutableMessageList();
+    messages.remove(index);
+    if (builders != null) {
+      SingleFieldBuilderV3<MType, BType, IType> entry = builders.remove(index);
+      if (entry != null) {
+        entry.dispose();
+      }
+    }
+    onChanged();
+    incrementModCounts();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.clear() instead.
+   */
+  @Deprecated
+  public void clear() {
+    messages = Collections.emptyList();
+    isMessagesListMutable = false;
+    if (builders != null) {
+      for (SingleFieldBuilderV3<MType, BType, IType> entry : builders) {
+        if (entry != null) {
+          entry.dispose();
+        }
+      }
+      builders = null;
+    }
+    onChanged();
+    incrementModCounts();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.build() instead.
+   */
+  @Deprecated
+  public List<MType> build() {
+    isClean = true;
+
+    if (!isMessagesListMutable && builders == null) {
+      return messages;
+    }
+
+    boolean allMessagesInSync = true;
+    if (!isMessagesListMutable) {
+      for (int i = 0; i < messages.size(); i++) {
+        Message message = messages.get(i);
+        SingleFieldBuilderV3<MType, BType, IType> builder = builders.get(i);
+        if (builder != null) {
+          if (builder.build() != message) {
+            allMessagesInSync = false;
+            break;
+          }
+        }
+      }
+      if (allMessagesInSync) {
+        return messages;
+      }
+    }
+    ensureMutableMessageList();
+    for (int i = 0; i < messages.size(); i++) {
+      messages.set(i, getMessage(i, true));
+    }
+    messages = Collections.unmodifiableList(messages);
+    isMessagesListMutable = false;
+    return messages;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getMessageList() instead.
+   */
+  @Deprecated
+  public List<MType> getMessageList() {
+    if (externalMessageList == null) {
+      externalMessageList = new MessageExternalList<MType, BType, IType>(this);
+    }
+    return externalMessageList;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getBuilderList() instead.
+   */
+  @Deprecated
+  public List<BType> getBuilderList() {
+    if (externalBuilderList == null) {
+      externalBuilderList = new BuilderExternalList<MType, BType, IType>(this);
+    }
+    return externalBuilderList;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.getMessageOrBuilderList() instead.
+   */
+  @Deprecated
+  public List<IType> getMessageOrBuilderList() {
+    if (externalMessageOrBuilderList == null) {
+      externalMessageOrBuilderList = new MessageOrBuilderExternalList<MType, BType, IType>(this);
+    }
+    return externalMessageOrBuilderList;
+  }
+
+  private void onChanged() {
+    if (isClean && parent != null) {
+      parent.markDirty();
+      isClean = false;
+    }
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * RepeatedFieldBuilder.markDirty() instead.
+   */
+  @Deprecated
+  @Override
+  public void markDirty() {
+    onChanged();
+  }
+
+  private void incrementModCounts() {
+    if (externalMessageList != null) {
+      externalMessageList.incrementModCount();
+    }
+    if (externalBuilderList != null) {
+      externalBuilderList.incrementModCount();
+    }
+    if (externalMessageOrBuilderList != null) {
+      externalMessageOrBuilderList.incrementModCount();
+    }
+  }
+
+  private static class MessageExternalList<
+          MType extends AbstractMessage,
+          BType extends AbstractMessage.Builder,
+          IType extends MessageOrBuilder>
+      extends AbstractList<MType> implements List<MType>, RandomAccess {
+
+    RepeatedFieldBuilderV3<MType, BType, IType> builder;
+
+    @Deprecated
+    MessageExternalList(RepeatedFieldBuilderV3<MType, BType, IType> builder) {
+      this.builder = builder;
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.MessageExternalList.size() instead.
+     */
+    @Deprecated
+    @Override
+    public int size() {
+      return this.builder.getCount();
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.MessageExternalList.get() instead.
+     */
+    @Deprecated
+    @Override
+    public MType get(int index) {
+      return builder.getMessage(index);
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.MessageExternalList.incrementModCount() instead.
+     */
+    @Deprecated
+    void incrementModCount() {
+      modCount++;
+    }
+  }
+
+  private static class BuilderExternalList<
+          MType extends AbstractMessage,
+          BType extends AbstractMessage.Builder,
+          IType extends MessageOrBuilder>
+      extends AbstractList<BType> implements List<BType>, RandomAccess {
+
+    RepeatedFieldBuilderV3<MType, BType, IType> builder;
+
+    @Deprecated
+    BuilderExternalList(RepeatedFieldBuilderV3<MType, BType, IType> builder) {
+      this.builder = builder;
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.BuilderExternalList.size() instead.
+     */
+    @Deprecated
+    @Override
+    public int size() {
+      return this.builder.getCount();
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.BuilderExternalList.get() instead.
+     */
+    @Deprecated
+    @Override
+    public BType get(int index) {
+      return builder.getBuilder(index);
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.BuilderExternalList.incrementModCount() instead.
+     */
+    @Deprecated
+    void incrementModCount() {
+      modCount++;
+    }
+  }
+
+  private static class MessageOrBuilderExternalList<
+          MType extends AbstractMessage,
+          BType extends AbstractMessage.Builder,
+          IType extends MessageOrBuilder>
+      extends AbstractList<IType> implements List<IType>, RandomAccess {
+
+    RepeatedFieldBuilderV3<MType, BType, IType> builder;
+
+    MessageOrBuilderExternalList(RepeatedFieldBuilderV3<MType, BType, IType> builder) {
+      this.builder = builder;
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.MessageOrBuilderExternalList.size() instead.
+     */
+    @Deprecated
+    @Override
+    public int size() {
+      return this.builder.getCount();
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.MessageOrBuilderExternalList.get() instead.
+     */
+    @Deprecated
+    @Override
+    public IType get(int index) {
+      return builder.getMessageOrBuilder(index);
+    }
+
+    /*
+     * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+     * (5.x). Users should update gencode to >= 4.26.x which uses
+     * RepeatedFieldBuilder.MessageOrBuilderExternalList.incrementModCount() instead.
+     */
+    @Deprecated
+    void incrementModCount() {
+      modCount++;
+    }
   }
 }

java/core/src/main/java/com/google/protobuf/SingleFieldBuilderV3.java

@@ -7,8 +7,12 @@
 
 package com.google.protobuf;
 
+import static com.google.protobuf.Internal.checkNotNull;
+
 /**
- * Stub for SingleFieldBuilderV3 wrapping SingleFieldBuilder for compatibility with older gencode.
+ * Stub for SingleFieldBuilderV3 matching SingleFieldBuilder for compatibility with older gencode.
+ * This cannot wrap SingleFieldBuilder directly due to SingleFieldBuilder having more restrictive
+ * extends GeneratedMessage for MType and BType.
  *
  * @deprecated This class is deprecated, and slated for removal in the next breaking change. Users
  *     should update gencode to >= 4.26.x which replaces SingleFieldBuilderV3 with
@@ -16,13 +20,175 @@ package com.google.protobuf;
  */
 @Deprecated
 public class SingleFieldBuilderV3<
-        MType extends GeneratedMessage,
-        BType extends GeneratedMessage.Builder,
+        MType extends AbstractMessage,
+        BType extends AbstractMessage.Builder,
         IType extends MessageOrBuilder>
-    extends SingleFieldBuilder<MType, BType, IType> {
+    implements AbstractMessage.BuilderParent {
+
+  private AbstractMessage.BuilderParent parent;
+
+  private BType builder;
+
+  private MType message;
 
+  private boolean isClean;
+
+  @Deprecated
   public SingleFieldBuilderV3(
-      MType message, GeneratedMessage.BuilderParent parent, boolean isClean) {
-    super(message, parent, isClean);
+      MType message, AbstractMessage.BuilderParent parent, boolean isClean) {
+    this.message = checkNotNull(message);
+    this.parent = parent;
+    this.isClean = isClean;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.dispose() instead.
+   */
+  @Deprecated
+  public void dispose() {
+    parent = null;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.getMessage() instead.
+   */
+  @Deprecated
+  @SuppressWarnings("unchecked")
+  public MType getMessage() {
+    if (message == null) {
+      message = (MType) builder.buildPartial();
+    }
+    return message;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.build() instead.
+   */
+  @Deprecated
+  public MType build() {
+    isClean = true;
+    return getMessage();
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.getbuilder() instead.
+   */
+  @Deprecated
+  @SuppressWarnings("unchecked")
+  public BType getBuilder() {
+    if (builder == null) {
+      builder = (BType) message.newBuilderForType(this);
+      builder.mergeFrom(message);
+      builder.markClean();
+    }
+    return builder;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.getMessageOrBuilder() instead.
+   */
+  @Deprecated
+  @SuppressWarnings("unchecked")
+  public IType getMessageOrBuilder() {
+    if (builder != null) {
+      return (IType) builder;
+    } else {
+      return (IType) message;
+    }
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.setMessage() instead.
+   */
+  @Deprecated
+  @CanIgnoreReturnValue
+  public SingleFieldBuilderV3<MType, BType, IType> setMessage(MType message) {
+    this.message = checkNotNull(message);
+    if (builder != null) {
+      builder.dispose();
+      builder = null;
+    }
+    onChanged();
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.mergeFrom() instead.
+   */
+  @Deprecated
+  @CanIgnoreReturnValue
+  public SingleFieldBuilderV3<MType, BType, IType> mergeFrom(MType value) {
+    if (builder == null && message == message.getDefaultInstanceForType()) {
+      message = value;
+    } else {
+      getBuilder().mergeFrom(value);
+    }
+    onChanged();
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.clear() instead.
+   */
+  @Deprecated
+  @SuppressWarnings("unchecked")
+  @CanIgnoreReturnValue
+  public SingleFieldBuilderV3<MType, BType, IType> clear() {
+    message =
+        (MType)
+            (message != null
+                ? message.getDefaultInstanceForType()
+                : builder.getDefaultInstanceForType());
+    if (builder != null) {
+      builder.dispose();
+      builder = null;
+    }
+    onChanged();
+    isClean = true;
+    return this;
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.onChanged() instead.
+   */
+  @Deprecated
+  private void onChanged() {
+    if (builder != null) {
+      message = null;
+    }
+    if (isClean && parent != null) {
+      parent.markDirty();
+
+      isClean = false;
+    }
+  }
+
+  /*
+   * @deprecated This method is deprecated, and slated for removal in the next Java breaking change
+   * (5.x). Users should update gencode to >= 4.26.x which uses
+   * SingleFieldBuilder.markDirty() instead.
+   */
+  @Deprecated
+  @Override
+  public void markDirty() {
+    onChanged();
   }
 }

java/internal/testing.bzl

@@ -45,6 +45,7 @@ def junit_tests(name, srcs, data = [], deps = [], package_name = "com.google.pro
         deps = deps,
         resources = data,
         data = data,
+        testonly = True,
     )
     test_names = []
     prefix = name.replace("-", "_") + "TestSuite"