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diff --git a/include/google/protobuf/implicit_weak_message.h b/include/google/protobuf/implicit_weak_message.h
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+++ b/include/google/protobuf/implicit_weak_message.h
@@ -0,0 +1,213 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef GOOGLE_PROTOBUF_IMPLICIT_WEAK_MESSAGE_H__
+#define GOOGLE_PROTOBUF_IMPLICIT_WEAK_MESSAGE_H__
+
+#include <string>
+
+#include <google/protobuf/io/coded_stream.h>
+#include <google/protobuf/arena.h>
+#include <google/protobuf/message_lite.h>
+#include <google/protobuf/repeated_field.h>
+
+#ifdef SWIG
+#error "You cannot SWIG proto headers"
+#endif
+
+// Must be included last.
+#include <google/protobuf/port_def.inc>
+
+// This file is logically internal-only and should only be used by protobuf
+// generated code.
+
+namespace google {
+namespace protobuf {
+namespace internal {
+
+// An implementation of MessageLite that treats all data as unknown. This type
+// acts as a placeholder for an implicit weak field in the case where the true
+// message type does not get linked into the binary.
+class PROTOBUF_EXPORT ImplicitWeakMessage : public MessageLite {
+ public:
+  ImplicitWeakMessage() : data_(new std::string) {}
+  explicit constexpr ImplicitWeakMessage(ConstantInitialized)
+      : data_(nullptr) {}
+  explicit ImplicitWeakMessage(Arena* arena)
+      : MessageLite(arena), data_(new std::string) {}
+
+  ~ImplicitWeakMessage() override {
+    // data_ will be null in the default instance, but we can safely call delete
+    // here because the default instance will never be destroyed.
+    delete data_;
+  }
+
+  static const ImplicitWeakMessage* default_instance();
+
+  std::string GetTypeName() const override { return ""; }
+
+  MessageLite* New(Arena* arena) const override {
+    return Arena::CreateMessage<ImplicitWeakMessage>(arena);
+  }
+
+  void Clear() override { data_->clear(); }
+
+  bool IsInitialized() const override { return true; }
+
+  void CheckTypeAndMergeFrom(const MessageLite& other) override {
+    const std::string* other_data =
+        static_cast<const ImplicitWeakMessage&>(other).data_;
+    if (other_data != nullptr) {
+      data_->append(*other_data);
+    }
+  }
+
+  const char* _InternalParse(const char* ptr, ParseContext* ctx) final;
+
+  size_t ByteSizeLong() const override {
+    return data_ == nullptr ? 0 : data_->size();
+  }
+
+  uint8_t* _InternalSerialize(uint8_t* target,
+                              io::EpsCopyOutputStream* stream) const final {
+    if (data_ == nullptr) {
+      return target;
+    }
+    return stream->WriteRaw(data_->data(), static_cast<int>(data_->size()),
+                            target);
+  }
+
+  int GetCachedSize() const override {
+    return data_ == nullptr ? 0 : static_cast<int>(data_->size());
+  }
+
+  typedef void InternalArenaConstructable_;
+
+ private:
+  // This std::string is allocated on the heap, but we use a raw pointer so that
+  // the default instance can be constant-initialized. In the const methods, we
+  // have to handle the possibility of data_ being null.
+  std::string* data_;
+  GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(ImplicitWeakMessage);
+};
+
+struct ImplicitWeakMessageDefaultType;
+extern ImplicitWeakMessageDefaultType implicit_weak_message_default_instance;
+
+// A type handler for use with implicit weak repeated message fields.
+template <typename ImplicitWeakType>
+class ImplicitWeakTypeHandler {
+ public:
+  typedef MessageLite Type;
+  static constexpr bool Moveable = false;
+
+  static inline MessageLite* NewFromPrototype(const MessageLite* prototype,
+                                              Arena* arena = nullptr) {
+    return prototype->New(arena);
+  }
+
+  static inline void Delete(MessageLite* value, Arena* arena) {
+    if (arena == nullptr) {
+      delete value;
+    }
+  }
+  static inline Arena* GetArena(MessageLite* value) {
+    return value->GetArena();
+  }
+  static inline void Clear(MessageLite* value) { value->Clear(); }
+  static void Merge(const MessageLite& from, MessageLite* to) {
+    to->CheckTypeAndMergeFrom(from);
+  }
+};
+
+}  // namespace internal
+
+template <typename T>
+struct WeakRepeatedPtrField {
+  using TypeHandler = internal::ImplicitWeakTypeHandler<T>;
+  constexpr WeakRepeatedPtrField() : weak() {}
+  explicit WeakRepeatedPtrField(Arena* arena) : weak(arena) {}
+  ~WeakRepeatedPtrField() { weak.template Destroy<TypeHandler>(); }
+
+  typedef internal::RepeatedPtrIterator<MessageLite> iterator;
+  typedef internal::RepeatedPtrIterator<const MessageLite> const_iterator;
+  typedef internal::RepeatedPtrOverPtrsIterator<MessageLite*, void*>
+      pointer_iterator;
+  typedef internal::RepeatedPtrOverPtrsIterator<const MessageLite* const,
+                                                const void* const>
+      const_pointer_iterator;
+
+  iterator begin() { return iterator(base().raw_data()); }
+  const_iterator begin() const { return iterator(base().raw_data()); }
+  const_iterator cbegin() const { return begin(); }
+  iterator end() { return begin() + base().size(); }
+  const_iterator end() const { return begin() + base().size(); }
+  const_iterator cend() const { return end(); }
+  pointer_iterator pointer_begin() {
+    return pointer_iterator(base().raw_mutable_data());
+  }
+  const_pointer_iterator pointer_begin() const {
+    return const_pointer_iterator(base().raw_mutable_data());
+  }
+  pointer_iterator pointer_end() {
+    return pointer_iterator(base().raw_mutable_data() + base().size());
+  }
+  const_pointer_iterator pointer_end() const {
+    return const_pointer_iterator(base().raw_mutable_data() + base().size());
+  }
+
+  MessageLite* AddWeak(const MessageLite* prototype) {
+    return base().AddWeak(prototype);
+  }
+  T* Add() { return weak.Add(); }
+  void Clear() { base().template Clear<TypeHandler>(); }
+  void MergeFrom(const WeakRepeatedPtrField& other) {
+    base().template MergeFrom<TypeHandler>(other.base());
+  }
+  void InternalSwap(WeakRepeatedPtrField* other) {
+    base().InternalSwap(&other->base());
+  }
+
+  const internal::RepeatedPtrFieldBase& base() const { return weak; }
+  internal::RepeatedPtrFieldBase& base() { return weak; }
+  // Union disables running the destructor. Which would create a strong link.
+  // Instead we explicitly destroy the underlying base through the virtual
+  // destructor.
+  union {
+    RepeatedPtrField<T> weak;
+  };
+};
+
+}  // namespace protobuf
+}  // namespace google
+
+#include <google/protobuf/port_undef.inc>
+
+#endif  // GOOGLE_PROTOBUF_IMPLICIT_WEAK_MESSAGE_H__