diff --git a/src/google/protobuf/compiler/cpp/field.cc b/src/google/protobuf/compiler/cpp/field.cc
index e9ccf46bbd..d2fee868e1 100644
--- a/src/google/protobuf/compiler/cpp/field.cc
+++ b/src/google/protobuf/compiler/cpp/field.cc
@@ -263,6 +263,7 @@ void HasBitVars(const FieldDescriptor* field, const Options& opts,
                 absl::optional<uint32_t> idx, std::vector<Sub>& vars) {
   if (!idx.has_value()) {
     vars.emplace_back("set_hasbit", "");
+    vars.emplace_back("this_set_hasbit", "");
     vars.emplace_back("clear_hasbit", "");
     return;
   }
@@ -283,6 +284,9 @@ void HasBitVars(const FieldDescriptor* field, const Options& opts,
   vars.emplace_back("has_hasbit", has);
   vars.emplace_back(Sub("set_hasbit", set).WithSuffix(";"));
   vars.emplace_back(Sub("clear_hasbit", clr).WithSuffix(";"));
+
+  set = absl::StrFormat("_this->%s[%d] |= %s;", has_bits, index, mask);
+  vars.emplace_back(Sub("this_set_hasbit", set).WithSuffix(";"));
 }
 
 void InlinedStringVars(const FieldDescriptor* field, const Options& opts,
diff --git a/src/google/protobuf/compiler/cpp/field.h b/src/google/protobuf/compiler/cpp/field.h
index 1bba5f27b5..d24d816d81 100644
--- a/src/google/protobuf/compiler/cpp/field.h
+++ b/src/google/protobuf/compiler/cpp/field.h
@@ -39,6 +39,11 @@ namespace cpp {
 // matter of clean composability.
 class FieldGeneratorBase {
  public:
+  // `GeneratorFunction` defines a subset of generator functions that may have
+  // additional optimizations or requirements such as 'uses a local `arena`
+  // variable instead of calling GetArena()'
+  enum class GeneratorFunction { kMergeFrom };
+
   FieldGeneratorBase(const FieldDescriptor* descriptor, const Options& options,
                      MessageSCCAnalyzer* scc_analyzer);
 
@@ -100,6 +105,10 @@ class FieldGeneratorBase {
     return has_default_constexpr_constructor_;
   }
 
+  // Returns true if this generator requires an 'arena' parameter on the
+  // given generator function.
+  virtual bool RequiresArena(GeneratorFunction) const { return false; }
+
   virtual std::vector<io::Printer::Sub> MakeVars() const { return {}; }
 
   virtual void GeneratePrivateMembers(io::Printer* p) const = 0;
@@ -230,6 +239,8 @@ class FieldGenerator {
   }
 
  public:
+  using GeneratorFunction = FieldGeneratorBase::GeneratorFunction;
+
   FieldGenerator(const FieldGenerator&) = delete;
   FieldGenerator& operator=(const FieldGenerator&) = delete;
   FieldGenerator(FieldGenerator&&) = default;
@@ -256,6 +267,11 @@ class FieldGenerator {
     return impl_->has_default_constexpr_constructor();
   }
 
+  // Requirements: see FieldGeneratorBase for documentation
+  bool RequiresArena(GeneratorFunction function) const {
+    return impl_->RequiresArena(function);
+  }
+
   // Prints private members needed to represent this field.
   //
   // These are placed inside the class definition.
diff --git a/src/google/protobuf/compiler/cpp/field_generators/message_field.cc b/src/google/protobuf/compiler/cpp/field_generators/message_field.cc
index 34c7733eac..83d2a21637 100644
--- a/src/google/protobuf/compiler/cpp/field_generators/message_field.cc
+++ b/src/google/protobuf/compiler/cpp/field_generators/message_field.cc
@@ -102,6 +102,8 @@ class SingularMessage : public FieldGeneratorBase {
     )cc");
   }
 
+  bool RequiresArena(GeneratorFunction function) const override;
+
   void GenerateNonInlineAccessorDefinitions(io::Printer* p) const override {}
 
   void GenerateAccessorDeclarations(io::Printer* p) const override;
@@ -415,15 +417,38 @@ void SingularMessage::GenerateMessageClearingCode(io::Printer* p) const {
   }
 }
 
+bool SingularMessage::RequiresArena(GeneratorFunction function) const {
+  switch (function) {
+    case GeneratorFunction::kMergeFrom:
+      return !(is_weak() || is_oneof() || should_split());
+  }
+  return false;
+}
+
 void SingularMessage::GenerateMergingCode(io::Printer* p) const {
   if (is_weak()) {
     p->Emit(
         "_Internal::mutable_$name$(_this)->CheckTypeAndMergeFrom(\n"
         "    _Internal::$name$(&from));\n");
-  } else {
+  } else if (is_oneof() || should_split()) {
     p->Emit(
         "_this->_internal_mutable_$name$()->$Submsg$::MergeFrom(\n"
         "    from._internal_$name$());\n");
+  } else {
+    // Important: we set `hasbits` after we copied the field. There are cases
+    // where people assign root values to child values or vice versa which
+    // are not always checked, so we delay this change becoming 'visibile'
+    // until after we copied the message.
+    // TODO enforces this as undefined behavior in debug builds.
+    p->Emit(R"cc(
+      $DCHK$(from.$field_$ != nullptr);
+      if (_this->$field_$ == nullptr) {
+        _this->$field_$ = CreateMaybeMessage<$Submsg$>(arena, *from.$field_$);
+      } else {
+        _this->$field_$->MergeFrom(*from.$field_$);
+      }
+      $this_set_hasbit$;
+    )cc");
   }
 }
 
diff --git a/src/google/protobuf/compiler/cpp/message.cc b/src/google/protobuf/compiler/cpp/message.cc
index 59d7c1e8f9..8dfff78caa 100644
--- a/src/google/protobuf/compiler/cpp/message.cc
+++ b/src/google/protobuf/compiler/cpp/message.cc
@@ -3488,6 +3488,15 @@ void MessageGenerator::GenerateMergeFrom(io::Printer* p) {
   }
 }
 
+bool MessageGenerator::RequiresArena(GeneratorFunction function) const {
+  for (const FieldDescriptor* field : FieldRange(descriptor_)) {
+    if (field_generators_.get(field).RequiresArena(function)) {
+      return true;
+    }
+  }
+  return false;
+}
+
 void MessageGenerator::GenerateClassSpecificMergeImpl(io::Printer* p) {
   if (HasSimpleBaseClass(descriptor_, options_)) return;
   // Generate the class-specific MergeFrom, which avoids the ABSL_CHECK and
@@ -3507,6 +3516,11 @@ void MessageGenerator::GenerateClassSpecificMergeImpl(io::Printer* p) {
         "  auto& from = static_cast<const $classname$&>(from_msg);\n");
   }
   format.Indent();
+  if (RequiresArena(GeneratorFunction::kMergeFrom)) {
+    p->Emit(R"cc(
+      ::$proto_ns$::Arena* arena = _this->GetArena();
+    )cc");
+  }
   format(
       "$annotate_mergefrom$"
       "// @@protoc_insertion_point(class_specific_merge_from_start:"
diff --git a/src/google/protobuf/compiler/cpp/message.h b/src/google/protobuf/compiler/cpp/message.h
index 4c404adcb9..e62bce2797 100644
--- a/src/google/protobuf/compiler/cpp/message.h
+++ b/src/google/protobuf/compiler/cpp/message.h
@@ -80,6 +80,7 @@ class MessageGenerator {
   const Descriptor* descriptor() const { return descriptor_; }
 
  private:
+  using GeneratorFunction = FieldGeneratorBase::GeneratorFunction;
   enum class InitType { kConstexpr, kArena, kArenaCopy };
 
   // Generate declarations and definitions of accessors for fields.
@@ -148,6 +149,10 @@ class MessageGenerator {
   void GenerateFieldClear(const FieldDescriptor* field, bool is_inline,
                           io::Printer* p);
 
+  // Returns true if any of the fields needs an `arena` variable containing
+  // the current message's arena, reducing `GetArena()` call churn.
+  bool RequiresArena(GeneratorFunction function) const;
+
   // Returns whether impl_ has a copy ctor.
   bool ImplHasCopyCtor() const;
 
diff --git a/src/google/protobuf/compiler/plugin.pb.cc b/src/google/protobuf/compiler/plugin.pb.cc
index d16ebb9888..dc37bd6e68 100644
--- a/src/google/protobuf/compiler/plugin.pb.cc
+++ b/src/google/protobuf/compiler/plugin.pb.cc
@@ -903,6 +903,7 @@ const ::_pbi::TcParseTable<3, 5, 3, 79, 2> CodeGeneratorRequest::_table_ = {
 void CodeGeneratorRequest::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<CodeGeneratorRequest*>(&to_msg);
   auto& from = static_cast<const CodeGeneratorRequest&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.compiler.CodeGeneratorRequest)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -919,8 +920,13 @@ void CodeGeneratorRequest::MergeImpl(::google::protobuf::Message& to_msg, const
       _this->_internal_set_parameter(from._internal_parameter());
     }
     if (cached_has_bits & 0x00000002u) {
-      _this->_internal_mutable_compiler_version()->::google::protobuf::compiler::Version::MergeFrom(
-          from._internal_compiler_version());
+      ABSL_DCHECK(from._impl_.compiler_version_ != nullptr);
+      if (_this->_impl_.compiler_version_ == nullptr) {
+        _this->_impl_.compiler_version_ = CreateMaybeMessage<::google::protobuf::compiler::Version>(arena, *from._impl_.compiler_version_);
+      } else {
+        _this->_impl_.compiler_version_->MergeFrom(*from._impl_.compiler_version_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000002u;
     }
   }
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
@@ -1233,6 +1239,7 @@ const ::_pbi::TcParseTable<2, 4, 1, 86, 2> CodeGeneratorResponse_File::_table_ =
 void CodeGeneratorResponse_File::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<CodeGeneratorResponse_File*>(&to_msg);
   auto& from = static_cast<const CodeGeneratorResponse_File&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.compiler.CodeGeneratorResponse.File)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -1250,8 +1257,13 @@ void CodeGeneratorResponse_File::MergeImpl(::google::protobuf::Message& to_msg,
       _this->_internal_set_content(from._internal_content());
     }
     if (cached_has_bits & 0x00000008u) {
-      _this->_internal_mutable_generated_code_info()->::google::protobuf::GeneratedCodeInfo::MergeFrom(
-          from._internal_generated_code_info());
+      ABSL_DCHECK(from._impl_.generated_code_info_ != nullptr);
+      if (_this->_impl_.generated_code_info_ == nullptr) {
+        _this->_impl_.generated_code_info_ = CreateMaybeMessage<::google::protobuf::GeneratedCodeInfo>(arena, *from._impl_.generated_code_info_);
+      } else {
+        _this->_impl_.generated_code_info_->MergeFrom(*from._impl_.generated_code_info_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000008u;
     }
   }
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
diff --git a/src/google/protobuf/descriptor.pb.cc b/src/google/protobuf/descriptor.pb.cc
index e690ce6cad..a9006879bf 100644
--- a/src/google/protobuf/descriptor.pb.cc
+++ b/src/google/protobuf/descriptor.pb.cc
@@ -3041,6 +3041,7 @@ constexpr ::_pbi::TcParseTable<4, 13, 7, 79, 2> FileDescriptorProto::_table_ = {
 void FileDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<FileDescriptorProto*>(&to_msg);
   auto& from = static_cast<const FileDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.FileDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -3069,12 +3070,22 @@ void FileDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const :
       _this->_internal_set_syntax(from._internal_syntax());
     }
     if (cached_has_bits & 0x00000008u) {
-      _this->_internal_mutable_options()->::google::protobuf::FileOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::FileOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000008u;
     }
     if (cached_has_bits & 0x00000010u) {
-      _this->_internal_mutable_source_code_info()->::google::protobuf::SourceCodeInfo::MergeFrom(
-          from._internal_source_code_info());
+      ABSL_DCHECK(from._impl_.source_code_info_ != nullptr);
+      if (_this->_impl_.source_code_info_ == nullptr) {
+        _this->_impl_.source_code_info_ = CreateMaybeMessage<::google::protobuf::SourceCodeInfo>(arena, *from._impl_.source_code_info_);
+      } else {
+        _this->_impl_.source_code_info_->MergeFrom(*from._impl_.source_code_info_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000010u;
     }
     if (cached_has_bits & 0x00000020u) {
       _this->_impl_.edition_ = from._impl_.edition_;
@@ -3371,6 +3382,7 @@ constexpr ::_pbi::TcParseTable<2, 3, 1, 0, 2> DescriptorProto_ExtensionRange::_t
 void DescriptorProto_ExtensionRange::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<DescriptorProto_ExtensionRange*>(&to_msg);
   auto& from = static_cast<const DescriptorProto_ExtensionRange&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.DescriptorProto.ExtensionRange)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -3379,8 +3391,13 @@ void DescriptorProto_ExtensionRange::MergeImpl(::google::protobuf::Message& to_m
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x00000007u) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_options()->::google::protobuf::ExtensionRangeOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::ExtensionRangeOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.start_ = from._impl_.start_;
@@ -4056,6 +4073,7 @@ constexpr ::_pbi::TcParseTable<4, 10, 8, 65, 2> DescriptorProto::_table_ = {
 void DescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<DescriptorProto*>(&to_msg);
   auto& from = static_cast<const DescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.DescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -4082,8 +4100,13 @@ void DescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::goo
       _this->_internal_set_name(from._internal_name());
     }
     if (cached_has_bits & 0x00000002u) {
-      _this->_internal_mutable_options()->::google::protobuf::MessageOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::MessageOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000002u;
     }
   }
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
@@ -4748,6 +4771,7 @@ constexpr ::_pbi::TcParseTable<3, 4, 4, 0, 12> ExtensionRangeOptions::_table_ =
 void ExtensionRangeOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<ExtensionRangeOptions*>(&to_msg);
   auto& from = static_cast<const ExtensionRangeOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.ExtensionRangeOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -4760,8 +4784,13 @@ void ExtensionRangeOptions::MergeImpl(::google::protobuf::Message& to_msg, const
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x00000003u) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.verification_ = from._impl_.verification_;
@@ -5279,6 +5308,7 @@ constexpr ::_pbi::TcParseTable<4, 11, 3, 96, 2> FieldDescriptorProto::_table_ =
 void FieldDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<FieldDescriptorProto*>(&to_msg);
   auto& from = static_cast<const FieldDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.FieldDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -5302,8 +5332,13 @@ void FieldDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const
       _this->_internal_set_json_name(from._internal_json_name());
     }
     if (cached_has_bits & 0x00000020u) {
-      _this->_internal_mutable_options()->::google::protobuf::FieldOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::FieldOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000020u;
     }
     if (cached_has_bits & 0x00000040u) {
       _this->_impl_.number_ = from._impl_.number_;
@@ -5574,6 +5609,7 @@ constexpr ::_pbi::TcParseTable<1, 2, 1, 49, 2> OneofDescriptorProto::_table_ = {
 void OneofDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<OneofDescriptorProto*>(&to_msg);
   auto& from = static_cast<const OneofDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.OneofDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -5585,8 +5621,13 @@ void OneofDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const
       _this->_internal_set_name(from._internal_name());
     }
     if (cached_has_bits & 0x00000002u) {
-      _this->_internal_mutable_options()->::google::protobuf::OneofOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::OneofOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000002u;
     }
   }
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
@@ -6130,6 +6171,7 @@ constexpr ::_pbi::TcParseTable<3, 5, 3, 61, 2> EnumDescriptorProto::_table_ = {
 void EnumDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<EnumDescriptorProto*>(&to_msg);
   auto& from = static_cast<const EnumDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.EnumDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -6146,8 +6188,13 @@ void EnumDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const :
       _this->_internal_set_name(from._internal_name());
     }
     if (cached_has_bits & 0x00000002u) {
-      _this->_internal_mutable_options()->::google::protobuf::EnumOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::EnumOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000002u;
     }
   }
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
@@ -6425,6 +6472,7 @@ constexpr ::_pbi::TcParseTable<2, 3, 1, 53, 2> EnumValueDescriptorProto::_table_
 void EnumValueDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<EnumValueDescriptorProto*>(&to_msg);
   auto& from = static_cast<const EnumValueDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.EnumValueDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -6436,8 +6484,13 @@ void EnumValueDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, co
       _this->_internal_set_name(from._internal_name());
     }
     if (cached_has_bits & 0x00000002u) {
-      _this->_internal_mutable_options()->::google::protobuf::EnumValueOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::EnumValueOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000002u;
     }
     if (cached_has_bits & 0x00000004u) {
       _this->_impl_.number_ = from._impl_.number_;
@@ -6714,6 +6767,7 @@ constexpr ::_pbi::TcParseTable<2, 3, 2, 51, 2> ServiceDescriptorProto::_table_ =
 void ServiceDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<ServiceDescriptorProto*>(&to_msg);
   auto& from = static_cast<const ServiceDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.ServiceDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -6727,8 +6781,13 @@ void ServiceDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, cons
       _this->_internal_set_name(from._internal_name());
     }
     if (cached_has_bits & 0x00000002u) {
-      _this->_internal_mutable_options()->::google::protobuf::ServiceOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::ServiceOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000002u;
     }
   }
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
@@ -7093,6 +7152,7 @@ constexpr ::_pbi::TcParseTable<3, 6, 1, 71, 2> MethodDescriptorProto::_table_ =
 void MethodDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<MethodDescriptorProto*>(&to_msg);
   auto& from = static_cast<const MethodDescriptorProto&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.MethodDescriptorProto)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -7110,8 +7170,13 @@ void MethodDescriptorProto::MergeImpl(::google::protobuf::Message& to_msg, const
       _this->_internal_set_output_type(from._internal_output_type());
     }
     if (cached_has_bits & 0x00000008u) {
-      _this->_internal_mutable_options()->::google::protobuf::MethodOptions::MergeFrom(
-          from._internal_options());
+      ABSL_DCHECK(from._impl_.options_ != nullptr);
+      if (_this->_impl_.options_ == nullptr) {
+        _this->_impl_.options_ = CreateMaybeMessage<::google::protobuf::MethodOptions>(arena, *from._impl_.options_);
+      } else {
+        _this->_impl_.options_->MergeFrom(*from._impl_.options_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000008u;
     }
     if (cached_has_bits & 0x00000010u) {
       _this->_impl_.client_streaming_ = from._impl_.client_streaming_;
@@ -7920,6 +7985,7 @@ constexpr ::_pbi::TcParseTable<5, 22, 3, 202, 12> FileOptions::_table_ = {
 void FileOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<FileOptions*>(&to_msg);
   auto& from = static_cast<const FileOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.FileOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -7962,8 +8028,13 @@ void FileOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google:
       _this->_internal_set_ruby_package(from._internal_ruby_package());
     }
     if (cached_has_bits & 0x00000400u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000400u;
     }
     if (cached_has_bits & 0x00000800u) {
       _this->_impl_.java_multiple_files_ = from._impl_.java_multiple_files_;
@@ -8382,6 +8453,7 @@ constexpr ::_pbi::TcParseTable<3, 7, 2, 0, 7> MessageOptions::_table_ = {
 void MessageOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<MessageOptions*>(&to_msg);
   auto& from = static_cast<const MessageOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.MessageOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -8392,8 +8464,13 @@ void MessageOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::goog
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x0000003fu) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.message_set_wire_format_ = from._impl_.message_set_wire_format_;
@@ -9172,6 +9249,7 @@ constexpr ::_pbi::TcParseTable<4, 13, 7, 0, 7> FieldOptions::_table_ = {
 void FieldOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<FieldOptions*>(&to_msg);
   auto& from = static_cast<const FieldOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.FieldOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -9185,8 +9263,13 @@ void FieldOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x000000ffu) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.ctype_ = from._impl_.ctype_;
@@ -9473,6 +9556,7 @@ constexpr ::_pbi::TcParseTable<2, 2, 2, 0, 7> OneofOptions::_table_ = {
 void OneofOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<OneofOptions*>(&to_msg);
   auto& from = static_cast<const OneofOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.OneofOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -9481,8 +9565,13 @@ void OneofOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google
   _this->_internal_mutable_uninterpreted_option()->MergeFrom(
       from._internal_uninterpreted_option());
   if ((from._impl_._has_bits_[0] & 0x00000001u) != 0) {
-    _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-        from._internal_features());
+    ABSL_DCHECK(from._impl_.features_ != nullptr);
+    if (_this->_impl_.features_ == nullptr) {
+      _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+    } else {
+      _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+    }
+    _this->_impl_._has_bits_[0] |= 0x00000001u;
   }
   _this->_impl_._extensions_.MergeFrom(internal_default_instance(), from._impl_._extensions_);
   _this->_internal_metadata_.MergeFrom<::google::protobuf::UnknownFieldSet>(from._internal_metadata_);
@@ -9809,6 +9898,7 @@ constexpr ::_pbi::TcParseTable<3, 5, 2, 0, 7> EnumOptions::_table_ = {
 void EnumOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<EnumOptions*>(&to_msg);
   auto& from = static_cast<const EnumOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.EnumOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -9819,8 +9909,13 @@ void EnumOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google:
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x0000000fu) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.allow_alias_ = from._impl_.allow_alias_;
@@ -10145,6 +10240,7 @@ constexpr ::_pbi::TcParseTable<3, 4, 2, 0, 7> EnumValueOptions::_table_ = {
 void EnumValueOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<EnumValueOptions*>(&to_msg);
   auto& from = static_cast<const EnumValueOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.EnumValueOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -10155,8 +10251,13 @@ void EnumValueOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::go
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x00000007u) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.deprecated_ = from._impl_.deprecated_;
@@ -10447,6 +10548,7 @@ constexpr ::_pbi::TcParseTable<2, 3, 2, 0, 12> ServiceOptions::_table_ = {
 void ServiceOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<ServiceOptions*>(&to_msg);
   auto& from = static_cast<const ServiceOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.ServiceOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -10457,8 +10559,13 @@ void ServiceOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::goog
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x00000003u) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.deprecated_ = from._impl_.deprecated_;
@@ -10782,6 +10889,7 @@ constexpr ::_pbi::TcParseTable<3, 4, 3, 0, 12> MethodOptions::_table_ = {
 void MethodOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<MethodOptions*>(&to_msg);
   auto& from = static_cast<const MethodOptions&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.MethodOptions)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -10792,8 +10900,13 @@ void MethodOptions::MergeImpl(::google::protobuf::Message& to_msg, const ::googl
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x00000007u) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.deprecated_ = from._impl_.deprecated_;
@@ -12064,6 +12177,7 @@ constexpr ::_pbi::TcParseTable<1, 2, 2, 0, 2> FeatureSetDefaults_FeatureSetEditi
 void FeatureSetDefaults_FeatureSetEditionDefault::MergeImpl(::google::protobuf::Message& to_msg, const ::google::protobuf::Message& from_msg) {
   auto* const _this = static_cast<FeatureSetDefaults_FeatureSetEditionDefault*>(&to_msg);
   auto& from = static_cast<const FeatureSetDefaults_FeatureSetEditionDefault&>(from_msg);
+  ::google::protobuf::Arena* arena = _this->GetArena();
   // @@protoc_insertion_point(class_specific_merge_from_start:google.protobuf.FeatureSetDefaults.FeatureSetEditionDefault)
   ABSL_DCHECK_NE(&from, _this);
   ::uint32_t cached_has_bits = 0;
@@ -12072,8 +12186,13 @@ void FeatureSetDefaults_FeatureSetEditionDefault::MergeImpl(::google::protobuf::
   cached_has_bits = from._impl_._has_bits_[0];
   if (cached_has_bits & 0x00000003u) {
     if (cached_has_bits & 0x00000001u) {
-      _this->_internal_mutable_features()->::google::protobuf::FeatureSet::MergeFrom(
-          from._internal_features());
+      ABSL_DCHECK(from._impl_.features_ != nullptr);
+      if (_this->_impl_.features_ == nullptr) {
+        _this->_impl_.features_ = CreateMaybeMessage<::google::protobuf::FeatureSet>(arena, *from._impl_.features_);
+      } else {
+        _this->_impl_.features_->MergeFrom(*from._impl_.features_);
+      }
+      _this->_impl_._has_bits_[0] |= 0x00000001u;
     }
     if (cached_has_bits & 0x00000002u) {
       _this->_impl_.edition_ = from._impl_.edition_;