diff --git a/src/google/protobuf/compiler/cpp/file.cc b/src/google/protobuf/compiler/cpp/file.cc
index e3e0130e93..4b77480997 100644
--- a/src/google/protobuf/compiler/cpp/file.cc
+++ b/src/google/protobuf/compiler/cpp/file.cc
@@ -529,6 +529,17 @@ void FileGenerator::GenerateSourcePrelude(io::Printer* p) {
   }
 }
 
+bool FileGenerator::IsFileDescriptorProto() const {
+  if (Namespace(file_, options_) !=
+      absl::StrCat("::", ProtobufNamespace(options_))) {
+    return false;
+  }
+  for (int i = 0; i < file_->message_type_count(); ++i) {
+    if (file_->message_type(i)->name() == "FileDescriptorProto") return true;
+  }
+  return false;
+}
+
 void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
   MessageGenerator* generator = message_generators_[idx].get();
 
@@ -569,24 +580,50 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
 
   generator->GenerateConstexprConstructor(p);
 
-  p->Emit(
-      {
-          {"type", DefaultInstanceType(generator->descriptor(), options_)},
-          {"name", DefaultInstanceName(generator->descriptor(), options_)},
-          {"class", ClassName(generator->descriptor())},
-      },
-      R"cc(
-        struct $type$ {
-          PROTOBUF_CONSTEXPR $type$() : _instance(::_pbi::ConstantInitialized{}) {}
-          ~$type$() {}
-          union {
-            $class$ _instance;
+  if (IsFileDescriptorProto()) {
+    p->Emit(
+        {
+            {"type", DefaultInstanceType(generator->descriptor(), options_)},
+            {"name", DefaultInstanceName(generator->descriptor(), options_)},
+            {"class", ClassName(generator->descriptor())},
+        },
+        R"cc(
+          struct $type$ {
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+            constexpr $type$() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+            $type$() {}
+            void Init() { ::new (&_instance) $class$(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+            ~$type$() {}
+            union {
+              $class$ _instance;
+            };
           };
-        };
 
-        PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
-            PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 $type$ $name$;
-      )cc");
+          PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
+              PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 $type$ $name$;
+        )cc");
+  } else {
+    p->Emit(
+        {
+            {"type", DefaultInstanceType(generator->descriptor(), options_)},
+            {"name", DefaultInstanceName(generator->descriptor(), options_)},
+            {"class", ClassName(generator->descriptor())},
+        },
+        R"cc(
+          struct $type$ {
+            PROTOBUF_CONSTEXPR $type$() : _instance(::_pbi::ConstantInitialized{}) {}
+            ~$type$() {}
+            union {
+              $class$ _instance;
+            };
+          };
+
+          PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
+              PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 $type$ $name$;
+        )cc");
+  }
 
   for (int i = 0; i < generator->descriptor()->field_count(); ++i) {
     const FieldDescriptor* field = generator->descriptor()->field(i);
@@ -1109,15 +1146,43 @@ void FileGenerator::GenerateReflectionInitializationCode(io::Printer* p) {
   // because in some situations that would otherwise pull in a lot of
   // unnecessary code that can't be stripped by --gc-sections. Descriptor
   // initialization will still be performed lazily when it's needed.
-  if (file_->name() == "net/proto2/proto/descriptor.proto") {
-    return;
+  if (file_->name() != "net/proto2/proto/descriptor.proto") {
+    p->Emit({{"dummy", UniqueName("dynamic_init_dummy", file_, options_)}},
+            R"cc(
+              // Force running AddDescriptors() at dynamic initialization time.
+              PROTOBUF_ATTRIBUTE_INIT_PRIORITY2
+              static ::_pbi::AddDescriptorsRunner $dummy$(&$desc_table$);
+            )cc");
   }
 
-  p->Emit({{"dummy", UniqueName("dynamic_init_dummy", file_, options_)}}, R"cc(
-    // Force running AddDescriptors() at dynamic initialization time.
-    PROTOBUF_ATTRIBUTE_INIT_PRIORITY2
-    static ::_pbi::AddDescriptorsRunner $dummy$(&$desc_table$);
-  )cc");
+  // However, we must provide a way to force initialize the default instances
+  // of FileDescriptorProto which will be used during registration of other
+  // files.
+  if (IsFileDescriptorProto()) {
+    NamespaceOpener ns(p);
+    ns.ChangeTo(absl::StrCat(ProtobufNamespace(options_), "::internal"));
+    p->Emit(
+        {{"dummy", UniqueName("dynamic_init_dummy", file_, options_)},
+         {"initializers", absl::StrJoin(message_generators_, "\n",
+                                        [&](std::string* out, const auto& gen) {
+                                          absl::StrAppend(
+                                              out,
+                                              DefaultInstanceName(
+                                                  gen->descriptor(), options_),
+                                              ".Init();");
+                                        })}},
+        R"cc(
+          //~ Emit wants an indented line, so give it a comment to strip.
+#if !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+          PROTOBUF_EXPORT void InitializeFileDescriptorDefaultInstancesSlow() {
+            $initializers$;
+          }
+          PROTOBUF_ATTRIBUTE_INIT_PRIORITY1
+          static std::true_type $dummy${
+              (InitializeFileDescriptorDefaultInstances(), std::true_type{})};
+#endif  // !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+        )cc");
+  }
 }
 
 class FileGenerator::ForwardDeclarations {
@@ -1240,6 +1305,16 @@ void FileGenerator::GenerateForwardDeclarations(io::Printer* p) {
   for (const auto& decl : decls) {
     decl.second.PrintTopLevelDecl(p, options_);
   }
+
+  if (IsFileDescriptorProto()) {
+    ns.ChangeTo(absl::StrCat(ProtobufNamespace(options_), "::internal"));
+    p->Emit(R"cc(
+      //~ Emit wants an indented line, so give it a comment to strip.
+#if !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+      PROTOBUF_EXPORT void InitializeFileDescriptorDefaultInstancesSlow();
+#endif  // !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+    )cc");
+  }
 }
 
 void FileGenerator::GenerateLibraryIncludes(io::Printer* p) {
diff --git a/src/google/protobuf/compiler/cpp/file.h b/src/google/protobuf/compiler/cpp/file.h
index 6514df048f..96cecd77d4 100644
--- a/src/google/protobuf/compiler/cpp/file.h
+++ b/src/google/protobuf/compiler/cpp/file.h
@@ -168,6 +168,8 @@ class FileGenerator {
 
   void GenerateProto2NamespaceEnumSpecializations(io::Printer* p);
 
+  bool IsFileDescriptorProto() const;
+
   // Sometimes the names we use in a .proto file happen to be defined as
   // macros on some platforms (e.g., macro/minor used in plugin.proto are
   // defined as macros in sys/types.h on FreeBSD and a few other platforms).
diff --git a/src/google/protobuf/descriptor.pb.cc b/src/google/protobuf/descriptor.pb.cc
index a67c00aa7a..a3c7065995 100644
--- a/src/google/protobuf/descriptor.pb.cc
+++ b/src/google/protobuf/descriptor.pb.cc
@@ -25,7 +25,12 @@ PROTOBUF_CONSTEXPR FileDescriptorSet::FileDescriptorSet(
     /*decltype(_impl_.file_)*/{}
   , /*decltype(_impl_._cached_size_)*/{}} {}
 struct FileDescriptorSetDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR FileDescriptorSetDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr FileDescriptorSetDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  FileDescriptorSetDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) FileDescriptorSet(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~FileDescriptorSetDefaultTypeInternal() {}
   union {
     FileDescriptorSet _instance;
@@ -66,7 +71,12 @@ PROTOBUF_CONSTEXPR FileDescriptorProto::FileDescriptorProto(
   , /*decltype(_impl_.options_)*/nullptr
   , /*decltype(_impl_.source_code_info_)*/nullptr} {}
 struct FileDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR FileDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr FileDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  FileDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) FileDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~FileDescriptorProtoDefaultTypeInternal() {}
   union {
     FileDescriptorProto _instance;
@@ -85,7 +95,12 @@ PROTOBUF_CONSTEXPR DescriptorProto_ExtensionRange::DescriptorProto_ExtensionRang
   , /*decltype(_impl_.end_)*/ 0
 } {}
 struct DescriptorProto_ExtensionRangeDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR DescriptorProto_ExtensionRangeDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr DescriptorProto_ExtensionRangeDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  DescriptorProto_ExtensionRangeDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) DescriptorProto_ExtensionRange(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~DescriptorProto_ExtensionRangeDefaultTypeInternal() {}
   union {
     DescriptorProto_ExtensionRange _instance;
@@ -103,7 +118,12 @@ PROTOBUF_CONSTEXPR DescriptorProto_ReservedRange::DescriptorProto_ReservedRange(
   , /*decltype(_impl_.end_)*/ 0
 } {}
 struct DescriptorProto_ReservedRangeDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR DescriptorProto_ReservedRangeDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr DescriptorProto_ReservedRangeDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  DescriptorProto_ReservedRangeDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) DescriptorProto_ReservedRange(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~DescriptorProto_ReservedRangeDefaultTypeInternal() {}
   union {
     DescriptorProto_ReservedRange _instance;
@@ -130,7 +150,12 @@ PROTOBUF_CONSTEXPR DescriptorProto::DescriptorProto(
 
   , /*decltype(_impl_.options_)*/nullptr} {}
 struct DescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR DescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr DescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  DescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) DescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~DescriptorProtoDefaultTypeInternal() {}
   union {
     DescriptorProto _instance;
@@ -160,7 +185,12 @@ PROTOBUF_CONSTEXPR ExtensionRangeOptions_Declaration::ExtensionRangeOptions_Decl
   , /*decltype(_impl_.repeated_)*/ false
 } {}
 struct ExtensionRangeOptions_DeclarationDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR ExtensionRangeOptions_DeclarationDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr ExtensionRangeOptions_DeclarationDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  ExtensionRangeOptions_DeclarationDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) ExtensionRangeOptions_Declaration(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~ExtensionRangeOptions_DeclarationDefaultTypeInternal() {}
   union {
     ExtensionRangeOptions_Declaration _instance;
@@ -179,7 +209,12 @@ PROTOBUF_CONSTEXPR ExtensionRangeOptions::ExtensionRangeOptions(
   , /*decltype(_impl_.verification_)*/ 1
 } {}
 struct ExtensionRangeOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR ExtensionRangeOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr ExtensionRangeOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  ExtensionRangeOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) ExtensionRangeOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~ExtensionRangeOptionsDefaultTypeInternal() {}
   union {
     ExtensionRangeOptions _instance;
@@ -224,7 +259,12 @@ PROTOBUF_CONSTEXPR FieldDescriptorProto::FieldDescriptorProto(
   , /*decltype(_impl_.type_)*/ 1
 } {}
 struct FieldDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR FieldDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr FieldDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  FieldDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) FieldDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~FieldDescriptorProtoDefaultTypeInternal() {}
   union {
     FieldDescriptorProto _instance;
@@ -243,7 +283,12 @@ PROTOBUF_CONSTEXPR OneofDescriptorProto::OneofDescriptorProto(
 
   , /*decltype(_impl_.options_)*/nullptr} {}
 struct OneofDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR OneofDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr OneofDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  OneofDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) OneofDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~OneofDescriptorProtoDefaultTypeInternal() {}
   union {
     OneofDescriptorProto _instance;
@@ -261,7 +306,12 @@ PROTOBUF_CONSTEXPR EnumDescriptorProto_EnumReservedRange::EnumDescriptorProto_En
   , /*decltype(_impl_.end_)*/ 0
 } {}
 struct EnumDescriptorProto_EnumReservedRangeDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR EnumDescriptorProto_EnumReservedRangeDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr EnumDescriptorProto_EnumReservedRangeDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  EnumDescriptorProto_EnumReservedRangeDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) EnumDescriptorProto_EnumReservedRange(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~EnumDescriptorProto_EnumReservedRangeDefaultTypeInternal() {}
   union {
     EnumDescriptorProto_EnumReservedRange _instance;
@@ -283,7 +333,12 @@ PROTOBUF_CONSTEXPR EnumDescriptorProto::EnumDescriptorProto(
 
   , /*decltype(_impl_.options_)*/nullptr} {}
 struct EnumDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR EnumDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr EnumDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  EnumDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) EnumDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~EnumDescriptorProtoDefaultTypeInternal() {}
   union {
     EnumDescriptorProto _instance;
@@ -304,7 +359,12 @@ PROTOBUF_CONSTEXPR EnumValueDescriptorProto::EnumValueDescriptorProto(
   , /*decltype(_impl_.number_)*/ 0
 } {}
 struct EnumValueDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR EnumValueDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr EnumValueDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  EnumValueDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) EnumValueDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~EnumValueDescriptorProtoDefaultTypeInternal() {}
   union {
     EnumValueDescriptorProto _instance;
@@ -324,7 +384,12 @@ PROTOBUF_CONSTEXPR ServiceDescriptorProto::ServiceDescriptorProto(
 
   , /*decltype(_impl_.options_)*/nullptr} {}
 struct ServiceDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR ServiceDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr ServiceDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  ServiceDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) ServiceDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~ServiceDescriptorProtoDefaultTypeInternal() {}
   union {
     ServiceDescriptorProto _instance;
@@ -355,7 +420,12 @@ PROTOBUF_CONSTEXPR MethodDescriptorProto::MethodDescriptorProto(
   , /*decltype(_impl_.server_streaming_)*/ false
 } {}
 struct MethodDescriptorProtoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR MethodDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr MethodDescriptorProtoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  MethodDescriptorProtoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) MethodDescriptorProto(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~MethodDescriptorProtoDefaultTypeInternal() {}
   union {
     MethodDescriptorProto _instance;
@@ -431,7 +501,12 @@ PROTOBUF_CONSTEXPR FileOptions::FileOptions(
   , /*decltype(_impl_.cc_enable_arenas_)*/ true
 } {}
 struct FileOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR FileOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr FileOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  FileOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) FileOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~FileOptionsDefaultTypeInternal() {}
   union {
     FileOptions _instance;
@@ -457,7 +532,12 @@ PROTOBUF_CONSTEXPR MessageOptions::MessageOptions(
   , /*decltype(_impl_.deprecated_legacy_json_field_conflicts_)*/ false
 } {}
 struct MessageOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR MessageOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr MessageOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  MessageOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) MessageOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~MessageOptionsDefaultTypeInternal() {}
   union {
     MessageOptions _instance;
@@ -495,7 +575,12 @@ PROTOBUF_CONSTEXPR FieldOptions::FieldOptions(
   , /*decltype(_impl_.target_obsolete_do_not_use_)*/ 0
 } {}
 struct FieldOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR FieldOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr FieldOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  FieldOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) FieldOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~FieldOptionsDefaultTypeInternal() {}
   union {
     FieldOptions _instance;
@@ -510,7 +595,12 @@ PROTOBUF_CONSTEXPR OneofOptions::OneofOptions(
   , /*decltype(_impl_.uninterpreted_option_)*/{}
   , /*decltype(_impl_._cached_size_)*/{}} {}
 struct OneofOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR OneofOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr OneofOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  OneofOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) OneofOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~OneofOptionsDefaultTypeInternal() {}
   union {
     OneofOptions _instance;
@@ -532,7 +622,12 @@ PROTOBUF_CONSTEXPR EnumOptions::EnumOptions(
   , /*decltype(_impl_.deprecated_legacy_json_field_conflicts_)*/ false
 } {}
 struct EnumOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR EnumOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr EnumOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  EnumOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) EnumOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~EnumOptionsDefaultTypeInternal() {}
   union {
     EnumOptions _instance;
@@ -550,7 +645,12 @@ PROTOBUF_CONSTEXPR EnumValueOptions::EnumValueOptions(
   , /*decltype(_impl_.deprecated_)*/ false
 } {}
 struct EnumValueOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR EnumValueOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr EnumValueOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  EnumValueOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) EnumValueOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~EnumValueOptionsDefaultTypeInternal() {}
   union {
     EnumValueOptions _instance;
@@ -568,7 +668,12 @@ PROTOBUF_CONSTEXPR ServiceOptions::ServiceOptions(
   , /*decltype(_impl_.deprecated_)*/ false
 } {}
 struct ServiceOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR ServiceOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr ServiceOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  ServiceOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) ServiceOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~ServiceOptionsDefaultTypeInternal() {}
   union {
     ServiceOptions _instance;
@@ -588,7 +693,12 @@ PROTOBUF_CONSTEXPR MethodOptions::MethodOptions(
   , /*decltype(_impl_.idempotency_level_)*/ 0
 } {}
 struct MethodOptionsDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR MethodOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr MethodOptionsDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  MethodOptionsDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) MethodOptions(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~MethodOptionsDefaultTypeInternal() {}
   union {
     MethodOptions _instance;
@@ -608,7 +718,12 @@ PROTOBUF_CONSTEXPR UninterpretedOption_NamePart::UninterpretedOption_NamePart(
   , /*decltype(_impl_.is_extension_)*/ false
 } {}
 struct UninterpretedOption_NamePartDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR UninterpretedOption_NamePartDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr UninterpretedOption_NamePartDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  UninterpretedOption_NamePartDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) UninterpretedOption_NamePart(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~UninterpretedOption_NamePartDefaultTypeInternal() {}
   union {
     UninterpretedOption_NamePart _instance;
@@ -641,7 +756,12 @@ PROTOBUF_CONSTEXPR UninterpretedOption::UninterpretedOption(
   , /*decltype(_impl_.double_value_)*/ 0
 } {}
 struct UninterpretedOptionDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR UninterpretedOptionDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr UninterpretedOptionDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  UninterpretedOptionDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) UninterpretedOption(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~UninterpretedOptionDefaultTypeInternal() {}
   union {
     UninterpretedOption _instance;
@@ -670,7 +790,12 @@ PROTOBUF_CONSTEXPR SourceCodeInfo_Location::SourceCodeInfo_Location(
   }
 } {}
 struct SourceCodeInfo_LocationDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR SourceCodeInfo_LocationDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr SourceCodeInfo_LocationDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  SourceCodeInfo_LocationDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) SourceCodeInfo_Location(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~SourceCodeInfo_LocationDefaultTypeInternal() {}
   union {
     SourceCodeInfo_Location _instance;
@@ -684,7 +809,12 @@ PROTOBUF_CONSTEXPR SourceCodeInfo::SourceCodeInfo(
     /*decltype(_impl_.location_)*/{}
   , /*decltype(_impl_._cached_size_)*/{}} {}
 struct SourceCodeInfoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR SourceCodeInfoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr SourceCodeInfoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  SourceCodeInfoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) SourceCodeInfo(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~SourceCodeInfoDefaultTypeInternal() {}
   union {
     SourceCodeInfo _instance;
@@ -711,7 +841,12 @@ PROTOBUF_CONSTEXPR GeneratedCodeInfo_Annotation::GeneratedCodeInfo_Annotation(
   , /*decltype(_impl_.semantic_)*/ 0
 } {}
 struct GeneratedCodeInfo_AnnotationDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR GeneratedCodeInfo_AnnotationDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr GeneratedCodeInfo_AnnotationDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  GeneratedCodeInfo_AnnotationDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) GeneratedCodeInfo_Annotation(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~GeneratedCodeInfo_AnnotationDefaultTypeInternal() {}
   union {
     GeneratedCodeInfo_Annotation _instance;
@@ -725,7 +860,12 @@ PROTOBUF_CONSTEXPR GeneratedCodeInfo::GeneratedCodeInfo(
     /*decltype(_impl_.annotation_)*/{}
   , /*decltype(_impl_._cached_size_)*/{}} {}
 struct GeneratedCodeInfoDefaultTypeInternal {
-  PROTOBUF_CONSTEXPR GeneratedCodeInfoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  constexpr GeneratedCodeInfoDefaultTypeInternal() : _instance(::_pbi::ConstantInitialized{}) {}
+#else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  GeneratedCodeInfoDefaultTypeInternal() {}
+  void Init() { ::new (&_instance) GeneratedCodeInfo(); };
+#endif  // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
   ~GeneratedCodeInfoDefaultTypeInternal() {}
   union {
     GeneratedCodeInfo _instance;
@@ -1532,6 +1672,47 @@ PROTOBUF_ATTRIBUTE_INIT_PRIORITY2
 static ::_pbi::AddDescriptorsRunner dynamic_init_dummy_google_2fprotobuf_2fdescriptor_2eproto(&descriptor_table_google_2fprotobuf_2fdescriptor_2eproto);
 namespace google {
 namespace protobuf {
+namespace internal {
+#if !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+PROTOBUF_EXPORT void InitializeFileDescriptorDefaultInstancesSlow() {
+  _FileDescriptorSet_default_instance_.Init();
+_FileDescriptorProto_default_instance_.Init();
+_DescriptorProto_ExtensionRange_default_instance_.Init();
+_DescriptorProto_ReservedRange_default_instance_.Init();
+_DescriptorProto_default_instance_.Init();
+_ExtensionRangeOptions_Declaration_default_instance_.Init();
+_ExtensionRangeOptions_default_instance_.Init();
+_FieldDescriptorProto_default_instance_.Init();
+_OneofDescriptorProto_default_instance_.Init();
+_EnumDescriptorProto_EnumReservedRange_default_instance_.Init();
+_EnumDescriptorProto_default_instance_.Init();
+_EnumValueDescriptorProto_default_instance_.Init();
+_ServiceDescriptorProto_default_instance_.Init();
+_MethodDescriptorProto_default_instance_.Init();
+_FileOptions_default_instance_.Init();
+_MessageOptions_default_instance_.Init();
+_FieldOptions_default_instance_.Init();
+_OneofOptions_default_instance_.Init();
+_EnumOptions_default_instance_.Init();
+_EnumValueOptions_default_instance_.Init();
+_ServiceOptions_default_instance_.Init();
+_MethodOptions_default_instance_.Init();
+_UninterpretedOption_NamePart_default_instance_.Init();
+_UninterpretedOption_default_instance_.Init();
+_SourceCodeInfo_Location_default_instance_.Init();
+_SourceCodeInfo_default_instance_.Init();
+_GeneratedCodeInfo_Annotation_default_instance_.Init();
+_GeneratedCodeInfo_default_instance_.Init();;
+}
+PROTOBUF_ATTRIBUTE_INIT_PRIORITY1
+static std::true_type dynamic_init_dummy_google_2fprotobuf_2fdescriptor_2eproto{
+    (InitializeFileDescriptorDefaultInstances(), std::true_type{})};
+#endif  // !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+}  // namespace internal
+}  // namespace protobuf
+}  // namespace google
+namespace google {
+namespace protobuf {
 const ::google::protobuf::EnumDescriptor* ExtensionRangeOptions_VerificationState_descriptor() {
   ::google::protobuf::internal::AssignDescriptors(&descriptor_table_google_2fprotobuf_2fdescriptor_2eproto);
   return file_level_enum_descriptors_google_2fprotobuf_2fdescriptor_2eproto[0];
diff --git a/src/google/protobuf/descriptor.pb.h b/src/google/protobuf/descriptor.pb.h
index 9d5ef15803..56a8c99b0f 100644
--- a/src/google/protobuf/descriptor.pb.h
+++ b/src/google/protobuf/descriptor.pb.h
@@ -195,6 +195,11 @@ template <>
 PROTOBUF_EXPORT ::google::protobuf::UninterpretedOption* Arena::CreateMaybeMessage<::google::protobuf::UninterpretedOption>(Arena*);
 template <>
 PROTOBUF_EXPORT ::google::protobuf::UninterpretedOption_NamePart* Arena::CreateMaybeMessage<::google::protobuf::UninterpretedOption_NamePart>(Arena*);
+namespace internal {
+#if !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+PROTOBUF_EXPORT void InitializeFileDescriptorDefaultInstancesSlow();
+#endif  // !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+}  // namespace internal
 }  // namespace protobuf
 }  // namespace google
 
diff --git a/src/google/protobuf/generated_message_reflection.cc b/src/google/protobuf/generated_message_reflection.cc
index 626672b515..bd3b4e07ce 100644
--- a/src/google/protobuf/generated_message_reflection.cc
+++ b/src/google/protobuf/generated_message_reflection.cc
@@ -103,6 +103,14 @@ Message* MaybeForceCopy(Arena* arena, Message* msg) {
 
 namespace internal {
 
+void InitializeFileDescriptorDefaultInstances() {
+#if !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+  static std::true_type init =
+      (InitializeFileDescriptorDefaultInstancesSlow(), std::true_type{});
+  (void)init;
+#endif  // !defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+}
+
 bool ParseNamedEnum(const EnumDescriptor* descriptor, absl::string_view name,
                     int* value) {
   const EnumValueDescriptor* d = descriptor->FindValueByName(name);
@@ -3570,6 +3578,7 @@ void AssignDescriptorsImpl(const DescriptorTable* table, bool eager) {
 void AddDescriptorsImpl(const DescriptorTable* table) {
   // Reflection refers to the default fields so make sure they are initialized.
   internal::InitProtobufDefaults();
+  internal::InitializeFileDescriptorDefaultInstances();
 
   // Ensure all dependent descriptors are registered to the generated descriptor
   // pool and message factory.
diff --git a/src/google/protobuf/generated_message_reflection.h b/src/google/protobuf/generated_message_reflection.h
index e0d87b33aa..61851566c1 100644
--- a/src/google/protobuf/generated_message_reflection.h
+++ b/src/google/protobuf/generated_message_reflection.h
@@ -355,6 +355,8 @@ PROTOBUF_EXPORT void UnknownFieldSetSerializer(const uint8_t* base,
                                                uint32_t has_offset,
                                                io::CodedOutputStream* output);
 
+PROTOBUF_EXPORT void InitializeFileDescriptorDefaultInstances();
+
 struct PROTOBUF_EXPORT AddDescriptorsRunner {
   explicit AddDescriptorsRunner(const DescriptorTable* table);
 };
diff --git a/src/google/protobuf/port_def.inc b/src/google/protobuf/port_def.inc
index f4c0bc59dc..082e012c1c 100644
--- a/src/google/protobuf/port_def.inc
+++ b/src/google/protobuf/port_def.inc
@@ -676,6 +676,7 @@ static_assert(PROTOBUF_CPLUSPLUS_MIN(201402L), "Protobuf only supports C++14 and
 # if defined(__cpp_constinit) && !defined(__CYGWIN__)
 #  define PROTOBUF_CONSTINIT constinit
 #  define PROTOBUF_CONSTEXPR constexpr
+#  define PROTOBUF_CONSTINIT_DEFAULT_INSTANCES
 // Some older Clang versions incorrectly raise an error about
 // constant-initializing weak default instance pointers. Versions 12.0 and
 // higher seem to work, except that XCode 12.5.1 shows the error even though it
@@ -686,9 +687,11 @@ static_assert(PROTOBUF_CPLUSPLUS_MIN(201402L), "Protobuf only supports C++14 and
      (!defined(__APPLE__) && PROTOBUF_CLANG_MIN(12, 0)))
 #  define PROTOBUF_CONSTINIT [[clang::require_constant_initialization]]
 #  define PROTOBUF_CONSTEXPR constexpr
+#  define PROTOBUF_CONSTINIT_DEFAULT_INSTANCES
 # elif PROTOBUF_GNUC_MIN(12, 2)
 #  define PROTOBUF_CONSTINIT __constinit
 #  define PROTOBUF_CONSTEXPR constexpr
+#  define PROTOBUF_CONSTINIT_DEFAULT_INSTANCES
 # endif
 #endif
 
diff --git a/src/google/protobuf/port_undef.inc b/src/google/protobuf/port_undef.inc
index c8aaabca26..89bf2f2034 100644
--- a/src/google/protobuf/port_undef.inc
+++ b/src/google/protobuf/port_undef.inc
@@ -91,6 +91,7 @@
 #undef PROTOBUF_BIG_ENDIAN
 #undef PROTOBUF_CONSTINIT
 #undef PROTOBUF_CONSTEXPR
+#undef PROTOBUF_CONSTINIT_DEFAULT_INSTANCES
 #undef PROTOBUF_ATTRIBUTE_WEAK
 #undef PROTOBUF_HAVE_ATTRIBUTE_WEAK
 #undef PROTOBUF_ATTRIBUTE_NO_DESTROY