diff --git a/CHANGES.txt b/CHANGES.txt
index 6491e2bf23f5c..13d5cbb8519c1 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -87,6 +87,7 @@ Unreleased Changes (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
   * Remove GetPointer() and explicit nullptr defaults.
   * add proto_h flag for speeding up large builds
   * Add missing overload for reference wrapped fields.
+  * Add MergedDescriptorDatabase::FindAllFileNames()
 
 
 2022-01-28 version 3.19.4 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
diff --git a/src/google/protobuf/any.pb.cc b/src/google/protobuf/any.pb.cc
index e1d129f7033ed..05f51a05f68f6 100644
--- a/src/google/protobuf/any.pb.cc
+++ b/src/google/protobuf/any.pb.cc
@@ -27,8 +27,8 @@ namespace _pbi = _pb::internal;
 PROTOBUF_NAMESPACE_OPEN
 constexpr Any::Any(
     ::_pbi::ConstantInitialized)
-  : type_url_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : type_url_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , _any_metadata_(&type_url_, &value_){}
 struct AnyDefaultTypeInternal {
   constexpr AnyDefaultTypeInternal()
diff --git a/src/google/protobuf/api.pb.cc b/src/google/protobuf/api.pb.cc
index f2baf912611a0..441f3aafb52ce 100644
--- a/src/google/protobuf/api.pb.cc
+++ b/src/google/protobuf/api.pb.cc
@@ -26,8 +26,8 @@ constexpr Api::Api(
   : methods_()
   , options_()
   , mixins_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , version_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , version_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , source_context_(nullptr)
   , syntax_(0)
 {}
@@ -43,9 +43,9 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr Method::Method(
     ::_pbi::ConstantInitialized)
   : options_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , request_type_url_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , response_type_url_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , request_type_url_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , response_type_url_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , request_streaming_(false)
   , response_streaming_(false)
   , syntax_(0)
@@ -61,8 +61,8 @@ struct MethodDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 MethodDefaultTypeInternal _Method_default_instance_;
 constexpr Mixin::Mixin(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , root_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string){}
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , root_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{}){}
 struct MixinDefaultTypeInternal {
   constexpr MixinDefaultTypeInternal()
       : _instance(::_pbi::ConstantInitialized{}) {}
diff --git a/src/google/protobuf/arenastring.h b/src/google/protobuf/arenastring.h
index a9f000d35e227..b8d31fec015b0 100644
--- a/src/google/protobuf/arenastring.h
+++ b/src/google/protobuf/arenastring.h
@@ -228,8 +228,8 @@ static_assert(std::is_trivial<TaggedStringPtr>::value,
 // held, and the mutable and ownership invariants for each type.
 struct PROTOBUF_EXPORT ArenaStringPtr {
   ArenaStringPtr() = default;
-  explicit constexpr ArenaStringPtr(
-      ExplicitlyConstructedArenaString* default_value)
+  constexpr ArenaStringPtr(ExplicitlyConstructedArenaString* default_value,
+                           ConstantInitialized)
       : tagged_ptr_(default_value) {}
 
   // Called from generated code / reflection runtime only. Resets value to point
diff --git a/src/google/protobuf/compiler/cpp/cpp_string_field.cc b/src/google/protobuf/compiler/cpp/cpp_string_field.cc
index fa28c629c5830..49bacb776ea4f 100644
--- a/src/google/protobuf/compiler/cpp/cpp_string_field.cc
+++ b/src/google/protobuf/compiler/cpp/cpp_string_field.cc
@@ -530,9 +530,11 @@ void StringFieldGenerator::GenerateConstinitInitializer(
     return;
   }
   if (descriptor_->default_value_string().empty()) {
-    format("$name$_(&::$proto_ns$::internal::fixed_address_empty_string)");
+    format(
+        "$name$_(&::_pbi::fixed_address_empty_string, "
+        "::_pbi::ConstantInitialized{})");
   } else {
-    format("$name$_(nullptr)");
+    format("$name$_(nullptr, ::_pbi::ConstantInitialized{})");
   }
 }
 
diff --git a/src/google/protobuf/compiler/plugin.pb.cc b/src/google/protobuf/compiler/plugin.pb.cc
index e9fdbe8b036a0..37c1a54a48a03 100644
--- a/src/google/protobuf/compiler/plugin.pb.cc
+++ b/src/google/protobuf/compiler/plugin.pb.cc
@@ -24,7 +24,7 @@ PROTOBUF_NAMESPACE_OPEN
 namespace compiler {
 constexpr Version::Version(
     ::_pbi::ConstantInitialized)
-  : suffix_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : suffix_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , major_(0)
   , minor_(0)
   , patch_(0){}
@@ -41,7 +41,7 @@ constexpr CodeGeneratorRequest::CodeGeneratorRequest(
     ::_pbi::ConstantInitialized)
   : file_to_generate_()
   , proto_file_()
-  , parameter_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , parameter_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , compiler_version_(nullptr){}
 struct CodeGeneratorRequestDefaultTypeInternal {
   constexpr CodeGeneratorRequestDefaultTypeInternal()
@@ -54,9 +54,9 @@ struct CodeGeneratorRequestDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 CodeGeneratorRequestDefaultTypeInternal _CodeGeneratorRequest_default_instance_;
 constexpr CodeGeneratorResponse_File::CodeGeneratorResponse_File(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , insertion_point_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , content_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , insertion_point_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , content_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , generated_code_info_(nullptr){}
 struct CodeGeneratorResponse_FileDefaultTypeInternal {
   constexpr CodeGeneratorResponse_FileDefaultTypeInternal()
@@ -70,7 +70,7 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr CodeGeneratorResponse::CodeGeneratorResponse(
     ::_pbi::ConstantInitialized)
   : file_()
-  , error_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , error_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , supported_features_(uint64_t{0u}){}
 struct CodeGeneratorResponseDefaultTypeInternal {
   constexpr CodeGeneratorResponseDefaultTypeInternal()
diff --git a/src/google/protobuf/descriptor.pb.cc b/src/google/protobuf/descriptor.pb.cc
index b17b5843d5f80..7fed5405a7cc7 100644
--- a/src/google/protobuf/descriptor.pb.cc
+++ b/src/google/protobuf/descriptor.pb.cc
@@ -42,9 +42,9 @@ constexpr FileDescriptorProto::FileDescriptorProto(
   , extension_()
   , public_dependency_()
   , weak_dependency_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , package_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , syntax_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , package_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , syntax_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr)
   , source_code_info_(nullptr){}
 struct FileDescriptorProtoDefaultTypeInternal {
@@ -93,7 +93,7 @@ constexpr DescriptorProto::DescriptorProto(
   , oneof_decl_()
   , reserved_range_()
   , reserved_name_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr){}
 struct DescriptorProtoDefaultTypeInternal {
   constexpr DescriptorProtoDefaultTypeInternal()
@@ -118,11 +118,11 @@ struct ExtensionRangeOptionsDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 ExtensionRangeOptionsDefaultTypeInternal _ExtensionRangeOptions_default_instance_;
 constexpr FieldDescriptorProto::FieldDescriptorProto(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , extendee_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , type_name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , default_value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , json_name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , extendee_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , type_name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , default_value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , json_name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr)
   , number_(0)
   , oneof_index_(0)
@@ -142,7 +142,7 @@ struct FieldDescriptorProtoDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 FieldDescriptorProtoDefaultTypeInternal _FieldDescriptorProto_default_instance_;
 constexpr OneofDescriptorProto::OneofDescriptorProto(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr){}
 struct OneofDescriptorProtoDefaultTypeInternal {
   constexpr OneofDescriptorProtoDefaultTypeInternal()
@@ -171,7 +171,7 @@ constexpr EnumDescriptorProto::EnumDescriptorProto(
   : value_()
   , reserved_range_()
   , reserved_name_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr){}
 struct EnumDescriptorProtoDefaultTypeInternal {
   constexpr EnumDescriptorProtoDefaultTypeInternal()
@@ -184,7 +184,7 @@ struct EnumDescriptorProtoDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 EnumDescriptorProtoDefaultTypeInternal _EnumDescriptorProto_default_instance_;
 constexpr EnumValueDescriptorProto::EnumValueDescriptorProto(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr)
   , number_(0){}
 struct EnumValueDescriptorProtoDefaultTypeInternal {
@@ -199,7 +199,7 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr ServiceDescriptorProto::ServiceDescriptorProto(
     ::_pbi::ConstantInitialized)
   : method_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr){}
 struct ServiceDescriptorProtoDefaultTypeInternal {
   constexpr ServiceDescriptorProtoDefaultTypeInternal()
@@ -212,9 +212,9 @@ struct ServiceDescriptorProtoDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 ServiceDescriptorProtoDefaultTypeInternal _ServiceDescriptorProto_default_instance_;
 constexpr MethodDescriptorProto::MethodDescriptorProto(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , input_type_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , output_type_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , input_type_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , output_type_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , options_(nullptr)
   , client_streaming_(false)
   , server_streaming_(false){}
@@ -230,16 +230,16 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr FileOptions::FileOptions(
     ::_pbi::ConstantInitialized)
   : uninterpreted_option_()
-  , java_package_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , java_outer_classname_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , go_package_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , objc_class_prefix_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , csharp_namespace_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , swift_prefix_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , php_class_prefix_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , php_namespace_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , php_metadata_namespace_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , ruby_package_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , java_package_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , java_outer_classname_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , go_package_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , objc_class_prefix_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , csharp_namespace_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , swift_prefix_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , php_class_prefix_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , php_namespace_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , php_metadata_namespace_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , ruby_package_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , java_multiple_files_(false)
   , java_generate_equals_and_hash_(false)
   , java_string_check_utf8_(false)
@@ -366,7 +366,7 @@ struct MethodOptionsDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 MethodOptionsDefaultTypeInternal _MethodOptions_default_instance_;
 constexpr UninterpretedOption_NamePart::UninterpretedOption_NamePart(
     ::_pbi::ConstantInitialized)
-  : name_part_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_part_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , is_extension_(false){}
 struct UninterpretedOption_NamePartDefaultTypeInternal {
   constexpr UninterpretedOption_NamePartDefaultTypeInternal()
@@ -380,9 +380,9 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr UninterpretedOption::UninterpretedOption(
     ::_pbi::ConstantInitialized)
   : name_()
-  , identifier_value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , string_value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , aggregate_value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , identifier_value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , string_value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , aggregate_value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , positive_int_value_(uint64_t{0u})
   , negative_int_value_(int64_t{0})
   , double_value_(0){}
@@ -402,8 +402,8 @@ constexpr SourceCodeInfo_Location::SourceCodeInfo_Location(
   , span_()
   , _span_cached_byte_size_(0)
   , leading_detached_comments_()
-  , leading_comments_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , trailing_comments_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string){}
+  , leading_comments_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , trailing_comments_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{}){}
 struct SourceCodeInfo_LocationDefaultTypeInternal {
   constexpr SourceCodeInfo_LocationDefaultTypeInternal()
       : _instance(::_pbi::ConstantInitialized{}) {}
@@ -429,7 +429,7 @@ constexpr GeneratedCodeInfo_Annotation::GeneratedCodeInfo_Annotation(
     ::_pbi::ConstantInitialized)
   : path_()
   , _path_cached_byte_size_(0)
-  , source_file_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , source_file_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , begin_(0)
   , end_(0){}
 struct GeneratedCodeInfo_AnnotationDefaultTypeInternal {
diff --git a/src/google/protobuf/descriptor_database.cc b/src/google/protobuf/descriptor_database.cc
index be1afcb57c264..203000d67d0be 100644
--- a/src/google/protobuf/descriptor_database.cc
+++ b/src/google/protobuf/descriptor_database.cc
@@ -34,6 +34,7 @@
 
 #include <google/protobuf/descriptor_database.h>
 
+#include <algorithm>
 #include <set>
 
 #include <google/protobuf/descriptor.pb.h>
@@ -1027,5 +1028,21 @@ bool MergedDescriptorDatabase::FindAllExtensionNumbers(
 }
 
 
+bool MergedDescriptorDatabase::FindAllFileNames(
+    std::vector<std::string>* output) {
+  bool implemented = false;
+  for (DescriptorDatabase* source : sources_) {
+    std::vector<std::string> source_output;
+    if (source->FindAllFileNames(&source_output)) {
+      output->reserve(output->size() + source_output.size());
+      for (auto& source : source_output) {
+        output->push_back(std::move(source));
+      }
+      implemented = true;
+    }
+  }
+  return implemented;
+}
+
 }  // namespace protobuf
 }  // namespace google
diff --git a/src/google/protobuf/descriptor_database.h b/src/google/protobuf/descriptor_database.h
index 665e4ed8c971e..f4f06bbfa67d8 100644
--- a/src/google/protobuf/descriptor_database.h
+++ b/src/google/protobuf/descriptor_database.h
@@ -381,6 +381,10 @@ class PROTOBUF_EXPORT MergedDescriptorDatabase : public DescriptorDatabase {
                                std::vector<int>* output) override;
 
 
+  // This function is best-effort. Returns true if at least one underlying
+  // DescriptorDatabase returns true.
+  bool FindAllFileNames(std::vector<std::string>* output) override;
+
  private:
   std::vector<DescriptorDatabase*> sources_;
   GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(MergedDescriptorDatabase);
diff --git a/src/google/protobuf/descriptor_database_unittest.cc b/src/google/protobuf/descriptor_database_unittest.cc
index 9e5cf5d8ce589..f58244808a1f8 100644
--- a/src/google/protobuf/descriptor_database_unittest.cc
+++ b/src/google/protobuf/descriptor_database_unittest.cc
@@ -799,6 +799,13 @@ TEST_F(MergedDescriptorDatabaseTest, FindAllExtensionNumbers) {
   }
 }
 
+TEST_F(MergedDescriptorDatabaseTest, FindAllFileNames) {
+  std::vector<std::string> files;
+  EXPECT_TRUE(forward_merged_.FindAllFileNames(&files));
+  EXPECT_THAT(files, ::testing::UnorderedElementsAre("foo.proto", "bar.proto",
+                                                     "baz.proto", "baz.proto"));
+}
+
 
 }  // anonymous namespace
 }  // namespace protobuf
diff --git a/src/google/protobuf/map_field.h b/src/google/protobuf/map_field.h
index 5bf54d4d4255a..2d8129d157340 100644
--- a/src/google/protobuf/map_field.h
+++ b/src/google/protobuf/map_field.h
@@ -338,7 +338,8 @@ class PROTOBUF_EXPORT MapFieldBase {
   // It uses a linker initialized mutex, so it is not compatible with regular
   // runtime instances.
   // Except in MSVC, where we can't have a constinit mutex.
-  explicit constexpr MapFieldBase(ConstantInitialized)
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  constexpr MapFieldBase(ConstantInitialized)
       : arena_(nullptr),
         repeated_field_(nullptr),
         mutex_(GOOGLE_PROTOBUF_LINKER_INITIALIZED),
@@ -497,7 +498,8 @@ class TypeDefinedMapFieldBase : public MapFieldBase {
   // This constructor is for constant initialized global instances.
   // It uses a linker initialized mutex, so it is not compatible with regular
   // runtime instances.
-  explicit constexpr TypeDefinedMapFieldBase(ConstantInitialized tag)
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  constexpr TypeDefinedMapFieldBase(ConstantInitialized tag)
       : MapFieldBase(tag) {}
   explicit TypeDefinedMapFieldBase(Arena* arena) : MapFieldBase(arena) {}
 
@@ -567,7 +569,8 @@ class MapField : public TypeDefinedMapFieldBase<Key, T> {
   // This constructor is for constant initialized global instances.
   // It uses a linker initialized mutex, so it is not compatible with regular
   // runtime instances.
-  explicit constexpr MapField(ConstantInitialized tag)
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  constexpr MapField(ConstantInitialized tag)
       : TypeDefinedMapFieldBase<Key, T>(tag), impl_() {}
   explicit MapField(Arena* arena)
       : TypeDefinedMapFieldBase<Key, T>(arena), impl_(arena) {}
diff --git a/src/google/protobuf/map_type_handler.h b/src/google/protobuf/map_type_handler.h
index a04408050d1ab..c210c63db519a 100644
--- a/src/google/protobuf/map_type_handler.h
+++ b/src/google/protobuf/map_type_handler.h
@@ -591,7 +591,8 @@ inline bool MapTypeHandler<WireFormatLite::TYPE_MESSAGE, Type>::IsInitialized(
   constexpr auto                                                              \
   MapTypeHandler<WireFormatLite::TYPE_##FieldType, Type>::Constinit()         \
       ->TypeOnMemory {                                                        \
-    return TypeOnMemory(&internal::fixed_address_empty_string);               \
+    return TypeOnMemory(&internal::fixed_address_empty_string,                \
+                        ConstantInitialized{});                               \
   }                                                                           \
   template <typename Type>                                                    \
   inline typename MapTypeHandler<WireFormatLite::TYPE_##FieldType,            \
diff --git a/src/google/protobuf/message_lite.h b/src/google/protobuf/message_lite.h
index 903fb1a9236bb..1c22f894e3636 100644
--- a/src/google/protobuf/message_lite.h
+++ b/src/google/protobuf/message_lite.h
@@ -84,12 +84,6 @@ namespace internal {
 
 class SwapFieldHelper;
 
-// Tag type used to invoke the constinit constructor overload of some classes.
-// Such constructors are internal implementation details of the library.
-struct ConstantInitialized {
-  explicit ConstantInitialized() = default;
-};
-
 // See parse_context.h for explanation
 class ParseContext;
 
diff --git a/src/google/protobuf/port.h b/src/google/protobuf/port.h
index 3201d4531db22..54f08dbf799f5 100644
--- a/src/google/protobuf/port.h
+++ b/src/google/protobuf/port.h
@@ -57,6 +57,13 @@ inline void SizedArrayDelete(void* p, size_t size) {
   ::operator delete[](p);
 #endif
 }
+
+// Tag type used to invoke the constinit constructor overload of some classes.
+// Such constructors are internal implementation details of the library.
+struct ConstantInitialized {
+  explicit ConstantInitialized() = default;
+};
+
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
diff --git a/src/google/protobuf/source_context.pb.cc b/src/google/protobuf/source_context.pb.cc
index 9b5785cbf0a14..009da1fcd78a2 100644
--- a/src/google/protobuf/source_context.pb.cc
+++ b/src/google/protobuf/source_context.pb.cc
@@ -23,7 +23,7 @@ namespace _pbi = _pb::internal;
 PROTOBUF_NAMESPACE_OPEN
 constexpr SourceContext::SourceContext(
     ::_pbi::ConstantInitialized)
-  : file_name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string){}
+  : file_name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{}){}
 struct SourceContextDefaultTypeInternal {
   constexpr SourceContextDefaultTypeInternal()
       : _instance(::_pbi::ConstantInitialized{}) {}
diff --git a/src/google/protobuf/type.pb.cc b/src/google/protobuf/type.pb.cc
index 5b1067c664aff..f4925ca386ba8 100644
--- a/src/google/protobuf/type.pb.cc
+++ b/src/google/protobuf/type.pb.cc
@@ -26,7 +26,7 @@ constexpr Type::Type(
   : fields_()
   , oneofs_()
   , options_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , source_context_(nullptr)
   , syntax_(0)
 {}
@@ -42,10 +42,10 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr Field::Field(
     ::_pbi::ConstantInitialized)
   : options_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , type_url_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , json_name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
-  , default_value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , type_url_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , json_name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
+  , default_value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , kind_(0)
 
   , cardinality_(0)
@@ -66,7 +66,7 @@ constexpr Enum::Enum(
     ::_pbi::ConstantInitialized)
   : enumvalue_()
   , options_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , source_context_(nullptr)
   , syntax_(0)
 {}
@@ -82,7 +82,7 @@ PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORIT
 constexpr EnumValue::EnumValue(
     ::_pbi::ConstantInitialized)
   : options_()
-  , name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  , name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , number_(0){}
 struct EnumValueDefaultTypeInternal {
   constexpr EnumValueDefaultTypeInternal()
@@ -95,7 +95,7 @@ struct EnumValueDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 EnumValueDefaultTypeInternal _EnumValue_default_instance_;
 constexpr Option::Option(
     ::_pbi::ConstantInitialized)
-  : name_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string)
+  : name_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{})
   , value_(nullptr){}
 struct OptionDefaultTypeInternal {
   constexpr OptionDefaultTypeInternal()
diff --git a/src/google/protobuf/wrappers.pb.cc b/src/google/protobuf/wrappers.pb.cc
index 57f7da33d1abd..925cb03145d06 100644
--- a/src/google/protobuf/wrappers.pb.cc
+++ b/src/google/protobuf/wrappers.pb.cc
@@ -107,7 +107,7 @@ struct BoolValueDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 BoolValueDefaultTypeInternal _BoolValue_default_instance_;
 constexpr StringValue::StringValue(
     ::_pbi::ConstantInitialized)
-  : value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string){}
+  : value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{}){}
 struct StringValueDefaultTypeInternal {
   constexpr StringValueDefaultTypeInternal()
       : _instance(::_pbi::ConstantInitialized{}) {}
@@ -119,7 +119,7 @@ struct StringValueDefaultTypeInternal {
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 StringValueDefaultTypeInternal _StringValue_default_instance_;
 constexpr BytesValue::BytesValue(
     ::_pbi::ConstantInitialized)
-  : value_(&::PROTOBUF_NAMESPACE_ID::internal::fixed_address_empty_string){}
+  : value_(&::_pbi::fixed_address_empty_string, ::_pbi::ConstantInitialized{}){}
 struct BytesValueDefaultTypeInternal {
   constexpr BytesValueDefaultTypeInternal()
       : _instance(::_pbi::ConstantInitialized{}) {}