Rollback the behavior of using `constexpr` on `_table_` instances for all types except for the types in descriptor.proto

DLLs get in the way of making these tables constexpr in the general case because cross-DLL references are not constant.
However, we need descriptor.proto tables to be constexpr so we keep constexpr there were we know it works. They do not reference other DLLs.

PiperOrigin-RevId: 531256398

src/google/protobuf/compiler/cpp/file.cc

@@ -529,17 +529,6 @@ 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();
 
@@ -580,7 +569,7 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
 
   generator->GenerateConstexprConstructor(p);
 
-  if (IsFileDescriptorProto()) {
+  if (IsFileDescriptorProto(file_, options_)) {
     p->Emit(
         {
             {"type", DefaultInstanceType(generator->descriptor(), options_)},
@@ -1158,7 +1147,7 @@ void FileGenerator::GenerateReflectionInitializationCode(io::Printer* p) {
   // 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()) {
+  if (IsFileDescriptorProto(file_, options_)) {
     NamespaceOpener ns(p);
     ns.ChangeTo(absl::StrCat(ProtobufNamespace(options_), "::internal"));
     p->Emit(
@@ -1306,7 +1295,7 @@ void FileGenerator::GenerateForwardDeclarations(io::Printer* p) {
     decl.second.PrintTopLevelDecl(p, options_);
   }
 
-  if (IsFileDescriptorProto()) {
+  if (IsFileDescriptorProto(file_, options_)) {
     ns.ChangeTo(absl::StrCat(ProtobufNamespace(options_), "::internal"));
     p->Emit(R"cc(
       //~ Emit wants an indented line, so give it a comment to strip.

src/google/protobuf/compiler/cpp/file.h

@@ -168,8 +168,6 @@ 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).

src/google/protobuf/compiler/cpp/helpers.cc

@@ -1648,6 +1648,17 @@ std::vector<io::Printer::Sub> AnnotatedAccessors(
   return vars;
 }
 
+bool IsFileDescriptorProto(const FileDescriptor* file, const Options& options) {
+  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;
+}
+
 }  // namespace cpp
 }  // namespace compiler
 }  // namespace protobuf

src/google/protobuf/compiler/cpp/helpers.h

@@ -1049,6 +1049,11 @@ std::vector<io::Printer::Sub> AnnotatedAccessors(
     absl::optional<google::protobuf::io::AnnotationCollector::Semantic> semantic =
         absl::nullopt);
 
+// Check whether `file` represents the .proto file FileDescriptorProto and
+// friends. This file needs special handling because it must be usable during
+// dynamic initialization.
+bool IsFileDescriptorProto(const FileDescriptor* file, const Options& options);
+
 }  // namespace cpp
 }  // namespace compiler
 }  // namespace protobuf

src/google/protobuf/compiler/cpp/parse_function_generator.cc

@@ -406,9 +406,16 @@ void ParseFunctionGenerator::GenerateTailCallTable(Formatter& format) {
   // unknown fields and potentially an extension range.
   auto field_num_to_entry_table = MakeNumToEntryTable(ordered_fields_);
   format(
-      "constexpr ::_pbi::TcParseTable<$1$, $2$, $3$, $4$, $5$> "
+      "$1$ ::_pbi::TcParseTable<$2$, $3$, $4$, $5$, $6$> "
       "$classname$::_table_ = "
       "{\n",
+      // FileDescriptorProto's table must be constant initialized. For MSVC this
+      // means using `constexpr`. However, we can't use `constexpr` for all
+      // tables because it breaks when crossing DLL boundaries.
+      // FileDescriptorProto is safe from this.
+      IsFileDescriptorProto(descriptor_->file(), options_)
+          ? "constexpr"
+          : "PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_INIT_PRIORITY1\nconst",
       tc_table_info_->table_size_log2, ordered_fields_.size(),
       tc_table_info_->aux_entries.size(),
       FieldNameDataSize(tc_table_info_->field_name_data),