Add a workaround for GCC constexpr bug

This turns the constexpr constructors into templates to silence errors when constexpr isn't valid.  We are also switching to 12.2 for GCC/cmake tests to prevent regressions (9.5 and 13.1 are already tested by GCC/bazel tests).

Fixes #12807

PiperOrigin-RevId: 532258101

.github/workflows/test_cpp.yml

@@ -234,7 +234,7 @@ jobs:
       - name: Run tests
         uses: protocolbuffers/protobuf-ci/docker@v1
         with:
-          image: us-docker.pkg.dev/protobuf-build/containers/test/linux/gcc:13.1-5.4.0-307caa02808127e49720f3e77d6a9f3b3ef5a915
+          image: us-docker.pkg.dev/protobuf-build/containers/test/linux/gcc:12.2-5.4.0-307caa02808127e49720f3e77d6a9f3b3ef5a915
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
           entrypoint: bash
           command: >-

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

@@ -1155,6 +1155,9 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
         "    ::$proto_ns$::internal::WireFormatLite::$val_wire_type$> "
         "SuperType;\n"
         "  $classname$();\n"
+        // Templatize constexpr constructor as a workaround for a bug in gcc 12
+        // (warning in gcc 13).
+        "  template <typename = void>\n"
         "  explicit PROTOBUF_CONSTEXPR $classname$(\n"
         "      ::$proto_ns$::internal::ConstantInitialized);\n"
         "  explicit $classname$(::$proto_ns$::Arena* arena);\n"
@@ -1248,6 +1251,9 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
     format("~$classname$() override;\n");
   }
   format(
+      // Templatize constexpr constructor as a workaround for a bug in gcc 12
+      // (warning in gcc 13).
+      "template<typename = void>\n"
       "explicit PROTOBUF_CONSTEXPR "
       "$classname$(::$proto_ns$::internal::ConstantInitialized);\n"
       "\n"
@@ -2449,13 +2455,19 @@ void MessageGenerator::GenerateConstexprConstructor(io::Printer* p) {
   Formatter format(p);
 
   if (IsMapEntryMessage(descriptor_) || !HasImplData(descriptor_, options_)) {
+    // Templatize constexpr constructor as a workaround for a bug in gcc 12
+    // (warning in gcc 13).
     format(
+        "template <typename>\n"
         "PROTOBUF_CONSTEXPR $classname$::$classname$(\n"
         "    ::_pbi::ConstantInitialized) {}\n");
     return;
   }
 
+  // Templatize constexpr constructor as a workaround for a bug in gcc 12
+  // (warning in gcc 13).
   format(
+      "template <typename>\n"
       "PROTOBUF_CONSTEXPR $classname$::$classname$(\n"
       "    ::_pbi::ConstantInitialized)");