Change how we decide which empty string implementation to use.

src/google/protobuf/port.cc

@@ -97,14 +97,9 @@ void RealDebugCounter::Register(absl::string_view name) {
   }
 }
 
-#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
-PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT const GlobalEmptyString
-    fixed_address_empty_string{};
-#else
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
     PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 GlobalEmptyString
         fixed_address_empty_string{};
-#endif
 
 }  // namespace internal
 }  // namespace protobuf

src/google/protobuf/port.h

@@ -494,20 +494,27 @@ class NoopDebugCounter {
 // Default empty string object. Don't use this directly. Instead, call
 // GetEmptyString() to get the reference. This empty string is aligned with a
 // minimum alignment of 8 bytes to match the requirement of ArenaStringPtr.
-#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
+
 // Take advantage of C++20 constexpr support in std::string.
-class alignas(8) GlobalEmptyString {
+class alignas(8) GlobalEmptyStringConstexpr {
  public:
   const std::string& get() const { return value_; }
   // Nothing to init, or destroy.
   std::string* Init() const { return nullptr; }
 
+  template <typename T = std::string, bool = (T(), true)>
+  static constexpr std::true_type HasConstexprDefaultConstructor(int) {
+    return {};
+  }
+  static constexpr std::false_type HasConstexprDefaultConstructor(char) {
+    return {};
+  }
+
  private:
   std::string value_;
 };
-PROTOBUF_EXPORT extern const GlobalEmptyString fixed_address_empty_string;
-#else
-class alignas(8) GlobalEmptyString {
+
+class alignas(8) GlobalEmptyStringDynamicInit {
  public:
   const std::string& get() const {
     return *reinterpret_cast<const std::string*>(internal::Launder(buffer_));
@@ -519,8 +526,12 @@ class alignas(8) GlobalEmptyString {
  private:
   alignas(std::string) char buffer_[sizeof(std::string)];
 };
+
+using GlobalEmptyString = std::conditional_t<
+    GlobalEmptyStringConstexpr::HasConstexprDefaultConstructor(0),
+    const GlobalEmptyStringConstexpr, GlobalEmptyStringDynamicInit>;
+
 PROTOBUF_EXPORT extern GlobalEmptyString fixed_address_empty_string;
-#endif
 
 }  // namespace internal
 }  // namespace protobuf