Change how we decide which empty string implementation to use. (#20708)

The feature macro seems to be lying on some toolchains. Instead, ask the
compiler to try to run the default constructor in constexpr context which
should fail if the implementation doesn't actually support it and fallback to
the dynamic one.

This should fix #20645

PiperOrigin-RevId: 737603382

Co-authored-by: Protobuf Team Bot <[email protected]>

Author: zhangskz

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