Made some small fixes for MinGW and for C++20 with GCC
Our use of constinit does not seem to work with GCC yet (see
https://github.com/protocolbuffers/protobuf/issues/8310), so this
commit disables it on GCC for now.
This commit also updates mutex.h to work around the fact that MinGW's
std::mutex apparently does not have a constexpr constructor. We already
have this kind of workaround for MSVC, so we can just use the same
workaround.
diff --git a/src/google/protobuf/port_def.inc b/src/google/protobuf/port_def.inc
index 3dc0ca7..0cba289 100644
--- a/src/google/protobuf/port_def.inc
+++ b/src/google/protobuf/port_def.inc
@@ -562,7 +562,9 @@
// by this flag is supposed to be removed after this experiment.
// #define PROTOBUF_MESSAGE_OWNED_ARENA_EXPERIMENT
-#if defined(__cpp_constinit)
+// Our use of constinit does not yet work with GCC:
+// https://github.com/protocolbuffers/protobuf/issues/8310
+#if defined(__cpp_constinit) && !defined(__GNUC__)
#define PROTOBUF_CONSTINIT constinit
#elif defined(__has_cpp_attribute)
#if __has_cpp_attribute(clang::require_constant_initialization)
diff --git a/src/google/protobuf/stubs/mutex.h b/src/google/protobuf/stubs/mutex.h
index 23f315f..82b62a6 100644
--- a/src/google/protobuf/stubs/mutex.h
+++ b/src/google/protobuf/stubs/mutex.h
@@ -126,7 +126,7 @@
private:
#if defined(GOOGLE_PROTOBUF_SUPPORT_WINDOWS_XP)
CallOnceInitializedMutex<CriticalSectionLock> mu_{};
-#elif defined(_MSC_VER)
+#elif defined(_WIN32)
CallOnceInitializedMutex<std::mutex> mu_{};
#else
std::mutex mu_{};
