1 files changed, 17 insertions, 12 deletions

diff --git a/src/corelib/kernel/qapplicationstatic.h b/src/corelib/kernel/qapplicationstatic.h
index d2e050a911..bf5e79b8bf 100644
--- a/src/corelib/kernel/qapplicationstatic.h
+++ b/src/corelib/kernel/qapplicationstatic.h
@@ -9,6 +9,8 @@
 #include <QtCore/qcoreapplication.h>
 #include <QtCore/qglobalstatic.h>
 
+#include <new>
+
 QT_BEGIN_NAMESPACE
 
 namespace QtGlobalStatic {
@@ -17,7 +19,7 @@ template <typename QAS> struct ApplicationHolder
     using Type = typename QAS::QAS_Type;
     using PlainType = std::remove_cv_t<Type>;
 
-    Q_CONSTINIT static inline std::aligned_union_t<1, PlainType> storage = {};
+    Q_CONSTINIT static inline struct { alignas(Type) unsigned char data[sizeof(Type)]; } storage = {};
     Q_CONSTINIT static inline QBasicAtomicInteger<qint8> guard = { QtGlobalStatic::Uninitialized };
     Q_CONSTINIT static inline QBasicMutex mutex {};
 
@@ -28,7 +30,8 @@ template <typename QAS> struct ApplicationHolder
     Q_DISABLE_COPY_MOVE(ApplicationHolder)
     ~ApplicationHolder()
     {
-        if (guard.loadRelaxed() == QtGlobalStatic::Initialized) {
+        if (guard.loadAcquire() == QtGlobalStatic::Initialized) {
+            // No mutex! Up to external code to ensure no race happens.
             guard.storeRelease(QtGlobalStatic::Destroyed);
             realPointer()->~PlainType();
         }
@@ -36,30 +39,32 @@ template <typename QAS> struct ApplicationHolder
 
     static PlainType *realPointer()
     {
-        return reinterpret_cast<PlainType *>(&storage);
+        return std::launder(reinterpret_cast<PlainType *>(&storage));
     }
 
     // called from QGlobalStatic::instance()
     PlainType *pointer() noexcept(MutexLockIsNoexcept && ConstructionIsNoexcept)
     {
-        if (guard.loadRelaxed() == QtGlobalStatic::Initialized)
+        if (guard.loadAcquire() == QtGlobalStatic::Initialized)
             return realPointer();
         QMutexLocker locker(&mutex);
         if (guard.loadRelaxed() == QtGlobalStatic::Uninitialized) {
-            QAS::innerFunction(realPointer());
-            QObject::connect(QCoreApplication::instance(), &QObject::destroyed, reset);
-            guard.storeRelaxed(QtGlobalStatic::Initialized);
+            QAS::innerFunction(&storage);
+            const auto *app = QCoreApplication::instance();
+            Q_ASSERT_X(app, Q_FUNC_INFO,
+                       "The application static was used without a QCoreApplication instance");
+            QObject::connect(app, &QObject::destroyed, app, reset, Qt::DirectConnection);
+            guard.storeRelease(QtGlobalStatic::Initialized);
         }
         return realPointer();
     }
 
     static void reset()
     {
-        if (guard.loadRelaxed() == QtGlobalStatic::Initialized) {
-            QMutexLocker locker(&mutex);
-            realPointer()->~PlainType();
-            guard.storeRelaxed(QtGlobalStatic::Uninitialized);
-        }
+        // we only synchronize using the mutex here, not the guard
+        QMutexLocker locker(&mutex);
+        realPointer()->~PlainType();
+        guard.storeRelaxed(QtGlobalStatic::Uninitialized);
     }
 };
 } // namespace QtGlobalStatic