QGlobalStatic: don't use a std::aligned_union if we can be a union

Simplifies further the code with C++11 unrestricted unions.

Pick-to: 6.3
Task-number: QTBUG-99122
Change-Id: Ib42b3adc93bf4d43bd55fffd16c0b6677441bf55
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

2 files changed, 14 insertions, 12 deletions

diff --git a/src/corelib/global/qglobalstatic.h b/src/corelib/global/qglobalstatic.h
index 01e06e5234..cf96e5ae8e 100644
--- a/src/corelib/global/qglobalstatic.h
+++ b/src/corelib/global/qglobalstatic.h
@@ -57,15 +57,17 @@ enum GuardValues {
     Initializing = 1
 };
 
-template <typename QGS> struct Holder
+template <typename QGS> union Holder
 {
     using Type = typename QGS::QGS_Type;
     using PlainType = std::remove_cv_t<Type>;
 
     static constexpr bool ConstructionIsNoexcept = noexcept(QGS::innerFunction(nullptr));
-    std::aligned_union_t<1, PlainType> storage;
     static inline QBasicAtomicInteger<qint8> guard = { QtGlobalStatic::Uninitialized };
 
+    // union's sole member
+    PlainType storage;
+
     Holder() noexcept(ConstructionIsNoexcept)
     {
         QGS::innerFunction(pointer());
@@ -81,7 +83,7 @@ template <typename QGS> struct Holder
 
     PlainType *pointer() noexcept
     {
-        return reinterpret_cast<PlainType *>(&storage);
+        return &storage;
     }
 
     Q_DISABLE_COPY_MOVE(Holder)
diff --git a/src/corelib/global/qglobalstatic.qdoc b/src/corelib/global/qglobalstatic.qdoc
index 5aea8fed06..99416ad102 100644
--- a/src/corelib/global/qglobalstatic.qdoc
+++ b/src/corelib/global/qglobalstatic.qdoc
@@ -234,15 +234,15 @@
     new and the variadic arguments.
 
     The majority of the work is done by the public \l QGlobalStatic class and
-    the private \c QtGlobalStatic::Holder class. The \c Holder class has a
-    non-static, trivial member of suitable size and alignment to hold \a TYPE
-    (a \c{std::aligned_union_t} or a \c{std::aligned_storage_t}). The
-    constructor calls the "QGS" type's static member function with a pointer to
-    this location so it can be initialized and the destructor calls the type's
-    destructor. The \c{Holder} type is therefore neither trivially
-    constructible nor trivially destructible. It is used as a function-local \c
-    static so its initialization is thread-safe due to C++11's requirement that
-    such variables be thread-safely initialized.
+    the private \c QtGlobalStatic::Holder class. The \c Holder union has a
+    single non-static member of type \a TYPE, but because this is a union, its
+    construction and destruction are explicitly controlled in the Holder's
+    constructor and destructor. The constructor calls the "QGS" type's static
+    member function with a pointer to this member so it can be initialized
+    and the destructor calls the type's destructor. The \c{Holder} type is
+    therefore neither trivially constructible nor trivially destructible. It is
+    used as a function-local \c static so its initialization is thread-safe due
+    to C++11's requirement that such variables be thread-safely initialized.
 
     Additionally, both the constructor and destructor modify a guard variable
     after construction and before destruction, respectively. The guard variable