Mark all ctors of [[nodiscard]] classes as Q_NODISCARD_CTOR

This works around around the difference in class-level [[nodiscard]]
behavior between Clang and GCC. While Clang already warns about

    QClass(~~~args~~~);

when QClass is marked as [[nodiscard]] at the class level, GCC does
not, and requires the ctor to be marked as [[nodiscard]] instead.

Fixes: QTBUG-104164
Change-Id: Ifd7076ee422fb9472ae8bbba43d9bfeee1968a78
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 603d6351cc32d07a5e4c349d6dc3db0889c3b120)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qfile.h

@@ -41,6 +41,7 @@ class [[nodiscard]] QNtfsPermissionCheckGuard
 {
     Q_DISABLE_COPY_MOVE(QNtfsPermissionCheckGuard)
 public:
+    Q_NODISCARD_CTOR
     QNtfsPermissionCheckGuard()
     {
         qEnableNtfsPermissionChecks();

src/corelib/kernel/qproperty.h

@@ -57,6 +57,7 @@ class [[nodiscard]] QScopedPropertyUpdateGroup
 {
     Q_DISABLE_COPY_MOVE(QScopedPropertyUpdateGroup)
 public:
+    Q_NODISCARD_CTOR
     QScopedPropertyUpdateGroup()
     { Qt::beginPropertyUpdateGroup(); }
     ~QScopedPropertyUpdateGroup() noexcept(false)
@@ -290,6 +291,7 @@ class [[nodiscard]] QPropertyChangeHandler : public QPropertyObserver
 {
     Functor m_handler;
 public:
+    Q_NODISCARD_CTOR
     QPropertyChangeHandler(Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
               auto This = static_cast<QPropertyChangeHandler<Functor>*>(self);
@@ -300,6 +302,7 @@ public:
     }
 
     template<typename Property, typename = typename Property::InheritsQUntypedPropertyData>
+    Q_NODISCARD_CTOR
     QPropertyChangeHandler(const Property &property, Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
               auto This = static_cast<QPropertyChangeHandler<Functor>*>(self);
@@ -315,8 +318,10 @@ class [[nodiscard]] QPropertyNotifier : public QPropertyObserver
 {
     std::function<void()> m_handler;
 public:
+    Q_NODISCARD_CTOR
     QPropertyNotifier() = default;
     template<typename Functor>
+    Q_NODISCARD_CTOR
     QPropertyNotifier(Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
             auto This = static_cast<QPropertyNotifier *>(self);
@@ -327,6 +332,7 @@ public:
     }
 
     template<typename Functor, typename Property, typename = typename Property::InheritsQUntypedPropertyData>
+    Q_NODISCARD_CTOR
     QPropertyNotifier(const Property &property, Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
             auto This = static_cast<QPropertyNotifier *>(self);

src/corelib/kernel/qproperty_p.h

@@ -99,6 +99,7 @@ struct [[nodiscard]] QPropertyObserverNodeProtector
     Q_DISABLE_COPY_MOVE(QPropertyObserverNodeProtector)
 
     QPropertyObserverBase m_placeHolder;
+    Q_NODISCARD_CTOR
     QPropertyObserverNodeProtector(QPropertyObserver *observer)
     {
         // insert m_placeholder after observer into the linked list

src/corelib/tools/qatomicscopedvaluerollback_p.h

@@ -52,11 +52,13 @@ public:
     //
     // std::atomic:
     //
+    Q_NODISCARD_CTOR
     explicit constexpr
     QAtomicScopedValueRollback(std::atomic<T> &var,
                                std::memory_order mo = std::memory_order_seq_cst)
         : m_atomic(var), m_value(var.load(mo)), m_mo(mo) {}
 
+    Q_NODISCARD_CTOR
     explicit constexpr
     QAtomicScopedValueRollback(std::atomic<T> &var, T value,
                                std::memory_order mo = std::memory_order_seq_cst)
@@ -65,11 +67,13 @@ public:
     //
     // Q(Basic)AtomicInteger:
     //
+    Q_NODISCARD_CTOR
     explicit constexpr
     QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var,
                                std::memory_order mo = std::memory_order_seq_cst)
         : QAtomicScopedValueRollback(var._q_value, mo) {}
 
+    Q_NODISCARD_CTOR
     explicit constexpr
     QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var, T value,
                                std::memory_order mo = std::memory_order_seq_cst)
@@ -78,11 +82,13 @@ public:
     //
     // Q(Basic)AtomicPointer:
     //
+    Q_NODISCARD_CTOR
     explicit constexpr
     QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var,
                                std::memory_order mo = std::memory_order_seq_cst)
         : QAtomicScopedValueRollback(var._q_value, mo) {}
 
+    Q_NODISCARD_CTOR
     explicit constexpr
     QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var, T value,
                                std::memory_order mo = std::memory_order_seq_cst)

src/corelib/tools/qscopedpointer.h

@@ -70,6 +70,7 @@ template <typename T, typename Cleanup = QScopedPointerDeleter<T> >
 class [[nodiscard]] QScopedPointer
 {
 public:
+    Q_NODISCARD_CTOR
     explicit QScopedPointer(T *p = nullptr) noexcept : d(p)
     {
     }
@@ -192,10 +193,12 @@ class [[nodiscard]] QScopedArrayPointer : public QScopedPointer<T, Cleanup>
     template <typename Ptr>
     using if_same_type = typename std::enable_if<std::is_same<typename std::remove_cv<T>::type, Ptr>::value, bool>::type;
 public:
+    Q_NODISCARD_CTOR
     inline QScopedArrayPointer() : QScopedPointer<T, Cleanup>(nullptr) {}
     inline ~QScopedArrayPointer() = default;
 
     template <typename D, if_same_type<D> = true>
+    Q_NODISCARD_CTOR
     explicit QScopedArrayPointer(D *p)
         : QScopedPointer<T, Cleanup>(p)
     {

src/corelib/tools/qscopedvaluerollback.h

@@ -12,11 +12,13 @@ template <typename T>
 class [[nodiscard]] QScopedValueRollback
 {
 public:
+    Q_NODISCARD_CTOR
     explicit constexpr QScopedValueRollback(T &var)
         : varRef(var), oldValue(var)
     {
     }
 
+    Q_NODISCARD_CTOR
     explicit constexpr QScopedValueRollback(T &var, T value)
         : varRef(var), oldValue(std::move(var)) // ### C++20: std::exchange(var, std::move(value))
     {

src/corelib/tools/qscopeguard.h

@@ -16,16 +16,19 @@ template <typename F>
 class [[nodiscard]] QScopeGuard
 {
 public:
+    Q_NODISCARD_CTOR
     explicit QScopeGuard(F &&f) noexcept
         : m_func(std::move(f))
     {
     }
 
+    Q_NODISCARD_CTOR
     explicit QScopeGuard(const F &f) noexcept
         : m_func(f)
     {
     }
 
+    Q_NODISCARD_CTOR
     QScopeGuard(QScopeGuard &&other) noexcept
         : m_func(std::move(other.m_func))
         , m_invoke(std::exchange(other.m_invoke, false))