Long live QT_DEFINE_TAG(_STRUCT)!

We seem to be using more and more of these, and their definition is a
bit subtle (need to have an explicit default ctor to avoid {} being
an initializer for the type).

Port the existing tag structs over to the new macro and add a test,
even though we found two users of QT_DEFINE_TAG in QtBase alone, one
of which is actually widely used (Disambiguated_t). There are more in
other modules.

Change-Id: I046bb2b70a2c7e79be2315d91c43e5fd2f0968a0
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 74a87a329498422db0dea3e469fb84704accbb2b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qnamespace.h

@@ -11,6 +11,7 @@
 
 #include <QtCore/qglobal.h>
 #include <QtCore/qcompare.h>
+#include <QtCore/qtclasshelpermacros.h>
 #include <QtCore/qtmetamacros.h>
 
 #if defined(__OBJC__) && !defined(__cplusplus)
@@ -1596,10 +1597,7 @@ namespace Qt {
     };
     inline constexpr Initialization Uninitialized = Initialization::Uninitialized;
 
-    struct Disambiguated_t {
-        explicit Disambiguated_t() = default;
-    };
-    inline constexpr Disambiguated_t Disambiguated{};
+    inline QT_DEFINE_TAG(Disambiguated);
 
     enum CoordinateSystem {
         DeviceCoordinates,

src/corelib/global/qtclasshelpermacros.h

@@ -74,6 +74,30 @@ QT_BEGIN_NAMESPACE
         return *this; \
     }
 
+/*
+    These macros can be used to define tag structs in the preferred way (ie.
+    with explicit default ctor).
+
+    The _STRUCT version only defines the tag type, no variable, while the
+    normal macro defines also a variable (and appends _t to the type name to
+    distinguish the two).
+
+    E.g. if we were std, we could use
+
+       QT_DEFINE_TAG(nullopt); // nullopt of type nullopt_t
+
+    The variable will be constexpr by default. If you want to make it static,
+    or inline, or both, prepend those keywords:
+
+      static QT_DEFINE_TAG(MyTag); // static constexpr
+      static inline QT_DEFINE_TAG(MyTag); // static inline constexpr
+*/
+#define QT_DEFINE_TAG_STRUCT(TAG) \
+    struct TAG { explicit TAG () = default; }
+#define QT_DEFINE_TAG(TAG) \
+    constexpr QT_DEFINE_TAG_STRUCT(TAG ## _t) TAG{}
+
+
 template <typename T> inline T *qGetPtrHelper(T *ptr) noexcept { return ptr; }
 template <typename Ptr> inline auto qGetPtrHelper(Ptr &ptr) noexcept -> decltype(ptr.get())
 { static_assert(noexcept(ptr.get()), "Smart d pointers for Q_DECLARE_PRIVATE must have noexcept get()"); return ptr.get(); }

src/corelib/global/qtdeprecationmarkers.h

@@ -5,6 +5,7 @@
 #define QTDEPRECATIONMARKERS_H
 
 #include <QtCore/qtconfigmacros.h>
+#include <QtCore/qtclasshelpermacros.h>
 #include <QtCore/qtdeprecationdefinitions.h>
 #include <QtCore/qtversionchecks.h>
 #include <QtCore/qcompilerdetection.h> // for Q_DECL_DEPRECATED
@@ -309,8 +310,7 @@ QT_BEGIN_NAMESPACE
 #ifdef __cplusplus
 // A tag to help mark stuff deprecated (cf. QStringViewLiteral)
 namespace QtPrivate {
-enum class Deprecated_t {};
-constexpr inline Deprecated_t Deprecated = {};
+inline QT_DEFINE_TAG(Deprecated);
 }
 #endif
 

src/corelib/kernel/qcoreevent.h

@@ -319,11 +319,11 @@ public:
     virtual QEvent *clone() const;
 
 protected:
-    struct InputEventTag { explicit InputEventTag() = default; };
+    QT_DEFINE_TAG_STRUCT(InputEventTag);
     QEvent(Type type, InputEventTag) : QEvent(type) { m_inputEvent = true; }
-    struct PointerEventTag { explicit PointerEventTag() = default; };
+    QT_DEFINE_TAG_STRUCT(PointerEventTag);
     QEvent(Type type, PointerEventTag) : QEvent(type, InputEventTag{}) { m_pointerEvent = true; }
-    struct SinglePointEventTag { explicit SinglePointEventTag() = default; };
+    QT_DEFINE_TAG_STRUCT(SinglePointEventTag);
     QEvent(Type type, SinglePointEventTag) : QEvent(type, PointerEventTag{}) { m_singlePointEvent = true; }
     quint16 t;
 

src/corelib/kernel/qtmetamacros.h

@@ -6,6 +6,7 @@
 #define QTMETAMACROS_H
 
 #include <QtCore/qglobal.h>
+#include <QtCore/qtclasshelpermacros.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -129,7 +130,7 @@ private: \
     Q_OBJECT_NO_ATTRIBUTES_WARNING \
     Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); \
     QT_WARNING_POP \
-    struct QPrivateSignal { explicit QPrivateSignal() = default; }; \
+    QT_DEFINE_TAG_STRUCT(QPrivateSignal); \
     QT_ANNOTATE_CLASS(qt_qobject, "")
 
 /* qmake ignore Q_OBJECT */

tests/auto/corelib/global/qglobal/tst_qglobal.cpp

@@ -75,6 +75,7 @@ private slots:
     void PRImacros();
     void testqToUnderlying();
     void nodiscard();
+    void tagStructDefinitions();
 };
 
 extern "C" {        // functions in qglobal.c
@@ -941,6 +942,23 @@ void tst_QGlobal::nodiscard()
     QCOMPARE(t2.get(), 42);
 }
 
+void tst_QGlobal::tagStructDefinitions()
+{
+    {
+        // predefined:
+        static_assert(std::is_same_v<decltype(QtPrivate::Deprecated), const QtPrivate::Deprecated_t>);
+        [[maybe_unused]] constexpr auto tag = QtPrivate::Deprecated;
+        static_assert(std::is_same_v<decltype(tag), const QtPrivate::Deprecated_t>);
+    }
+    {
+        // self-defined:
+        QT_DEFINE_TAG(MyTag);
+        static_assert(std::is_same_v<decltype(MyTag), const MyTag_t>);
+        [[maybe_unused]] constexpr auto tag = MyTag;
+        static_assert(std::is_same_v<decltype(tag), const MyTag_t>);
+    }
+}
+
 QT_BEGIN_NAMESPACE
 
 // Compile-time typeinfo tests