QtGlobal: introduce a helper macro for declaring the RO5 SMFs as defaulted

If a class honors the RO0 we usually just leave a comment in the class'
body, and don't explicitly redeclare any of the RO5 special member
functions.

In some cases we may need to redeclare (some of) them. The main use-case
so far is to declare a protected destructor for a base class that is
not polymorphic, in order to prevent slicing; the compiler-provided
destructor is always public.

We can easily declare and default such a protected destructor, but that
comes with the problem that now we're violating the RO5 (as far as C++
is concerned, declaring a SMF counts towards the RO5, even if it's
immediately defaulted).

Specifically: by declaring the destructor, the class loses the
compiler-generated move operations, and the copy operations are
generated but deprecated. Clang >= 18 warns about this.

In such a scenario we *must* redeclare all five SMFs... and default them
all. This is boilerplate, therefore I'm adding a macro to streamline it.

Apply the new macro to a couple of cases were manual refactoring has
already been done.

Pick-to: 6.8 6.5
Change-Id: I5dc0ff9770621fbec0b1057c164d0623f901e3e9
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit fcb57391f73d36914e10ba964dbd9b01fe6f3af2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qtclasshelpermacros.h

@@ -83,6 +83,31 @@ QT_BEGIN_NAMESPACE
         return *this; \
     }
 
+/*
+    This macro defines the RO5 special member functions (destructor,
+    copy+move constructors and assignment operators) as defaulted.
+
+    Normally we don't use this macro if we're fine with these functions
+    to be public; we instead leave a comment in the class declaration,
+    something like:
+
+    // compiler-generated special member functions are fine!
+
+    In some cases a class may need to redeclare these functions, for
+    instance if it wants to change their accessibility. Since
+    defaulting all five is boilerplate, use this macro instead.
+
+    Note that the default constructor is not covered, and this macro
+    will prevented its automatic generation.
+*/
+
+#define QT_DECLARE_RO5_SMF_AS_DEFAULTED(Class) \
+    ~Class() = default; \
+    Class(const Class &) = default; \
+    Class(Class &&) = default; \
+    Class &operator=(const Class &) = default; \
+    Class &operator=(Class &&) = default;
+
 /*
     These macros can be used to define tag structs in the preferred way (ie.
     with explicit default ctor).

src/corelib/global/qxpfunctional.h

@@ -72,11 +72,7 @@ template <bool noex, class Const, class R, class... ArgTypes>
 class function_ref_base
 {
 protected:
-    ~function_ref_base() = default;
-    function_ref_base(const function_ref_base &) = default;
-    function_ref_base(function_ref_base &&) = default;
-    function_ref_base &operator=(const function_ref_base &) = default;
-    function_ref_base &operator=(function_ref_base &&) = default;
+    QT_DECLARE_RO5_SMF_AS_DEFAULTED(function_ref_base)
 
     using BoundEntityType = detail::BoundEntityType<Const>;
 

src/corelib/tools/qlist.h

@@ -32,11 +32,7 @@ template <typename T> struct QListSpecialMethodsBase
 {
 protected:
     QListSpecialMethodsBase() = default;
-    ~QListSpecialMethodsBase() = default;
-    QListSpecialMethodsBase(const QListSpecialMethodsBase &) = default;
-    QListSpecialMethodsBase(QListSpecialMethodsBase &&) = default;
-    QListSpecialMethodsBase &operator=(const QListSpecialMethodsBase &) = default;
-    QListSpecialMethodsBase &operator=(QListSpecialMethodsBase &&) = default;
+    QT_DECLARE_RO5_SMF_AS_DEFAULTED(QListSpecialMethodsBase)
 
     using Self = QList<T>;
     Self *self() { return static_cast<Self *>(this); }
@@ -58,11 +54,7 @@ template <typename T> struct QListSpecialMethods : QListSpecialMethodsBase<T>
 {
 protected:
     QListSpecialMethods() = default;
-    ~QListSpecialMethods() = default;
-    QListSpecialMethods(const QListSpecialMethods &) = default;
-    QListSpecialMethods(QListSpecialMethods &&) = default;
-    QListSpecialMethods &operator=(const QListSpecialMethods &) = default;
-    QListSpecialMethods &operator=(QListSpecialMethods &&) = default;
+    QT_DECLARE_RO5_SMF_AS_DEFAULTED(QListSpecialMethods)
 
 public:
     using QListSpecialMethodsBase<T>::indexOf;