Short live Q_NODISCARD_(CTOR_)X!

Wrappers around P1301 [[nodiscard("reason")]].

[ChangeLog][QtCore][Q_NODISCARD_X/Q_NODISCARD_CTOR_X] Added as
wrappers around C++20 [[nodiscard("reason")]].

Task-number: QTBUG-114767
Change-Id: Ie566d9c9d500ef632c7e243af97081f83506a752
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/global/qcompilerdetection.h

@@ -959,6 +959,13 @@
 #  ifndef Q_NODISCARD_CTOR
 #    define Q_NODISCARD_CTOR [[nodiscard]]
 #  endif
+// [[nodiscard("reason")]] (P1301)
+#  ifndef Q_NODISCARD_X
+#    define Q_NODISCARD_X(message) [[nodiscard(message)]]
+#  endif
+#  ifndef Q_NODISCARD_CTOR_X
+#    define Q_NODISCARD_CTOR_X(message) [[nodiscard(message)]]
+#  endif
 #endif
 
 #if __has_cpp_attribute(maybe_unused)
@@ -1010,9 +1017,15 @@
 #ifndef Q_REQUIRED_RESULT
 #  define Q_REQUIRED_RESULT
 #endif
+#ifndef Q_NODISCARD_X
+#  define Q_NODISCARD_X(message) Q_REQUIRED_RESULT
+#endif
 #ifndef Q_NODISCARD_CTOR
 #  define Q_NODISCARD_CTOR
 #endif
+#ifndef Q_NODISCARD_CTOR_X
+#  define Q_NODISCARD_CTOR_X(message) Q_NODISCARD_CTOR
+#endif
 #ifndef Q_DECL_DEPRECATED
 #  define Q_DECL_DEPRECATED
 #endif

src/corelib/global/qcompilerdetection.qdoc

@@ -409,4 +409,19 @@
     implement the feature. If your supported platforms all allow \c{[[nodiscard]]}
     on constructors, we strongly recommend you use the C++ attribute directly instead
     of this macro.
+
+    \sa Q_NODISCARD_CTOR_X
+*/
+
+/*!
+    \macro Q_NODISCARD_X(message)
+    \macro Q_NODISCARD_CTOR_X(message)
+    \relates <QtCompilerDetection>
+    \since 6.7
+
+    \brief Expand to \c{[[nodiscard(message)]]} on compilers that support the feature.
+
+    Otherwise they expand to [[nodiscard]] and Q_NODISCARD_CTOR, respectively.
+
+    \sa`Q_NODISCARD_CTOR
 */

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

@@ -37,7 +37,7 @@ private slots:
     void qRoundDoubles();
     void PRImacros();
     void testqToUnderlying();
-    void nodiscardConstructor();
+    void nodiscard();
 };
 
 extern "C" {        // functions in qglobal.c
@@ -692,16 +692,19 @@ void tst_QGlobal::testqToUnderlying()
     QCOMPARE(qToUnderlying(EE2), 456UL);
 }
 
-void tst_QGlobal::nodiscardConstructor()
+void tst_QGlobal::nodiscard()
 {
-    // Syntax-only test, just to make sure that Q_NODISCARD_CTOR compiles
+    // Syntax-only test, just to make sure that the Q_NODISCARD_* compile
     // on all platforms.
     // Other code is just to silence all various compiler warnings about
     // unused private members or methods.
     class Test {
     public:
-        Q_NODISCARD_CTOR explicit Test(int val) : m_val(val) {}
+        Q_NODISCARD_CTOR_X("Why construct a Test instead of just passing the int through?")
+        explicit Test(int val) : m_val(val) {}
+        Q_NODISCARD_CTOR explicit Test(float val) : m_val(int(val)) {}
 
+        Q_NODISCARD_X("Why call get() if you don't use the returned value, hu?") // NOT idiomatic use!
         int get() const { return m_val; }
 
     private:
@@ -710,6 +713,8 @@ void tst_QGlobal::nodiscardConstructor()
 
     Test t{42};
     QCOMPARE(t.get(), 42);
+    Test t2{42.0f};
+    QCOMPARE(t2.get(), 42);
 }
 
 QTEST_APPLESS_MAIN(tst_QGlobal)