Short live Q_NODISCARD_CTOR

[ChangeLog][QtCore] Introduced Q_NODISCARD_CTOR which resolves to
[[nodiscard]] attribute for constructors on compilers that support
it, and does nothing on other compilers.

Using [[nodiscard]] attribute on a constructor is a C++20 feature,
however in practice it is supported on most of the compilers that
we use in Qt 6. Clang generates a [-Wunused-value] warning, GCC
and MinGW generate a [-Wunused-result] warnings, and MSVC
generates a C4834 warning.
However, there are some exceptions.

The Integrity compiler provides the following warning:
 "tst_qglobal.cpp", line 699: warning #3435-D:
           the "nodiscard" attribute doesn't apply to constructors,
           destructors, or routines with void return type
           [[nodiscard]] explicit Test(int val) : m_val(val) {}

The QNX compiler (QCC 8.3.0) and GCC 9.3.1 on OpenSUSE generate
the [-Wattributes] warning:
 tst_qglobal.cpp: In member function
   'void tst_QGlobal::nodiscardConstructor()':
 tst_qglobal.cpp:699:44: warning: 'nodiscard' attribute applied to
   'tst_QGlobal::nodiscardConstructor()::Test::Test(int)' with void
    return type [-Wattributes]
          [[nodiscard]] explicit Test(int val) : m_val(val) {}

These warnings will lead to build failures when compiled with
-warnings-are-errors flag, so for these compilers the macro
does not do anything.

An attempt to use __attribute__((__warn_unused_result__)) was
also unsuccessful on these compilers, so this patch goes for
an easy solution, and simply checks
 __has_cpp_attribute(nodiscard) >= 201907L
to decide if the attribute is supported or not.

This commit also introduces a syntax-only test, and also applies
the new macro to QMutexLocker, because not all platforms in the
CI build and run unit tests.

Fixes: QTBUG-104161
Change-Id: Ib4230661a5ad5e8af0d67b21b034486ebcd67562
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/global/qcompilerdetection.h

@@ -970,6 +970,15 @@
 #define Q_DECL_ENUMERATOR_DEPRECATED Q_DECL_DEPRECATED
 #define Q_DECL_ENUMERATOR_DEPRECATED_X(x) Q_DECL_DEPRECATED_X(x)
 
+// [[nodiscard]] constructor
+#ifndef Q_NODISCARD_CTOR
+#  if __has_cpp_attribute(nodiscard) >= 201907L
+#    define Q_NODISCARD_CTOR [[nodiscard]]
+#  else
+#    define Q_NODISCARD_CTOR
+#  endif
+#endif
+
 /*
  * Fallback macros to certain compiler features
  */

src/corelib/thread/qmutex.h

@@ -231,6 +231,7 @@ template <typename Mutex>
 class [[nodiscard]] QMutexLocker
 {
 public:
+    Q_NODISCARD_CTOR
     inline explicit QMutexLocker(Mutex *mutex) QT_MUTEX_LOCK_NOEXCEPT
     {
         m_mutex = mutex;
@@ -240,6 +241,7 @@ public:
         }
     }
 
+    Q_NODISCARD_CTOR
     inline QMutexLocker(QMutexLocker &&other) noexcept
         : m_mutex(std::exchange(other.m_mutex, nullptr)),
           m_isLocked(std::exchange(other.m_isLocked, false))
@@ -326,6 +328,7 @@ template <typename Mutex>
 class [[nodiscard]] QMutexLocker
 {
 public:
+    Q_NODISCARD_CTOR
     inline explicit QMutexLocker(Mutex *) noexcept {}
     inline ~QMutexLocker() noexcept {}
 

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

@@ -37,6 +37,7 @@ private slots:
     void qRoundDoubles();
     void PRImacros();
     void testqToUnderlying();
+    void nodiscardConstructor();
 };
 
 extern "C" {        // functions in qglobal.c
@@ -691,5 +692,25 @@ void tst_QGlobal::testqToUnderlying()
     QCOMPARE(qToUnderlying(EE2), 456UL);
 }
 
+void tst_QGlobal::nodiscardConstructor()
+{
+    // Syntax-only test, just to make sure that Q_NODISCARD_CTOR compiles
+    // 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) {}
+
+        int get() const { return m_val; }
+
+    private:
+        int m_val;
+    };
+
+    Test t{42};
+    QCOMPARE(t.get(), 42);
+}
+
 QTEST_APPLESS_MAIN(tst_QGlobal)
 #include "tst_qglobal.moc"