qcompilerdetection.h: Introduce Q_DECL_CONSTEXPR_DTOR macro

Add this macro to be able to declare explicit destructors constexpr.
This became possible starting from C++20, where constexpr >= 201907L.

Introduce a syntax test in test_qglobal.cpp.

Document the new macro.

Replace "#if __cpp_constexpr >= 201907L constexpr" with the macro
(QScopedValueRollback has this).

[ChangeLog][QtCore] Introduced Q_DECL_CONSTEXPR_DTOR which
resolves to constexpr when __cpp_constexpr >= 201907L, otherwise
it resolves to inline.

Task-number: QTBUG-124462
Change-Id: I763ce1e4674fdc6d7307a63df9a45cc4db253fc4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/global/qcompilerdetection.h

@@ -514,6 +514,8 @@
  *
  * For a full listing of feature test macros, see
  *  https://en.cppreference.com/w/cpp/feature_test
+ * Exceptions:
+ *  Q_DECL_CONSTEXPR_DTOR           constexpr in C++20 for explicit destructors __cpp_constexpr >= 201907L
  *
  * C++ extensions:
  *  Q_COMPILER_RESTRICTED_VLA       variable-length arrays, prior to __cpp_runtime_arrays
@@ -1003,6 +1005,14 @@
 #define Q_DECL_ENUMERATOR_DEPRECATED Q_DECL_DEPRECATED
 #define Q_DECL_ENUMERATOR_DEPRECATED_X(x) Q_DECL_DEPRECATED_X(x)
 
+#ifndef Q_DECL_CONSTEXPR_DTOR
+#  if __cpp_constexpr >= 201907L
+#    define Q_DECL_CONSTEXPR_DTOR constexpr
+#  else
+#    define Q_DECL_CONSTEXPR_DTOR inline
+#  endif
+#endif
+
 /*
  * Fallback macros to certain compiler features
  */

src/corelib/global/qcompilerdetection.qdoc

@@ -275,7 +275,7 @@
     This macro can be used to declare variable that should be constructed at compile-time,
     or an inline function that can be computed at compile-time.
 
-    \sa Q_DECL_RELAXED_CONSTEXPR
+    \sa Q_DECL_RELAXED_CONSTEXPR, Q_DECL_CONSTEXPR_DTOR
 */
 
 /*!
@@ -286,7 +286,20 @@
     This macro can be used to declare an inline function that can be computed
     at compile-time according to the relaxed rules from C++14.
 
-    \sa Q_DECL_CONSTEXPR
+    \sa Q_DECL_CONSTEXPR, Q_DECL_CONSTEXPR_DTOR
+*/
+
+/*!
+    \macro Q_DECL_CONSTEXPR_DTOR
+    \relates <QtCompilerDetection>
+    \since 6.9
+
+    Expands to \c{constexpr} on compilers that support C++20, where
+    \c{__cpp_constexpr} has a value greater than or equal to \c{201907L}.
+    Otherwise, it expands to \c{inline}.
+
+    Use this macro to declare a destructor that can be computed at
+    compile-time in C++20 or later.
 */
 
 /*!

src/corelib/tools/qscopedvaluerollback.h

@@ -25,9 +25,7 @@ public:
         var = std::move(value);
     }
 
-#if __cpp_constexpr >= 201907L
-    constexpr
-#endif
+    Q_DECL_CONSTEXPR_DTOR
     ~QScopedValueRollback()
     {
         varRef = std::move(oldValue);

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

@@ -97,6 +97,7 @@ private slots:
     void testqToUnderlying();
     void nodiscard();
     void tagStructDefinitions();
+    void CXX20_constexpr_dtor();
 };
 
 extern "C" {        // functions in qglobal.c
@@ -995,6 +996,23 @@ void tst_QGlobal::tagStructDefinitions()
     }
 }
 
+class DestructorTest {
+public:
+    explicit constexpr DestructorTest(int *arr) : m_arr(arr) {}
+    Q_DECL_CONSTEXPR_DTOR ~DestructorTest() { delete[] m_arr; }
+private:
+    int *m_arr;
+};
+
+[[maybe_unused]] Q_DECL_CONSTEXPR_DTOR DestructorTest dt(nullptr);
+
+void tst_QGlobal::CXX20_constexpr_dtor()
+{
+#if __cpp_constexpr >= 201907L
+    [[maybe_unused]] constexpr DestructorTest tmp(nullptr);
+#endif
+}
+
 QT_BEGIN_NAMESPACE
 
 // Compile-time typeinfo tests