qcompilerdetection.h: Introduce Q_CONSTEXPR_DTOR macro for variables

Add this macro to declare constexpr variables that have destructors
marked with Q_DECL_CONSTEXPR_DTOR.

Declare variables using the new Q_CONSTEXPR_DTOR macro instead of
Q_DECL_CONSTEXPR_DTOR in syntax tests. Move the declarations inside the
functions as Q_DECL_CONSTEXPR_DTOR could only be used to declare
non-local variables.

[ChangeLog][QtCore] Introduced Q_CONSTEXPR_DTOR for variables, which
resolves to constexpr when __cpp_constexpr >= 201907L, otherwise it
resolves to const.

Task-number: QTBUG-124462
Change-Id: I06cd99bb956d9af560e92c326a7c82aa4c8852d1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qcompilerdetection.h

@@ -516,6 +516,7 @@
  *  https://en.cppreference.com/w/cpp/feature_test
  * Exceptions:
  *  Q_DECL_CONSTEXPR_DTOR           constexpr in C++20 for explicit destructors __cpp_constexpr >= 201907L
+ *  Q_CONSTEXPR_DTOR                constexpr in C++20 for variables __cpp_constexpr >= 201907L otherwise const
  *  Q_DECL_EQ_DELETE_X(message)     = delete("reason"), __cpp_deleted_function >= 202403L
  *
  * C++ extensions:
@@ -1014,6 +1015,14 @@
 #  endif
 #endif
 
+#ifndef Q_CONSTEXPR_DTOR
+#  if __cpp_constexpr >= 201907L
+#    define Q_CONSTEXPR_DTOR constexpr
+#  else
+#    define Q_CONSTEXPR_DTOR const
+#  endif
+#endif
+
 #ifndef Q_DECL_EQ_DELETE_X
 #  if defined(__cpp_deleted_function) && __cpp_deleted_function >= 202403L
 #    define Q_DECL_EQ_DELETE_X(reason) = delete(reason)

src/corelib/global/qcompilerdetection.qdoc

@@ -300,6 +300,23 @@
 
     Use this macro to declare a destructor that can be computed at
     compile-time in C++20 or later.
+
+    \sa Q_CONSTEXPR_DTOR
+*/
+
+/*!
+    \macro Q_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{const}.
+
+    Use this macro to declare a variable that can be constructed at compile-time
+    in C++20 or later.
+
+    \sa Q_DECL_CONSTEXPR_DTOR
 */
 
 /*!

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

@@ -1004,10 +1004,9 @@ private:
     int *m_arr;
 };
 
-[[maybe_unused]] Q_DECL_CONSTEXPR_DTOR DestructorTest dt(nullptr);
-
 void tst_QGlobal::CXX20_constexpr_dtor()
 {
+    [[maybe_unused]] Q_CONSTEXPR_DTOR DestructorTest tmp0(nullptr);
 #if __cpp_constexpr >= 201907L
     [[maybe_unused]] constexpr DestructorTest tmp(nullptr);
 #endif