Long live qToUnderlying

"Cherry-pick" of C++2b's std::to_underlying.

[ChangeLog][QtCore][QtGlobal] The qToUnderlying function has been
added, to convert an value of enumeration type to its underlying
value.

Change-Id: Ia46bd8e4496e55174171ac2f0799eacbcca02cf9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -3877,6 +3877,15 @@ bool qunsetenv(const char *varName)
     that qExchange() returns a non-const object, so Qt containers may detach.
 */
 
+/*!
+    \fn template <typename Enum> std::underlying_type_t<Enum> qToUnderlying(Enum e)
+    \relates <QtGlobal>
+    \since 6.2
+
+    Converts the enumerator \e to the equivalent value expressed in its
+    enumeration's underlying type.
+*/
+
 /*!
     \macro QT_TR_NOOP(sourceText)
     \relates <QtGlobal>

src/corelib/global/qglobal.h

@@ -1177,6 +1177,13 @@ constexpr T qExchange(T &t, U &&newValue)
     return old;
 }
 
+// like std::to_underlying
+template <typename Enum>
+constexpr std::underlying_type_t<Enum> qToUnderlying(Enum e) noexcept
+{
+    return static_cast<std::underlying_type_t<Enum>>(e);
+}
+
 #ifdef __cpp_conditional_explicit
 #define Q_IMPLICIT explicit(false)
 #else

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

@@ -60,6 +60,7 @@ private slots:
     void qRoundDoubles_data();
     void qRoundDoubles();
     void PRImacros();
+    void testqToUnderlying();
 };
 
 extern "C" {        // functions in qglobal.c
@@ -698,5 +699,24 @@ void tst_QGlobal::PRImacros()
     }
 }
 
+void tst_QGlobal::testqToUnderlying()
+{
+    enum class E {
+        E1 = 123,
+        E2 = 456,
+    };
+    static_assert(std::is_same_v<decltype(qToUnderlying(E::E1)), int>);
+    QCOMPARE(qToUnderlying(E::E1), 123);
+    QCOMPARE(qToUnderlying(E::E2), 456);
+
+    enum EE : unsigned long {
+        EE1 = 123,
+        EE2 = 456,
+    };
+    static_assert(std::is_same_v<decltype(qToUnderlying(EE1)), unsigned long>);
+    QCOMPARE(qToUnderlying(EE1), 123UL);
+    QCOMPARE(qToUnderlying(EE2), 456UL);
+}
+
 QTEST_APPLESS_MAIN(tst_QGlobal)
 #include "tst_qglobal.moc"