Simplify the Q_FOREACH macro when using C++17

This way there is only one for loop, which is more optimizer friendly

Change-Id: Iaa02026627d5259c3eea1ff5664e8f22664eef73
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.h

@@ -1005,6 +1005,15 @@ QForeachContainer<typename std::decay<T>::type> qMakeForeachContainer(T &&t)
 }
 
 }
+
+#if __cplusplus >= 201703L
+// Use C++17 if statement with initializer. User's code ends up in a else so
+// scoping of different ifs is not broken
+#define Q_FOREACH(variable, container)                                   \
+for (auto _container_ = QtPrivate::qMakeForeachContainer(container);     \
+     _container_.i != _container_.e;  ++_container_.i)                   \
+    if (variable = *_container_.i; false) {} else
+#else
 // Explanation of the control word:
 //  - it's initialized to 1
 //  - that means both the inner and outer loops start
@@ -1019,7 +1028,7 @@ for (auto _container_ = QtPrivate::qMakeForeachContainer(container); \
      _container_.control && _container_.i != _container_.e;         \
      ++_container_.i, _container_.control ^= 1)                     \
     for (variable = *_container_.i; _container_.control; _container_.control = 0)
-
+#endif
 #endif // QT_NO_FOREACH
 
 #define Q_FOREVER for(;;)

tests/auto/corelib/global/qglobal/qglobal.pro

@@ -2,3 +2,4 @@ CONFIG += testcase
 TARGET = tst_qglobal
 QT = core testlib
 SOURCES = tst_qglobal.cpp qglobal.c
+contains(QT_CONFIG, c++1z): CONFIG += c++1z

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

@@ -126,6 +126,46 @@ void tst_QGlobal::for_each()
         QCOMPARE(i, counter++);
     }
     QCOMPARE(counter, list.count());
+
+    // Should also work with an existing variable
+    int local;
+    counter = 0;
+    foreach (local, list) {
+        QCOMPARE(local, counter++);
+    }
+    QCOMPARE(counter, list.count());
+    QCOMPARE(local, counter - 1);
+
+    // Test the macro does not mess if/else conditions
+    counter = 0;
+    if (true)
+        foreach (int i, list)
+            QCOMPARE(i, counter++);
+    else
+        QFAIL("If/Else mismatch");
+    QCOMPARE(counter, list.count());
+
+    counter = 0;
+    if (false)
+        foreach (int i, list)
+            if (i) QFAIL("If/Else mismatch");
+            else QFAIL("If/Else mismatch");
+    else
+        foreach (int i, list)
+            if (false) { }
+            else QCOMPARE(i, counter++);
+    QCOMPARE(counter, list.count());
+
+    // break and continue
+    counter = 0;
+    foreach (int i, list) {
+        if (i == 0)
+            continue;
+        QCOMPARE(i, (counter++) + 1);
+        if (i == 3)
+            break;
+    }
+    QCOMPARE(counter, 3);
 }
 
 void tst_QGlobal::qassert()