QSizePolicy: make (Policy,Policy) ctor constexpr

Unfortunately, that ctor also takes a ControlType argument (defaulted),
and calls the non-constexpr, non-inline function setControlType().

In order to make at least the two-arg version constexpr, I added
a use of the ternary operator to check for type == DefaultType,
making all calls of the ctor that use type == DefaultType
constexpr. For init'ing an aggregate type without ctor in the
ctor-init-list, I needed to require uniform initialization, too.

C++11-style constexpr cannot call void functions, so I needed
to extract the transformation part of setControlType() into a
new function that returns the result instead of storing it directly.

Saves a surprising 2K in QtWidgets text size on GCC 4.9, AMD64 Linux
stripped release builds.

Change-Id: Ib4adf5fd6e54d5345dbfe1c298554278faf13c58
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/widgets/kernel/qsizepolicy.cpp

@@ -254,6 +254,11 @@ QSizePolicy::ControlType QSizePolicy::controlType() const
     \sa QStyle::layoutSpacing()
 */
 void QSizePolicy::setControlType(ControlType type)
+{
+    bits.ctype = toControlTypeFieldValue(type);
+}
+
+quint32 QSizePolicy::toControlTypeFieldValue(ControlType type) Q_DECL_NOTHROW
 {
     /*
         The control type is a flag type, with values 0x1, 0x2, 0x4, 0x8, 0x10,
@@ -271,10 +276,8 @@ void QSizePolicy::setControlType(ControlType type)
 
     int i = 0;
     while (true) {
-        if (type & (0x1 << i)) {
-            bits.ctype = i;
-            return;
-        }
+        if (type & (0x1 << i))
+            return i;
         ++i;
     }
 }

src/widgets/kernel/qsizepolicy.h

@@ -119,12 +119,19 @@ public:
 
     QT_SIZEPOLICY_CONSTEXPR QSizePolicy() : data(0) { }
 
+#ifdef Q_COMPILER_UNIFORM_INIT
+    QT_SIZEPOLICY_CONSTEXPR QSizePolicy(Policy horizontal, Policy vertical, ControlType type = DefaultType)
+        : bits{0, 0, quint32(horizontal), quint32(vertical),
+               type == DefaultType ? 0 : toControlTypeFieldValue(type), 0, 0, 0}
+    {}
+#else
     QSizePolicy(Policy horizontal, Policy vertical, ControlType type = DefaultType)
         : data(0) {
         bits.horPolicy = horizontal;
         bits.verPolicy = vertical;
         setControlType(type);
     }
+#endif // uniform-init
     QT_SIZEPOLICY_CONSTEXPR Policy horizontalPolicy() const { return static_cast<Policy>(bits.horPolicy); }
     QT_SIZEPOLICY_CONSTEXPR Policy verticalPolicy() const { return static_cast<Policy>(bits.verPolicy); }
     ControlType controlType() const;
@@ -176,6 +183,8 @@ private:
     struct Bits;
     QT_SIZEPOLICY_CONSTEXPR explicit QSizePolicy(Bits b) Q_DECL_NOTHROW : bits(b) { }
 
+    static quint32 toControlTypeFieldValue(ControlType type) Q_DECL_NOTHROW;
+
     struct Bits {
         quint32 horStretch : 8;
         quint32 verStretch : 8;

tests/auto/widgets/kernel/qsizepolicy/tst_qsizepolicy.cpp

@@ -113,6 +113,8 @@ void tst_QSizePolicy::constExpr()
     // check that certain ctors are constexpr (compile-only):
     { Q_CONSTEXPR QSizePolicy sp; Q_UNUSED(sp); }
     { Q_CONSTEXPR QSizePolicy sp = QSizePolicy(); Q_UNUSED(sp); }
+    { Q_CONSTEXPR QSizePolicy sp = QSizePolicy(QSizePolicy::Fixed, QSizePolicy::Preferred); Q_UNUSED(sp); }
+    { Q_CONSTEXPR QSizePolicy sp = QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Expanding, QSizePolicy::DefaultType); Q_UNUSED(sp); }
 #else
     QSKIP("QSizePolicy cannot be constexpr with this version of the compiler.");
 #endif