QSizePolicy: inline toControlTypeFieldValue()

The implementation now uses the relaxed-constexpr
qCountTrailingZeroBits() function from QtAlgorithms, making the
QSizePolicy(Policy, Policy, ControlType) constructor constexpr on
C++14 compilers. The explicit check for DefaultType remains to keep
the constructor C++11-constexpr when called with just (Policy,
Policy).

Extend the constExpr tests a bit.

Change-Id: I59690f0921d9bdee08e3615d0d1f4a9b92870c32
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/widgets/kernel/qsizepolicy.cpp

@@ -258,30 +258,6 @@ void QSizePolicy::setControlType(ControlType type) Q_DECL_NOTHROW
     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,
-        etc. In memory, we pack it onto the available bits (CTSize) in
-        setControlType(), and unpack it here.
-
-        Example:
-
-            0x00000001 maps to 0
-            0x00000002 maps to 1
-            0x00000004 maps to 2
-            0x00000008 maps to 3
-            etc.
-    */
-
-    int i = 0;
-    while (true) {
-        if (type & (0x1 << i))
-            return i;
-        ++i;
-    }
-}
-
 /*!
     \fn void QSizePolicy::setHeightForWidth(bool dependent)
 

src/widgets/kernel/qsizepolicy.h

@@ -42,6 +42,7 @@
 
 #include <QtWidgets/qtwidgetsglobal.h>
 #include <QtCore/qobject.h>
+#include <QtCore/qalgorithms.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -175,7 +176,24 @@ private:
     struct Bits;
     QT_SIZEPOLICY_CONSTEXPR explicit QSizePolicy(Bits b) Q_DECL_NOTHROW : bits(b) { }
 
-    static quint32 toControlTypeFieldValue(ControlType type) Q_DECL_NOTHROW;
+    static Q_DECL_RELAXED_CONSTEXPR quint32 toControlTypeFieldValue(ControlType type) Q_DECL_NOTHROW
+    {
+        /*
+          The control type is a flag type, with values 0x1, 0x2, 0x4, 0x8, 0x10,
+          etc. In memory, we pack it onto the available bits (CTSize) in
+          setControlType(), and unpack it here.
+
+          Example:
+
+          0x00000001 maps to 0
+          0x00000002 maps to 1
+          0x00000004 maps to 2
+          0x00000008 maps to 3
+          etc.
+        */
+
+        return qCountTrailingZeroBits(static_cast<quint32>(type));
+    }
 
     struct Bits {
         quint32 horStretch : 8;

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

@@ -114,7 +114,10 @@ void tst_QSizePolicy::constExpr()
     { 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); }
+    { Q_CONSTEXPR QSizePolicy sp = QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Expanding, QSizePolicy::DefaultType);
+      Q_CONSTEXPR QSizePolicy tp = sp.transposed(); Q_UNUSED(tp); }
+    { Q_RELAXED_CONSTEXPR auto sp = QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Fixed, QSizePolicy::CheckBox);
+      Q_RELAXED_CONSTEXPR auto tp = sp.transposed(); Q_UNUSED(tp); }
 #else
     QSKIP("QSizePolicy cannot be constexpr with this version of the compiler.");
 #endif