QPair: add constexpr liberally

On templates, adding constexpr makes a given instantiation constexpr
if it can be.

This turns qMakePair(0,0), say, into a compile-time constant.

The effects on existing code are small, but exist:

  $ size lib/*{-baseline,-paircexp} | sort -nr
  6516727  211192    2608 6730527  66b31f lib/libQt5Widgets.so.5.4.0-baseline
  6516711  211192    2608 6730511  66b30f lib/libQt5Widgets.so.5.4.0-paircexp
  5373720   44492   15976 5434188  52eb4c lib/libQt5Core.so.5.4.0-baseline
  5373504   44492   15976 5433972  52ea74 lib/libQt5Core.so.5.4.0-paircexp
  5107206  125072    6080 5238358  4fee56 lib/libQt5Gui.so.5.4.0-baseline
  5107030  125072    6080 5238182  4feda6 lib/libQt5Gui.so.5.4.0-paircexp
  1341290   30180    2600 1374070  14f776 lib/libQt5Network.so.5.4.0-baseline
  1341210   30180    2600 1373990  14f726 lib/libQt5Network.so.5.4.0-paircexp
  # no other libraries benefit

[ChangeLog][QtCore][QPair] Can now be used in C++11 constexpr contexts.

Change-Id: I3872e6aa33a7d02a168516f4dfa7119efcac8c40
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

2 files changed, 27 insertions, 12 deletions

diff --git a/src/corelib/tools/qpair.h b/src/corelib/tools/qpair.h
index 9b8691a3d8..cfc8035a48 100644
--- a/src/corelib/tools/qpair.h
+++ b/src/corelib/tools/qpair.h
@@ -53,18 +53,18 @@ struct QPair
     typedef T1 first_type;
     typedef T2 second_type;
 
-    QPair() : first(), second() {}
-    QPair(const T1 &t1, const T2 &t2) : first(t1), second(t2) {}
+    Q_DECL_CONSTEXPR QPair() : first(), second() {}
+    Q_DECL_CONSTEXPR QPair(const T1 &t1, const T2 &t2) : first(t1), second(t2) {}
     // compiler-generated copy/move ctor/assignment operators are fine!
 
     template <typename TT1, typename TT2>
-    QPair(const QPair<TT1, TT2> &p) : first(p.first), second(p.second) {}
+    Q_DECL_CONSTEXPR QPair(const QPair<TT1, TT2> &p) : first(p.first), second(p.second) {}
     template <typename TT1, typename TT2>
     QPair &operator=(const QPair<TT1, TT2> &p)
     { first = p.first; second = p.second; return *this; }
 #ifdef Q_COMPILER_RVALUE_REFS
     template <typename TT1, typename TT2>
-    QPair(QPair<TT1, TT2> &&p) : first(std::move(p.first)), second(std::move(p.second)) {}
+    Q_DECL_CONSTEXPR QPair(QPair<TT1, TT2> &&p) : first(std::move(p.first)), second(std::move(p.second)) {}
     template <typename TT1, typename TT2>
     QPair &operator=(QPair<TT1, TT2> &&p)
     { first = std::move(p.first); second = std::move(p.second); return *this; }
@@ -80,39 +80,39 @@ template<class T1, class T2>
 class QTypeInfo<QPair<T1, T2> > : public QTypeInfoMerger<QPair<T1, T2>, T1, T2> {}; // Q_DECLARE_TYPEINFO
 
 template <class T1, class T2>
-Q_INLINE_TEMPLATE bool operator==(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
+Q_DECL_CONSTEXPR Q_INLINE_TEMPLATE bool operator==(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 { return p1.first == p2.first && p1.second == p2.second; }
 
 template <class T1, class T2>
-Q_INLINE_TEMPLATE bool operator!=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
+Q_DECL_CONSTEXPR Q_INLINE_TEMPLATE bool operator!=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 { return !(p1 == p2); }
 
 template <class T1, class T2>
-Q_INLINE_TEMPLATE bool operator<(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
+Q_DECL_CONSTEXPR Q_INLINE_TEMPLATE bool operator<(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 {
     return p1.first < p2.first || (!(p2.first < p1.first) && p1.second < p2.second);
 }
 
 template <class T1, class T2>
-Q_INLINE_TEMPLATE bool operator>(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
+Q_DECL_CONSTEXPR Q_INLINE_TEMPLATE bool operator>(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 {
     return p2 < p1;
 }
 
 template <class T1, class T2>
-Q_INLINE_TEMPLATE bool operator<=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
+Q_DECL_CONSTEXPR Q_INLINE_TEMPLATE bool operator<=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 {
     return !(p2 < p1);
 }
 
 template <class T1, class T2>
-Q_INLINE_TEMPLATE bool operator>=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
+Q_DECL_CONSTEXPR Q_INLINE_TEMPLATE bool operator>=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 {
     return !(p1 < p2);
 }
 
 template <class T1, class T2>
-Q_OUTOFLINE_TEMPLATE QPair<T1, T2> qMakePair(const T1 &x, const T2 &y)
+Q_DECL_CONSTEXPR Q_OUTOFLINE_TEMPLATE QPair<T1, T2> qMakePair(const T1 &x, const T2 &y)
 {
     return QPair<T1, T2>(x, y);
 }
diff --git a/tests/auto/corelib/tools/qpair/tst_qpair.cpp b/tests/auto/corelib/tools/qpair/tst_qpair.cpp
index 05031dbbf8..59b0ffb95e 100644
--- a/tests/auto/corelib/tools/qpair/tst_qpair.cpp
+++ b/tests/auto/corelib/tools/qpair/tst_qpair.cpp
@@ -42,12 +42,13 @@
 #include <QtTest/QtTest>
 
 #include <QPair>
+#include <QSize>
 
 class tst_QPair : public QObject
 {
     Q_OBJECT
 private Q_SLOTS:
-    void dummy() {}
+    void testConstexpr();
 };
 
 class C { char _[4]; };
@@ -101,5 +102,19 @@ Q_STATIC_ASSERT(!QTypeInfo<QPairPP>::isDummy  );
 Q_STATIC_ASSERT(!QTypeInfo<QPairPP>::isPointer);
 
 
+void tst_QPair::testConstexpr()
+{
+    Q_CONSTEXPR QPair<int, double> pID = qMakePair(0, 0.0);
+    Q_UNUSED(pID);
+
+    Q_CONSTEXPR QPair<double, double> pDD  = qMakePair(0.0, 0.0);
+    Q_CONSTEXPR QPair<double, double> pDD2 = qMakePair(0, 0.0);   // involes (rvalue) conversion ctor
+    Q_CONSTEXPR bool equal = pDD2 == pDD;
+    QVERIFY(equal);
+
+    Q_CONSTEXPR QPair<QSize, int> pSI = qMakePair(QSize(4, 5), 6);
+    Q_UNUSED(pSI);
+}
+
 QTEST_APPLESS_MAIN(tst_QPair)
 #include "tst_qpair.moc"