QVariant::fromValue: Add rvalue optimization

When passing an rvalue-reference to QVariant, there is no reason to make
a copy if the type is moveable. Moreover, we know that the pointer which
we construct from the object passed to fromValue non-null. We make use
of both facts by parametrizing custom_construct on
non-nullness and availability of a move-ctor, and then dispatching to
the suitable template.
We need to keep the const T& overload, as otherwise code which
explicitly specializes fromValue and passes a const lvalue to it would
stop to compile.

[ChangeLog][QtCore][QVariant] Added fromValue() overload taking rvalues.

Change-Id: I44fb757d516ef364fe7967bc103b3f98278b4919
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 23 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
index feb03a029f..addf7ceec9 100644
--- a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
+++ b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
@@ -5573,6 +5573,16 @@ void tst_QVariant::mutableView()
     QCOMPARE(extracted.text, nullptr);
 }
 
+struct MoveTester
+{
+    bool wasMoved = false;
+    MoveTester() = default;
+    MoveTester(const MoveTester &) {}; // non-trivial on purpose
+    MoveTester(MoveTester &&other) { other.wasMoved = true; }
+    MoveTester& operator=(const MoveTester &) = default;
+    MoveTester& operator=(MoveTester &&other) {other.wasMoved = true; return *this;}
+};
+
 void tst_QVariant::moveOperations()
 {
     {
@@ -5594,6 +5604,19 @@ void tst_QVariant::moveOperations()
     v = QVariant::fromValue(list);
     v2 = std::move(v);
     QVERIFY(v2.value<std::list<int>>() == list);
+
+    {
+        MoveTester tester;
+        QVariant::fromValue(tester);
+        QVERIFY(!tester.wasMoved);
+        QVariant::fromValue(std::move(tester));
+        QVERIFY(tester.wasMoved);
+    }
+    {
+        const MoveTester tester;
+        QVariant::fromValue(std::move(tester));
+        QVERIFY(!tester.wasMoved); // we don't want to move from const variables
+    }
 }
 
 class NoMetaObject : public QObject {};