QVariant::value/qvariant_cast: add rvalue optimization

If we have a rvalue reference to an unshared QVariant, we can avoid
potentially expensive copies, and use move semantics instead.

[ChangeLog][QtCore][QVariant] Added rvalue QVariant overloads of qvariant_cast<T>() and QVariant::value<T>().

[ChangeLog][Potentially Source-Incompatible Changes][QVariant] It is no
longer possible to take the address of a specialization of
qvariant_cast; consider using a lambda function instead.

Change-Id: Ifc74991eadcc31387b755c45484224a3200bb0ba
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 11 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
index 23fa969edd..d1d47b329b 100644
--- a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
+++ b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
@@ -5662,6 +5662,17 @@ void tst_QVariant::moveOperations()
         QVariant::fromValue(std::move(tester));
         QVERIFY(!tester.wasMoved); // we don't want to move from const variables
     }
+    {
+        QVariant var(std::in_place_type<MoveTester>);
+        const auto p = get_if<MoveTester>(&var);
+        QVERIFY(p);
+        auto &tester = *p;
+        QVERIFY(!tester.wasMoved);
+        [[maybe_unused]] auto copy = var.value<MoveTester>();
+        QVERIFY(!tester.wasMoved);
+        [[maybe_unused]] auto moved = std::move(var).value<MoveTester>();
+        QVERIFY(tester.wasMoved);
+    }
 }
 
 class NoMetaObject : public QObject {};