QVariant: add rvalue overload of fromStdVariant()

Extract Method fromStdVariantImpl() to share the otherwise
more-or-less identical implementation between the two overloads.

Don't use a constrained template version of fromStdVariantImpl() as
fromStdVariant(), because the constraint would have to be very complex
to continue allowing subclasses of std::variant to be passed.

As a drive-by, mark the valueless_by_exception() path Q_UNLIKELY.

[ChangeLog][QtCore][QVariant] Added overload of fromStdVariant()
taking rvalue std::variant<>s.

Fixes: QTBUG-114134
Change-Id: Ia1c7ae93ab421e6689dc9f2d8d0c2295b23cbbf6
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qvariant.cpp

@@ -2721,6 +2721,12 @@ QT_WARNING_POP
     \sa fromValue()
 */
 
+/*!
+    \fn template<typename... Types> QVariant QVariant::fromStdVariant(std::variant<Types...> &&value)
+    \since 6.6
+    \overload
+*/
+
 /*!
     \fn template<typename T> T qvariant_cast(const QVariant &value)
     \relates QVariant

src/corelib/kernel/qvariant.h

@@ -569,9 +569,13 @@ public:
     template<typename... Types>
     static inline QVariant fromStdVariant(const std::variant<Types...> &value)
     {
-        if (value.valueless_by_exception())
-            return QVariant();
-        return std::visit([](const auto &arg) { return QVariant::fromValue(arg); }, value);
+        return fromStdVariantImpl(value);
+    }
+
+    template<typename... Types>
+    static QVariant fromStdVariant(std::variant<Types...> &&value)
+    {
+        return fromStdVariantImpl(std::move(value));
     }
 
     template<typename T>
@@ -585,6 +589,17 @@ public:
     static QPartialOrdering compare(const QVariant &lhs, const QVariant &rhs);
 
 private:
+    template <typename StdVariant>
+    static QVariant fromStdVariantImpl(StdVariant &&v)
+    {
+        if (Q_UNLIKELY(v.valueless_by_exception()))
+            return QVariant();
+        auto visitor = [](auto &&arg) {
+            return QVariant::fromValue(q23::forward_like<StdVariant>(arg));
+        };
+        return std::visit(visitor, std::forward<StdVariant>(v));
+    }
+
     friend inline bool operator==(const QVariant &a, const QVariant &b)
     { return a.equals(b); }
     friend inline bool operator!=(const QVariant &a, const QVariant &b)

tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp

@@ -5392,6 +5392,16 @@ void tst_QVariant::shouldDeleteVariantDataWorksForAssociative()
 
 void tst_QVariant::fromStdVariant()
 {
+#define CHECK_EQUAL(lhs, rhs, type) do { \
+        QCOMPARE(lhs.typeId(), rhs.typeId()); \
+        if (lhs.isNull()) { \
+            QVERIFY(rhs.isNull()); \
+        } else { \
+            QVERIFY(!rhs.isNull()); \
+            QCOMPARE(get< type >(lhs), get< type >(rhs)); \
+        } \
+    } while (false)
+
     {
         typedef std::variant<int, bool> intorbool_t;
         intorbool_t stdvar = 5;
@@ -5399,21 +5409,38 @@ void tst_QVariant::fromStdVariant()
         QVERIFY(!qvar.isNull());
         QCOMPARE(qvar.typeId(), QMetaType::Int);
         QCOMPARE(qvar.value<int>(), std::get<int>(stdvar));
+        {
+            const auto qv2 = QVariant::fromStdVariant(std::move(stdvar));
+            CHECK_EQUAL(qv2, qvar, int);
+        }
+
         stdvar = true;
         qvar = QVariant::fromStdVariant(stdvar);
         QVERIFY(!qvar.isNull());
         QCOMPARE(qvar.typeId(), QMetaType::Bool);
         QCOMPARE(qvar.value<bool>(), std::get<bool>(stdvar));
+        {
+            const auto qv2 = QVariant::fromStdVariant(std::move(stdvar));
+            CHECK_EQUAL(qv2, qvar, bool);
+        }
     }
     {
         std::variant<std::monostate, int> stdvar;
         QVariant qvar = QVariant::fromStdVariant(stdvar);
         QVERIFY(!qvar.isValid());
+        {
+            const auto qv2 = QVariant::fromStdVariant(std::move(stdvar));
+            CHECK_EQUAL(qv2, qvar, int); // fake type, they're empty
+        }
         stdvar = -4;
         qvar = QVariant::fromStdVariant(stdvar);
         QVERIFY(!qvar.isNull());
         QCOMPARE(qvar.typeId(), QMetaType::Int);
         QCOMPARE(qvar.value<int>(), std::get<int>(stdvar));
+        {
+            const auto qv2 = QVariant::fromStdVariant(std::move(stdvar));
+            CHECK_EQUAL(qv2, qvar, int);
+        }
     }
     {
         std::variant<int, bool, QChar> stdvar = QChar::fromLatin1(' ');
@@ -5421,7 +5448,25 @@ void tst_QVariant::fromStdVariant()
         QVERIFY(!qvar.isNull());
         QCOMPARE(qvar.typeId(), QMetaType::QChar);
         QCOMPARE(qvar.value<QChar>(), std::get<QChar>(stdvar));
+        {
+            const auto qv2 = QVariant::fromStdVariant(std::move(stdvar));
+            CHECK_EQUAL(qv2, qvar, QChar);
+        }
     }
+    // rvalue fromStdVariant() actually moves:
+    {
+        const auto foo = u"foo"_s;
+        std::variant<QString, QByteArray> stdvar = foo;
+        QVariant qvar = QVariant::fromStdVariant(std::move(stdvar));
+        const auto ps = get_if<QString>(&stdvar);
+        QVERIFY(ps);
+        QVERIFY(ps->isNull()); // QString was moved from
+        QVERIFY(!qvar.isNull());
+        QCOMPARE(qvar.typeId(), QMetaType::QString);
+        QCOMPARE(get<QString>(qvar), foo);
+    }
+
+#undef CHECK_EQUAL
 }
 
 void tst_QVariant::qt4UuidDataStream()