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>

2 files changed, 24 insertions, 3 deletions

diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index 8e2457350c..30da12c95b 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -2722,6 +2722,12 @@ QT_WARNING_POP
 */
 
 /*!
+    \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
 
diff --git a/src/corelib/kernel/qvariant.h b/src/corelib/kernel/qvariant.h
index 582f135fab..fe99ad6bdd 100644
--- a/src/corelib/kernel/qvariant.h
+++ b/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)