Add support for converting from std::variant

Adds inline handling of the C++17 type std::variant, so the type will
be resolved if converted into a QVariant.

Change-Id: I31809d70d7f347277389d42a3695836ec7a32d02
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qvariant.cpp

@@ -3892,6 +3892,19 @@ QDebug operator<<(QDebug dbg, const QVariant::Type p)
     \sa setValue(), value()
 */
 
+/*! \fn static inline QVariant fromStdVariant(const std::variant<T, Types...> &value)
+    \since 5.11
+
+    Returns a QVariant with the type and value of the active variant of \a value. If
+    the active type is std::monostate a default QVariant is returned.
+
+    \note With this method you do not need to register the variant as a Qt metatype,
+    since the std::variant is resolved before being stored. The component types
+    should be registered however.
+
+    \sa fromValue()
+*/
+
 /*!
     \fn template<typename T> QVariant qVariantFromValue(const T &value)
     \relates QVariant

src/corelib/kernel/qvariant.h

@@ -53,6 +53,10 @@
 #include <QtCore/qbytearraylist.h>
 #endif
 
+#if QT_HAS_INCLUDE(<variant>) && __cplusplus >= 201703L
+#include <variant>
+#endif
+
 QT_BEGIN_NAMESPACE
 
 
@@ -355,6 +359,16 @@ class Q_CORE_EXPORT QVariant
     static inline QVariant fromValue(const T &value)
     { return qVariantFromValue(value); }
 
+#if QT_HAS_INCLUDE(<variant>) && __cplusplus >= 201703L
+    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 fromValue(arg); }, value);
+    }
+#endif
+
     template<typename T>
     bool canConvert() const
     { return canConvert(qMetaTypeId<T>()); }
@@ -503,6 +517,11 @@ inline QVariant qVariantFromValue(const T &t)
 template <>
 inline QVariant qVariantFromValue(const QVariant &t) { return t; }
 
+#if QT_HAS_INCLUDE(<variant>) && __cplusplus >= 201703L
+template <>
+inline QVariant qVariantFromValue(const std::monostate &) { return QVariant(); }
+#endif
+
 template <typename T>
 inline void qVariantSetValue(QVariant &v, const T &t)
 {

tests/auto/corelib/kernel/qvariant/qvariant.pro

@@ -5,7 +5,8 @@ INCLUDEPATH += $$PWD/../../../other/qvariant_common
 SOURCES = tst_qvariant.cpp
 RESOURCES += qvariant.qrc
 DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0
-qtConfig(c++11): CONFIG += c++11
+qtConfig(c++14): CONFIG += c++14
+qtConfig(c++1z): CONFIG += c++1z
 !qtConfig(doubleconversion):!qtConfig(system-doubleconversion) {
     DEFINES += QT_NO_DOUBLECONVERSION
 }

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

@@ -45,7 +45,9 @@
 #include <limits.h>
 #include <float.h>
 #include <cmath>
-
+#if QT_HAS_INCLUDE(<variant>) && __cplusplus >= 201703L
+#include <variant>
+#endif
 #include <QLinkedList>
 #include <QRegularExpression>
 #include <QDir>
@@ -281,6 +283,8 @@ private slots:
 
     void accessSequentialContainerKey();
 
+    void fromStdVariant();
+
 private:
     void dataStream_data(QDataStream::Version version);
     void loadQVariantFromDataStream(QDataStream::Version version);
@@ -4944,5 +4948,41 @@ void tst_QVariant::accessSequentialContainerKey()
     QCOMPARE(nameResult, QStringLiteral("Seven"));
 }
 
+void tst_QVariant::fromStdVariant()
+{
+#if QT_HAS_INCLUDE(<variant>) && __cplusplus >= 201703L
+    {
+        typedef std::variant<int, bool> intorbool_t;
+        intorbool_t stdvar = 5;
+        QVariant qvar = QVariant::fromStdVariant(stdvar);
+        QVERIFY(!qvar.isNull());
+        QCOMPARE(qvar.type(), QVariant::Int);
+        QCOMPARE(qvar.value<int>(), std::get<int>(stdvar));
+        stdvar = true;
+        qvar = QVariant::fromStdVariant(stdvar);
+        QVERIFY(!qvar.isNull());
+        QCOMPARE(qvar.type(), QVariant::Bool);
+        QCOMPARE(qvar.value<bool>(), std::get<bool>(stdvar));
+    }
+    {
+        std::variant<std::monostate, int> stdvar;
+        QVariant qvar = QVariant::fromStdVariant(stdvar);
+        QVERIFY(!qvar.isValid());
+        stdvar = -4;
+        qvar = QVariant::fromStdVariant(stdvar);
+        QVERIFY(!qvar.isNull());
+        QCOMPARE(qvar.type(), QVariant::Int);
+        QCOMPARE(qvar.value<int>(), std::get<int>(stdvar));
+    }
+    {
+        std::variant<int, bool, QChar> stdvar = QChar::fromLatin1(' ');
+        QVariant qvar = QVariant::fromStdVariant(stdvar);
+        QVERIFY(!qvar.isNull());
+        QCOMPARE(qvar.type(), QVariant::Char);
+        QCOMPARE(qvar.value<QChar>(), std::get<QChar>(stdvar));
+    }
+#endif
+}
+
 QTEST_MAIN(tst_QVariant)
 #include "tst_qvariant.moc"