QVariant::fromValue<T>: require T to be copy constructible

In Qt 5, QVariant::fromValue<T> would not compile unless
Q_DECLARE_METATYPE(T) was used, and Q_DECLARE_METATYPE(T) would lead to
a compile error if T were not copy constructible.
In Qt 6, we do not require Q_DECLARE_METATYPE before using fromValue,
and QMetaType itself works with non-copy constructible types just fine.
However, QVariant still requires it, thus we need to now enforce this in
fromValue itself.

Change-Id: Ib6964a438d8c46033dd3a037b9d871de2b42e175
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qvariant.h

@@ -392,7 +392,12 @@ class Q_CORE_EXPORT QVariant
     }
 
     template<typename T>
+#ifndef Q_CLANG_QDOC
+    static inline auto fromValue(const T &value) ->
+    std::enable_if_t<std::is_copy_constructible_v<T>, QVariant>
+#else
     static inline QVariant fromValue(const T &value)
+#endif
     {
         return QVariant(QMetaType::fromType<T>(), std::addressof(value));
     }

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

@@ -68,6 +68,21 @@
 
 class CustomNonQObject;
 
+template<typename T, typename  = void>
+struct QVariantFromValueCompiles
+{
+    static inline constexpr bool value = false;
+};
+
+template<typename T>
+struct QVariantFromValueCompiles<T, std::void_t<decltype (QVariant::fromValue(std::declval<T>()))>>
+{
+    static inline constexpr bool value = true;
+};
+
+static_assert(QVariantFromValueCompiles<int>::value);
+static_assert(!QVariantFromValueCompiles<QObject>::value);
+
 class tst_QVariant : public QObject
 {
     Q_OBJECT