This changes enables passing stateful lambdas to qmlRegisterSingletonType, which helps when porting away from setContextProperty. Unfortunately, we cannot directly add an overload for std::function, as this causes ambiguity in the overload set when a lambda of the form auto f = [](QQmlEngine*, QJSEngine*) -> QObject* is passed to the function (which is what the examples do) We therefore use a template to support abribtrary callables f, then SFINAE them out if f is not convertible to the desired std::function, or when f is convertible to a plain C function pointer, thus removing the ambiguity Change-Id: I6ca95ad692d8bb785e420b85bf3d8c1d0007ce17 Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>