On Wednesday, 22 May 2019 at 22:33:52 UTC, Alex wrote: > auto x = (GdkEventButton* e, Widget w) ... > X.addOnButtonPress(x); > > Why is x not a delegate? Because you didn't ask for one and it didn't have to be. Just add the delegate keyword to ask for one: auto x = delegate(GdkEventButton* e, Widget w) {}; ^^^^^^^^