I don't think that would work in general: trait methods can have signatures for which you can't synthesize an implementation for the sum type (something that ta

I don't think that would work in general: trait methods can have signatures for which you can't synthesize an implementation for the sum type (something that takes a second Self as an argument, like std::ops::Add, comes to mind). For object-safe traits where this would be possible, you can at least do this at the cost of an allocation, as you probably know: fn some_func() -> Box<dyn SomeTrait> { if some_condition { Box::new(a) } else { Box::new(b) } }