Yes and no - you can't have bi-variant argument types as shown here and be sound, but that does not mean you can't type those idioms. You just need the type sys

Yes and no - you can't have bi-variant argument types as shown here and be sound, but that does not mean you can't type those idioms. You just need the type system to be able to express a dependency between the name of the event handler and the type of the associated function. It is not exactly straightforward (this is a complex type system feature) though, I don't blame the TS devs for going for an escape hatch there. (Edit: to be clear, allowing to refine function types in the way they do is part of the solution, the unsound part is that it is not checked afaik)