As a library maintainer, you can't copy-paste a line for a type that isn't written yet. As a library user, you can't copy-paste a line for a type you just crea

As a library maintainer, you can't copy-paste a line for a type that isn't written yet. As a library user, you can't copy-paste a line for a type you just created in a library you don't maintain. Being able to do what you describe lets a maintainer create an interface for all the future types that satisfy given requirements.