Do you ever wish for enum subtypes?

I'm prototyping a programming language and considering what features I want to support for union types / enums there. One sentiment I've heard about Rust several times is how nice it would be to be able to "subtype" enums - I think there are even some RFCs or pre-RFCs that try to tackle this. Specifically, given a enum like

enum MyLibraryError {
    InvariantViolation(String),
    OutOfMemory,
    UnknownError
}

You would be able to define its "subtype" like this:

enum KnownErrors = MyLibraryError::{InvariantViolation, OutOfMemory}

My question to y'all: do you ever wish for such a feature? What are the practical problems it would help you solve?

I'd appreciate any thoughts!