Magic type `super !` that implements every trait

[SkiFire13]

@nik-rev

Since all methods on Value take self as an argument, I was able to provide an impl Value for Infallible like this:

impl Value for Infallible {
    fn as_list(&self) -> Option<impl Iterator<Item = impl Value>> {
        None::<std::iter::Empty<std::convert::Infallible>>
    }

    fn as_map<'a>(&'a self) -> Option<impl Iterator<Item = (Spanned<Key<'a>>, impl Value)>> {
        None::<std::iter::Empty<(Spanned<Key<'a>>, std::convert::Infallible)>>
    }
}

As you can see, In order to provide this impl I once again need to specify a completely arbitrary type that implements those traits for None.

You should be able to implement both bodies as match *self {}.

@nik-rev

Rule 2: You can only use the super ! type in the bodies of trait methods, where the body is unreachable. This should be OK, because unlike free functions: you cannot just use the return type of a particular implementation of a trait method

I'm not sure what makes you think this is the case. In the example above I can just use <Infallible as Value>::as_list to refer to a function that internally returns Option<super!>, then pass that function as parameter to a function that is then able to name the return type and call methods on it, just like it was shown in the other thread.

@nik-rev

What do you think? Could super ! actually exist, or is there some fundamental reason this type is impossible due to something I missed?

IMO something similar to it could exist, in that the compiler could generate a dummy implementation for some traits where it can prove that it is safe, possible to write manually, and not possible to call the autogenerated methods.

I don't believe super! should exist as a type though because it would be able to do things that are unsound for any other type, e.g. implementing conflicting traits.

---

Sidenote: isn't this discussion a duplicate of Should we have another non-type never?, which you even linked to in your first post ? What does it add that it warrants its own discussion?