Extension types should be able to omit their bodys.

[Cat-sushi]

Currently, extension types can't omit their body.
An extension type declaration must have a body, even if it is empty.

As a smart alternative of typedef, onliner terminated by semicolon seems important.

[Cat-sushi]

Ex.

extension type NaturalNumber(int value);

[lrhn]

Not planned for the first version.

It's something that can be allowed for classes too, and it's a likely change if we get primary constructors for classes.

I can see a use-case for an extension type which has, or adds, no members, as an opaque wrapper that only exists to distinguish a particular role for the representation object.

(It'll only save one character, so it's not going to be a high priority. I'd rather not introduce it for extension types only, because then it feels weird that you can't do the same thing for classes. Mixins without members is technically possible, but it's only a marker, so you could use an interface class instead, enums without a body won't work, extension without a body has no use. So maybe it really is just class.)

[Cat-sushi]

FYI
Primary constructors will be able to omit their empty bodys.

language/working/2364 - primary constructors/feature-specification.md

Line 83 in ed8f940

Note that an empty class body, `{}`, can be replaced by `;`.