Is there really no love for macro annotations here? (ping @xeno-by)
It would be straightforward to make the syntax proposed by @galedric work with an @opaque macro to annotate class definitions (which would then expand into opaque types).
@opaque class T(val a: A) { ... } // expands into opaque type with helpers
Why make the spec of the language more and more complicated, when the problem is simply boilerplate, a problem that is basically solved by macro annotations? IMHO the language should continue in its original goal of being scalable, providing simple but powerful tools to be used as primitives for building advanced features –– as opposed to being an ad-hoc assembly of specific features from someone’s wish-list at instant t.