Making a protocol's associated type concrete via inheritance

[Joe_Groff]

The restriction still exists. There's a pitch thread about removing this restriction in general:

Lifting the "Self or associated type" constraint on existentials

When the first seed of Swift came out many years ago, there were technical reasons for the "self or associated type" constraint on protocol existentials: At that time, protocol witness tables did not carry associated type information, so it was impossible to re-open the dynamic type in order to dispatch methods on an existential when a protocol had associated types. This was fixed a while ago in order to allow for recursive protocol constraints, but we held on to the restriction on existentials …

We also have a patch from @anthonylatsis that lifts this restriction specifically on protocols that bind away all of the associated types:

[Sema][AST][WIP] Support existentials with concrete assoc. types

To make this change official, we would still need a proposal to review.