Allow non-inherited conformances

Previous discussion (c. 2015): Making protocol conformance inheritance controllable

Sometimes it makes sense for a class to conform to a protocol, but *not* for its subclasses. An example is the ExpressibleBy…Literal protocols. It is entirely reasonable for a superclass to have one stored property, and thus be expressible by a literal, whereas a subclass adds more properties and thus cannot be expressed by a literal.

Here is what I want to write:

class A : ExpressibleByIntegerLiteral {
  var n: Int
  init(n: Int) { self.n = n }

  convenience init(integerLiteral value: Int) {
    self.init(n: value)

class B : A {
  var s: String
  init(n: Int, s: String) {
    self.s = s
    super.init(n: n)

However that doesn’t work. There are multiple errors:

• The protocol requirement init(integerLiteral:) must be required since A is not final.
• Therefore, B must supply its own implementation of init(integerLiteral:).

But B *shouldn’t* provide that initializer, because it is *not* supposed to be expressible by an integer literal!

• • •

It seems to me that when a protocol’s requirements are all initializers (and/or static), it ought to be possible to conform a class to that protocol without requiring that its subclasses also conform.

In the previous thread, @Joe_Groff pointed out that it is also possible for conformance to a protocol with instance-member requirements to be non-inherited, since “instances can implicitly upconvert to their conforming base class”, but I am not certain we want to go that far.

A related issue, also mentioned by @Joe_Groff, is that currently “methods often must return dynamic Self when they'd really prefer to return the base class”.

• • •

Are non-inherited conformances something you would find useful?

Should they be possible for all protocols, or only those with all requirements static / init?

Does it make sense to mark the non-inheritedness on the protocol definition, or on the conforming class?

How important is the dynamic-Self issue to address?

Any other ideas on the matter?