SE-0267 — `where` clauses on contextually generic declarations

[Joe_Groff]

The review period for this proposal ended last Thursday, October 31, 2019. The core team has decided to accept this proposal with one modification. The proposal addresses the issue of conditional protocol requirements by disallowing constraints involving Self from being applied to protocol requirements, but it should also do so for non-final class methods, to avoid the same problem with conditional dynamically-dispatched methods in classes.

Brent Royal-Gordon raised the valid concern that this change exposes more opportunities for surprising ABI breaks:

@beccadax

While this doesn’t change the ABI, I think it will open up new surface area in the ABI, because these two declarations:

struct Box {
    func sequence() -> [Box<Wrapped.Element>] where Wrapped: Sequence { ... }
}

extension Box where Wrapped: Sequence {
    func sequence() -> [Box<Wrapped.Element>] { ... }
}

Will, AFAIK, have different name manglings. In a binary framework, switching from one to the other will be an ABI-breaking change.

However, as Jordan Rose noted, this problem already exists in the mangling implementation today:

@jrose

The whole bottom row represents a second way to write things already expressible by the top-right cell. We didn't bother to do that for the bottom-right row, so we already have this problem today:

extension Foo where T: Hashable {
  func bar<U>(_: U) where T: Comparable {}
}

extension Foo where T: Hashable, T: Comparable {
  func bar<U>(_: U) {} // could be equivalent to above, but isn't
}

SE-0267 exposes the problem in a lot more places, and while there's an obvious way to canonicalize the newly-allowed declarations to an existing syntax (bottom-left cell -> top-right cell*), we have both the top-right and bottom-right cells in existence today.

So this is not a new problem, and the Core Team does not think that SE-267 makes it substantially worse. The ABI concern should not prevent implementation of this proposal.

Thanks to everyone who participated in the review!