[SE-0352] Amendment to open existential arguments for optional parameters

[DougGregor]

SE-0352 "Implicitly Opened Existentials" has a limitation that prevents the opening of an existential argument when the corresponding parameter is optional. This amendment proposes to change that behavior, so that such a call will succeed when a (non-optional) existential argument is passed to a parameter of optional type:

func acceptOptional<T: P>(_ x: T?) { }
func test(p: any P, pOpt: (any P)?) {
  acceptOptional(p) // currently does not open "p", this amendment will open "p" and bind "T" to its underlying type
  acceptOptiona(pOpt) // does not open "pOpt", because there is no "T" to bind to when "pOpt" is "nil"
}

The rationale for not opening the existential p in the first call was to ensure consistent behavior with the second call, in an effort to avoid confusion. However, experience with implicitly-opened existentials has shown that opening an existential argument in the first case is important, because many functions accept optional parameters. Additionally, the workaround requires a lot of boilerplate, writing a separate generic function.

There is an implementation of this amendment at swiftlang/swift#61321

[rjmccall]

SE-0352 has been released, right? I think this needs to be run as a new proposal.

[DougGregor]

SE-0352 has been released, right? I think this needs to be run as a new proposal.

Yes, SE-0352 was in Swift 5.7. I'll turn this into a standalone proposal.