URL.init(string:) handling of IPv6 addresses has changed in Swift 6

[rnro]

Initializing a URL from an IPv6 address in Swift 6 on Linux now does not strip enclosing square-brackets. In earlier Swift versions and on macOS the square-brackets are removed.

As well as being a behavior change this can cause issues when passing the host component on e.g. to inet_pton which doesn't tolerate the square braces.

On Linux Swift 5.10:

  1> import Foundation
  2> let u = Foundation.URL(string: "http://[::1]")
  [snip]
  3> print(u!.host)
Optional("::1")

On Linux Swift 6.0.1:

  1> import Foundation
  2> let u = Foundation.URL(string: "http://[::1]")
  [snip]
  3> print(u!.host)
Optional("[::1]")

[jrflat]

This is expected from the update of URL to RFC 3986, which defines:

host          = IP-literal / IPv4address / reg-name
IP-literal    = "[" ( IPv6address / IPvFuture  ) "]"

with host including the [ and ]. We'd like to maintain this behavior for .host, as it unifies the behaviors of URL.host, URL.host(), and URLComponents.host, but we'll keep an eye out if there's more bincompat issues from this change.

[YOCKOW]

Something about this that may impact existing code is discrepancy between macOS and Linux:
URL(string: "http://[::1]")!.host! is "::1" (without square brackets) on macOS even in Swift 6.

I think we'd better to resolve such discord at least.