encodedOffsetとinit(encodedOffset:)を@available(swift, deprecated: 4.2, obsoleted: 5.0,…)にして、String.Index.utf16CodeUnitOffset(with:), String.Index.utf8CodeUnitOffset(with:), String.Index.init(utf16CodeUnitOffset:within:), String.Index.init(utf16CodeUnitOffset:within:)を新設するPR来た！ https://github.com/apple/swift/pull/22108 (edited)

[WIP DO NOT MERGE][String.Index] Deprecate/obsolete encodedOffset ...

Replace the encodedOffset initializer and var with a pair of initializers and vars specifying the actual encoding. encodedOffset was meant to be used opaquely, but common usage makes assumptions ab...