The printout for after may indicate an additional issue – U+00A9 is the copyright symbol; what is it doing there?

after: String = "\u{a9}"

@lorentey A9 is the last byte of the UTF-8 sequence, so that's where it's coming from:

café.utf8.map { String($0, radix: 16, uppercase: true) }
// ["63", "61", "66", "C3", "A9"]

But if you print after it will (correctly) print � (the replacement character for an ill-formed sequence), not the copyright symbol. Maybe the debugger is using a different way to show a String's contents?

It definitely seems like a bug that café[index...] can produce an invalid string that contains just a single UTF-8 continuation byte.

Beyond that, the printing of "\u{a9}" in the swift LLDB repl seems like a separate bug. I believe this is using a custom LLDB formatter, so that must be buggy in the presence of invalid UTF-8 sequences, but it seems the debugDescription implementation is also buggy because passing before and after to debugPrint produces

"cafÀ"
"©"

Granted, I'm not quite sure what this should print, because there's no way to write a string literal that would reproduce the original malformed string, but the best option is probably to just use U+FFFD replacement characters instead of this (the © appears to be interpreting the a9 byte as U+A9, but the À is mystifying; the UTF-8 byte here is c3 but À is U+C0).

It seems String.UTF16View.index(before:) is also not compatible between Swift 4.x and Swift 5.

$ pbpaste
func printUnicodeScalars<S: StringProtocol>(_ string: S) {
    print(string.unicodeScalars.map {
        "\\u{\(String($0.value, radix: 16))}"
    }.joined())
}
// https://emojipedia.org/family-man-woman-girl-boy/
let family = "\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}\u{1f466}"
printUnicodeScalars(family)

let utf16Index = family.utf16.index(before: family.utf16.endIndex)
let utf16Before = String(family[..<utf16Index])
printUnicodeScalars(utf16Before)

let utf16After = String(family[utf16Index...])
printUnicodeScalars(utf16After)

printUnicodeScalars(utf16Before + utf16After)
print(utf16After.utf16.count)
$ pbpaste|xcrun --toolchain org.swift.42120181030a swift -
\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}\u{1f466}
\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}\u{fffd}
\u{fffd}
\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}\u{1f466}
1
$ pbpaste|xcrun --toolchain org.swift.5020190129a swift -
\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}\u{1f466}
\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}
\u{1f466}
\u{1f468}\u{200d}\u{1f469}\u{200d}\u{1f467}\u{200d}\u{1f466}
2

filed separated issue https://bugs.swift.org/browse/SR-9820 for String.UTF16View.index(before:)

UnicodeScalar boundaries are:

• respected by:

-   `String.UTF8View.index(before:)` on Swift 4

-   `String.UTF16View.index(before:)` on Swift 5

• not respected by

-   `String.UTF16View.index(before:)` on Swift 4

-   `String.UTF8View.index(before:)` on Swift 5

At the same time Native Encoding is switched from UTF-16 to UTF-8, the behavior is also swapped.

It seems I misunderstood.
The problem is how does subscript handle Range with Index that is not aligned with unicodeScalar, not index(before:).

Did this ever get resolved? @lorentey, @milseman

Not yet, I can look at it this week.

(I am investigating this and a whole slew of issues as part of https://bugs.swift.org/browse/SR-10124)

#23834