[SR-9955] String.Index.init?<S: StringProtocol>(_ sourcePosition: Swift.String.Index, within target: S) is missing

[norio-nomura]

Previous ID	SR-9955
Radar	rdar://problem/32340799
Original Reporter	@norio-nomura
Type	New Feature
Status	Resolved
Resolution	Done

Environment

swift-5.0-DEVELOPMENT-SNAPSHOT-2019-02-17-a
swift-DEVELOPMENT-SNAPSHOT-2019-02-14-a

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	New Feature
Assignee	@milseman
Priority	Medium

md5: 6ba3b0d9475f51dabd794e1b6e524822

Issue Description:

That makes it impossible to write the code below.

extension StringProtocol where Index == String.Index, UTF16View.Index == String.Index {
    func toUTF16Indices(_ range: NSRange) -> Range<Index>? {
        precondition(range.location != NSNotFound)
        guard let start = utf16.index(utf16.startIndex, offsetBy: range.lowerBound, limitedBy: utf16.endIndex),
            let end = utf16.index(start, offsetBy: range.length, limitedBy: utf16.endIndex),
            let lowerBound = String.Index(start, within: self),
            let upperBound = String.Index(end, within: self)
            else { return nil }
        return Range(uncheckedBounds: (lower: lowerBound, upper: upperBound))
    }
}

[belkadan]

cc @milseman

[milseman]

You filed this as a bug. Is this a regression in 5.0, or is this an API request? The above code emits the same errors for me in 4.2 as in 5.0.

If this is an API request, then I agree that we should make a generic initializer on String.Index over StringProtocol, but we should also make https://github.com/apple/swift/blob/master/stdlib/public/Darwin/Foundation/NSRange.swift#L178 generic, so that you don't even need to write this code.

[norio-nomura]

Sorry, it should be filed as a new feature request that we should make a generic initializer on String.Index over StringProtocol.

so that you don't even need to write this code.

I know that API, but I also encountered to this issue on writing another similar implementation for UTF-8.

extension StringProtocol where Index == String.Index, UTF8View.Index == String.Index {
    func toUTF8Indices(_ range: UTF8Range) -> Range<Index>? {
        precondition(!range.isNotFound)
        guard let start = utf8.index(utf8.startIndex, offsetBy: range.lowerBound, limitedBy: utf8.endIndex),
            let end = utf8.index(start, offsetBy: range.utf8Length, limitedBy: utf8.endIndex),
            let lowerBound = String.Index(start, within: self),
            let upperBound = String.Index(end, within: self)
            else { return nil }
        return Range(uncheckedBounds: (lower: lowerBound, upper: upperBound))
    }
}

For writing above, it needs a generic initializer on String.Index over StringProtocol.

[milseman]

#22869 and https://forums.swift.org/t/pitch-string-gaps-and-missing-apis/20984

[milseman]

SE-0248