Why does String.subscript(_:) require the types `String.Index` and `Int` to be equal when there is no `Int` involved?

Question

I fail to understand the problem Xcode is confronting me with in this line:

iteration.template = template[iterationSubstring.endIndex...substring.startIndex]

template is a String and iterationSubstring and substring are Substrings of template. Xcode highlights the opening square bracket with the following message:

Subscript 'subscript(_:)' requires the types 'Substring.Index' and 'Int' be equivalent

The error message does not make any sense to me. I try to obtain a Substring by creating a Range<String.Index> with the [template.startIndex...template.endIndex] subscript. How is this related to Int? And why does the same pattern work elsewhere?

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Xcode playground code reproducing the problem:

import Foundation
let template = "This is an ordinary string literal."

let firstSubstringStart = template.index(template.startIndex, offsetBy: 5)
let firstSubstringEnd = template.index(template.startIndex, offsetBy: 7)
let firstSubstring = template[firstSubstringStart...firstSubstringEnd]

let secondSubstringStart = template.index(template.startIndex, offsetBy: 10)
let secondSubstringEnd = template.index(template.startIndex, offsetBy: 12)
let secondSubstring = template[secondSubstringStart...secondSubstringEnd]

let part: String = template[firstSubstring.endIndex...secondSubstring.startIndex]

After all I have a template string and two substrings of it. I want to get a String ranging from the end of the first Substring to the start of the second Substring.

Answer 1

The current version of Swift works with the Substring struct which is a sliced String.

The error seems to be misleading and occurs if you are going to assign a (range-subscripted) Substring to a String variable.

To fix the error create a String from the Substring

iteration.template = String(template[iterationSubstring.endIndex...substring.startIndex])

Nevertheless you are strongly discouraged from creating ranges with indices from different strings (iterationSubstring and substring). Slice the main string, the indices are preserved.

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The crash in the second (meanwhile deleted) example occurred because the last character of a string is at index before endIndex, it's

template[template.startIndex..<template.endIndex] 

or shorter

template[template.startIndex...]