Issue Description:

Description

An NSRangeException arises and the application crashes when getting substring with a Swift range via subscript brackets from a specific Foundation's inner string class instance.

This issue occurs only when a compile optimization is enabled and under macOS 10.14. When runs the code without optimization or under macOS 10.15, it works correctly (I haven't tried macOS 10.13 or other environments yet).

Furthermore, if the same code is written in Objective-C it works even under the same conditions. So, something would go wrong with the range translation between Swift <-> ObjC.

Conditions

The conditions I found so far to reproduce this phenomenon are:

• The execution environment is macOS 10.14 (not macOS 10.15).

• Optimization -O is enabled.

• The actual string instance class is not a pure Swift.String or __NSCFString but NSPathStore2 or NSBigMutableString at least.

• The range to obtain substring contains the last index.

• The range to obtain substring was made from a Foundation method (not like string.startIndex..<string.endIndex).

• Obtaining substring with Range<String.Index> via subscript (not PartialRangeFrom<String.Index>).

In addition, the same code works when I used Xcode 10.3 with Swift 5. But I'm not sure from when (from Xcode 11, 11.1, or from Swift 5.1?) it has been broken.

If not satisfy one of those conditions, the code works as excepted without throwing any exception.

Expected Result

Succeed in obtaining substring without any exception.

Steps to Reproduce the Issue

The followings are sample codes to reproduce those issues.

Run it with -O option under macOS 10.14.x.

$swift -O foo.swift

It will crash due to an NSRangeException.

Sample code for NSPathStore2

import Foundation

// create NSPathStore2 instance
let string = NSString(string: "foo").appendingPathComponent("bar")
print((string as NSString).className)  // -> NSPathStore2

let range = string.range(of: "foo/bar")!
let substring = string[range]  // -[NSPathStore2 characterAtIndex:]: index (7) beyond bounds (7)
print(substring)

Sample code for NSBigMutableString

import Foundation
import AppKit.NSTextStorage

// create NSBigMutableString instance via NSTextStorage
let bigString = String(repeating: "a", count: 513)  // 512+1 for NSBigMutableString
let string = NSTextStorage(string: bigString).string
print((string as NSString).className)  // -> NSBigMutableString

let range = string.range(of: "a", options: .backwards)!
let substring = string[range]  // -[NSBigMutableString characterAtIndex:]: Index 513 out of bounds; string length 513'
print(substring)