Program is crashing when trying to access sliced data via subscript

[flexlixrup]

Describe the bug

When trying to access a Data slice via a subscript, the program crashes with Signal 5. When I first copy the slice into new Data access works

To Reproduce
Sample code to reproduce the behavior.

import Foundation

let data = Data([0x06, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D])
let slice = data[1 ..< 14]
let sliceNew = Data(slice)
let notCrashing = sliceNew[0]
print("Not crashing correct: \(notCrashing)")
let crashing = slice[0]
print("Crashing: \(crashing)")

Expected behavior

We don't crash when we access the data slice

Configuration (please complete the following information):

Apple Swift version 6.2.1 (swift-6.2.1-RELEASE)
Target: arm64-apple-macosx26.0
Build config: +assertions
Darwin Felix-MacBook.local 25.1.0 Darwin Kernel Version 25.1.0: Mon Oct 20 19:32:41 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6000 arm64

[jmschonfeld]

This is expected behavior as technically index 0 is out of bounds in the slice. While Data uses integer indices (so you can subscript via an integer literal like someData[0], it does not guarantee that 0 is the start index. In particular, when you slice a Data, since Data is a self-slicing type you get back a Data that shares the same index values as the original Data, just with a subset of valid indices. In other words, in your example slice shares the same indices as data but only indices 1 ..< 14 are valid. You'll notice that slice.startIndex is 1 and not 0, indicating that the first element of slice is identified by index 1.

This is definitely something that's not easily noticeable in the API surface and often catches people by surprise, but in general the main consideration to keep in mind is that Data's startIndex is not guaranteed to be 0. The same type of behavior is exhibited by array:

var array = [1, 2, 3, 4]
var slice = array[1 ..< 3]
print(slice.startIndex) // 1
slice[0] // Crash

But Array has a different slice type (ArraySlice) which makes the API surface a bit clearer about when you're potentially working with a slice / when indices might not start at 0.

[flexlixrup]

Thanks for the clarification. I think this should be added as a note in the documentation that the return type is technically a DataSlice and not Data itself, so we know, that it behaves the same way as Arrays.