Data slicing a slice produces incorrect Data

[jasonbobier]

Description

Passing sliced data to a function that slices further doesn't result in a correct data slice.

Reproduction

let d = Data([0x02, 0x00, 0x00, 0x00, 0x00, 0x00])

func test(data: Data) {
	let d1 = data[4...]  // this should be 0 bytes, but is 2 bytes
}

test(data: d[2...])

Expected behavior

Slicing would work when slicing a slice of Data since it returns Data.

Environment

Xcode 16 RC

Additional information

No response

[oscbyspro]

A Data subsequence shares indices with its base sequence, so the current behavior is correct. Subsequence index sharing is required by protocols such as Collection. data[4...] requests a subsequence starting at index 4. data.dropFirst(4) is what you're looking for.

let data = Data(repeating: 0, count: 12)
print(data[2...].startIndex) // 2
print(data[2...][4...].startIndex) // 4
print(data[2...].dropFirst(4).startIndex) // 6

[jasonbobier]

This doesn't seem correct because func test(data: Data) could be anywhere. It just takes a Data and has no idea that it is a slice. From its point of view, it is just Data to take bytes from. The other sequences deal with this by having dedicated slice types.

I would argue that the slicing behavior isn't a very good design, but in Data's case, there isn't even a type difference.

[jasonbobier]

(btw... this just bit me because I spaced wrapping the Data slice in a Data and passed it on. The compiler was completely fine with it and then produced the wrong values several layers down in a common function that expected Data.)