Non-copyable: compiler produces a copy of non-copyable value returned by pointer using 'unsafeAddress' accessor in a subscript

[hyp]

Description

Using the unsafeAddress accessor in a subscript that returns a non-copyable type makes Swift copy the value when it's returned by a subscript get access that intends only to borrow the value. The following error is produced:

error: copy of noncopyable typed value. This is a compiler bug. Please file a bug with a small example of the bug
    _ = borrowSwiftNC(holder[0])
                            ^

This accessor is needed for C++ interop, to support use of non-copyable return references in accessors that get synthesized for an imported operated [].

Reproduction

public struct SwiftNonCopyable: ~Swift.Copyable {
    let x: Int
}

public struct SwiftNonCopyableHolderConstSubscript: ~Swift.Copyable {
    let x: SwiftNonCopyable
}

extension SwiftNonCopyableHolderConstSubscript {
    subscript(_ i: Int) -> SwiftNonCopyable {
        unsafeAddress {
            var ptr: UnsafePointer<SwiftNonCopyable>?
            withUnsafePointer(to: self.x) {
                ptr = $0
            }
            return ptr!
        }
    }
}

func borrowSwiftNC(_ x: borrowing SwiftNonCopyable) -> Int {
    return x.x
}

func test() {
    let holder = SwiftNonCopyableHolderConstSubscript(x: SwiftNonCopyable(x: 23))
    _ = borrowSwiftNC(holder[0])
}

test()

Expected behavior

The code should compile, as it should be okay to borrow the address value returned from the subscript accessor.

Environment

Swift main @:

commit 7680332 (origin/main, origin/HEAD)
Merge: 975519b 46c4da3
Date: Thu Nov 16 10:06:03 2023 -0800

Additional information

No response

[hyp]

Note : the sample code needs -enable-experimental-feature NoncopyableGenerics at the moment.

[hyp]

It also works for property accessors, but not for subscripts :(