[omochi@omochi-iMac-PC43 check]$ cat c3.swift
public protocol P { }
private struct Impl: P { }

public struct Vendor {
    private var storage: [Impl] = [Impl()]
    
    public var count: Int {
        return storage.count
    }
    
    public subscript(index: Int) -> some P {
        get {
            return storage[index]
        }
        set (newValue) {
            storage[index] = newValue
        }
    }
}

var vendor = Vendor()
vendor[0] = vendor[2] // ok: can move elements around
[omochi@omochi-iMac-PC43 check]$ swift c3.swift
c3.swift:16:30: error: cannot assign value of type 'some P' to type 'Impl'
            storage[index] = newValue
                             ^~~~~~~~
                                      as! Impl