[SR-2978] "is" or "as?" operator doesn't work correctly for protocol for struct?

[swift-ci]

Previous ID	SR-2978
Radar	None
Original Reporter	mshibanami (JIRA User)
Type	Bug

Attachment: Download

Environment

Xcode 8.3.1 beta (8T29o)

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Bug, Runtime | |Assignee | None | |Priority | Medium | md5: 212788bca1d39fae8e28f366c2274c2d

Issue Description:

See the following code or the attached project.

// ViewController.swift
protocol MyProtocol {}
struct MyStruct: MyProtocol {}

class ViewController: NSViewController {
    @IBOutlet weak var outlineVIew: NSOutlineView!
}

extension ViewController: NSOutlineViewDataSource {

    public func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int {
        return 1;
    }

    public func outlineView(_ outlineView: NSOutlineView, child index: Int, ofItem item: Any?) -> Any {
        return MyStruct()
    }

    public func outlineView(_ outlineView: NSOutlineView, isItemExpandable item: Any) -> Bool {
        return false;
    }
}

extension ViewController: NSOutlineViewDelegate
{
    func outlineView(_ outlineView: NSOutlineView, viewFor tableColumn: NSTableColumn?, item: Any) -> NSView?
    {
        let cellView = outlineView.make(withIdentifier: "cell", owner: self)

        guard item is MyStruct else {
            fatalError("'item' should be a MyStruct instance.")
        }

        guard item is MyProtocol else {
            fatalError("'item' should implement MyProtocol")
        }

        return cellView
    }
}

When I ran this code, it reached to fatalError() in the 2nd guard clause.
I thought this is weird. Because the valuable "item" is a instance of MyStruct, and MyStruct implements MyProtocol.
And if I changed MyStruct to class from struct, It didn't happen.

It also happend If I changed 2nd guard like this.

guard let i = item as? MyProtocol else {
    fatalError("'item' should implement MyProtocol")
}

I'm afraid if this is just a newbee mistake or a AppKit's bug...

[belkadan]

cc @jckarter

[jckarter]

This is probably an instance of the general problem that we don't try to bridge ObjC objects back to Swift structs (or vice versa) when casting to protocol types.