[SR-1894] Return type of 'NSCoding.Protocol' is considered ObjC-compatible, but crashes

[belkadan]

Previous ID	SR-1894
Radar	None
Original Reporter	@belkadan
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompilerCrash, StarterBug
Assignee	gal (JIRA)
Priority	Medium

md5: 7c3c3d2da857e9ccac5a0aca9b6a8702

Issue Description:

import Foundation
class Test: NSObject {
  @objc func foo() -> NSCoding.Protocol { return NSCoding.self }
}

Compiling this with Swift 3 (preview 1) leads to a crash in SILGen trying to emit an ObjC thunk for foo. We should probably just not consider this to be Objective-C-compatible; it doesn't buy us much over just returning Protocol (which works fine, though despite the name it's limited to Objective-C protocols).

Banning this means altering the logic in lib/AST/Type.cpp. There's actually an explicit check marked "@objc protocol metatypes"; that entire block can be removed to make the whole function return false.

[belkadan]

Since this bug has a 100%-clear fix and yet doesn't come up very often, let's leave it for a first-time committer to get their bearings!

[swift-ci]

Comment by Aleksey Gaponov (JIRA)

Created a PR

[jckarter]

This is not a bug. Single `@objc` `Protocol` metatypes bridge to ObjC's `Protocol` class.

[jckarter]

Or at least, considering the protocol to be @objc isn't a bug in and of itself; the crash should be fixed.

[jckarter]

It shouldn't lose type information, since the Protocol object knows what protocol it represents, and `*.Protocol` types are unitary in Swift.

[belkadan]

(deleted comment) Yes, but we don't actually implement that, and it loses type information anyway. (We do allow it for classes, though, so we could reconsider.)

(new response) PrintAsObjC has no way to repesent it.

[belkadan]

And as stated above, writing "Protocol" gives you the Objective-C type if you do need Objective-C compatibility.

[jckarter]

I suppose it's of marginal utility; however, as you noted, we do allow this for class metatypes despite there being no way to represent a specific `Class` in ObjC.

[belkadan]

We can always add it back later; right now the easiest and safest thing to do is to ban it.

[belkadan]

Aleksey's PR was merged. :-)