[SR-4451] type(of:) behaviour change from Swift 3.0.2 to Swift 3.1.

Description

import Foundation

internal extension Set where Element: NSObject {
    
    internal func items <T: NSObject> (ofType itemType: T.Type) -> Set<T>? {
        return NSSet(set: self).filtered(using: NSPredicate { (oItem: Any /* Note Any instead of Any? */, _) -> Bool in
            return nil != (type(of: oItem) as? T.Type)  // Swift 3.0.2 => true, Swift 3.1 => false.
            //                        return oItem is T // Swift 3.0.2 => true, Swift 3.1 => true.
        }) as? Set<T>
    }
}

let stringsOfTypeNSObject = (["1", "2"] as Set<NSString>).items(ofType: NSObject.self)

print(stringsOfTypeNSObject ?? []) // Swift 3.0.2 => [1, 2], Swift 3.1 => [].

Contributor guide

Tech stack

swift

Domain

backend

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

half day

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

mostly clear

Prerequisites

Swift basicsunderstanding of dynamic type checkingcompiler build environment

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

60

Research direction

Investigate the change in type(of:) behavior between Swift 3.0.2 and 3.1. Focus on the type checker's handling of optional Any parameters in closures. The relevant code is in the Swift compiler's type checker or SILGen. Look for changes in dynamic casting and metatype extraction for optional values. Use the provided test case to verify.