import Foundation

class User: NSObject {
}

@objc extension User {
    var name: String {
        get {
            return "No name"
        }
        set {
            // Nothing
        }
    }
}

Stack dump:
0.  While type-checking extension of User at /var/folders/lk/wf8805dn77s3hjyqm399tnqc0000gp/T/playground1-cc22d8..swift:14:7
1.  While type-checking declaration 0x7f96fe396f40 at /var/folders/lk/wf8805dn77s3hjyqm399tnqc0000gp/T/playground1-cc22d8..swift:15:2
2.  While validating 'name' at /var/folders/lk/wf8805dn77s3hjyqm399tnqc0000gp/T/playground1-cc22d8..swift:15:6
3.  While validating materializeForSet for name at /var/folders/lk/wf8805dn77s3hjyqm399tnqc0000gp/T/playground1-cc22d8..swift:15:6
4.  While type-checking materializeForSet for name at /var/folders/lk/wf8805dn77s3hjyqm399tnqc0000gp/T/playground1-cc22d8..swift:15:6
 
Assertion failed: (FD->isGetterOrSetter() && "missing diags for other accessors"), function isRepresentableInObjC, file /Users/buildnode/jenkins/workspace/oss-swift-4.0-package-osx/swift/lib/Sema/TypeCheckType.cpp, line 3558.