swift3compilercrash2.swift

import Foundation

@objc protocol FooProtocol {
    var bar: Set<String> { get set }
}

class Foo : FooProtocol {
    var bar = Set<String>()
}

func toggle(foo: inout FooProtocol, item: String) {
    if !foo.bar.contains(item) {
        foo.bar.insert(item)
    } else {
        foo.bar.remove(item)
    }
}

var a: FooProtocol = Foo()
toggle(foo: &a, item: "x")
print(a.bar)
toggle(foo: &a, item: "x")
print(a.bar)