Soundness hole in isolated protocol conformances

Smaller version of test function gets to crash it either for me:

func test() {
    let s = S()
    func open(_ foo: some Foo) {
        foo.bar()
    }
    open(s)
}

results in Fatal error: Incorrect actor executor assumption; Expected 'UnownedSerialExecutor(executor: (Opaque Value))' executor.