[swift-evolution] Required Callback

[Ben Rimmington]

What are the differences between 
<https://github.com/apple/swift-evolution/pull/502> and 
<https://github.com/apple/swift-evolution/blob/master/proposals/0073-noescape-once.md>
?

application(_:handleEventsForBackgroundURLSession:completionHandler:) 
has an @escaping `completionHandler`, so can @required be enforced?

<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160125/008478.html>

-- Ben