[swift-evolution] Question about SE-0057 "Importing Objective-C Lightweight Generics"

[Michael Peternell]

https://github.com/apple/swift-evolution/blob/master/proposals/0057-importing-objc-generics.md

How can Swift know if an Objective-C class implements +/- classForGenericArgumentAtIndex: at compile time? Does it look at the object code? Or does it look at the header file? Or maybe there is a third option?

I'm asking, because this may lead to misunderstandings if the method needs to be there in the header file, but Objective-C programmers are not aware that they have to declare that method.

-Michael