[cwillu]

If I make changes to a GPL3 library, but never distribute it, and you come along and leak it, you've distributed my changes without a license. Those changes are toxic, and the result cannot be distributed by anyone. It's not really any different than the usual dance one has to perform to combine two components with incompatible licenses: you can mix the ingredients yourself, you can tell people how to mix the ingredients themselves, but you can't distribute the cake.