I actually really like the idea of a license that forces changes to the licensed code to be made available to be merged back upstream, at least in theory. The i

I actually really like the idea of a license that forces changes to the licensed code to be made available to be merged back upstream, at least in theory. The issue I have with the GPL is that it doesn't, in my opinion, do a good enough job of distinguishing between "changes to the licensed code" and "changes to unrelated code which happens to be integrated with the licensed code". I would prefer not to impose restrictions on the latter, because in my mind that effectively restricts what people are allowed to DO with my code by making it harder for them to use it with code that's available under a different license. For example, if I were writing some kind of calendar application which contained a date parsing module within it and someone wanted to copy some code out of that date parsing module and paste it into their own application, I'd be perfectly fine with that. If they later make improvements to that code they copied, I'd prefer it if they'd contribute those changes back, but I absolutely don't want to try to force them to make their entire application open-source, or if it's already open source, to force them to re-license it under my license. I recognize that the line between "unrelated code" and "improvements to my code" can get really blurry, which is why right now I prefer to err on the side of freedom by using permissive licenses like the MIT, but I wouldn't mind using a copyleft license that took greater pains to ensure it isn't "viral" like the GPL.