[swift-evolution] Default Generic Arguments

[Alexis]

I don’t have much skin in the nuance of PU vs DWIM since, as far as I can tell, it’s backwards compatible to update from PU to DWIM. So we could conservatively adopt PU and then migrate to DWIM if that's found to be intolerable. I expect it will be intolerable, though.

Also, language subtlety thing here: there’s lots of things which are *strictly* source breaking changes, but tend to work out 99% of the time anyway because of things like inference. I’m not at all opposed to making things work out 99.9% of the time instead. For instance, if I changed the Iterator type some collection yielded, almost no one would notice because they just pass it into a for loop or call a standard Sequence method on it. Still, strictly a source breaking change. Someone’s code could stop compiling.