I can see the point of explicitly calling out the syntax like this, and, while it doesn’t really feel necessary to me, it doesn’t feel at all onerous either. So if there are people who feel strongly against the single colon and like the double colon, I’m fine with the double colon.

(I’m still not sure we need to do anything. And I don’t love extending dict displays, and think that if we do need a solution, maybe there’s a better one lurking out there that nobody has hit on. But I definitely dislike extending dict displays less than extending some but not other uses of **unpacking, or having a mode switch in the middle of call syntax, or anything else proposed so far.)