In previous phases of this proposal, I've favored the ?? + bottom Never alternative, because it gives us several different constructs like this. However, my thinking has evolved and I now support this proposal.
Instead of writing a lot of boring words about why this proposal is a good idea, I've prepared a visual aid:
Basically, I think that !! guides people into better use of force-unwraps. Merely asking people to "explain why this cannot be nil" may help people learn when a force-unwrap is appropriate and when they should do something else instead.
And that teaching advantage is one that ?? fatalError() can't match. This is why my opinion on !! changed: Even if we supported Never values on the right side of ??—and I still think we should!—the !! operator would still be a good idea purely as a shorthand for a best practice we very strongly recommend.
I have a couple of minor concerns:
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I'm worried that infix
!!may look confusingly similar to infix||. One way to address this would be to change the operator to a single!; that would still be confusable with the|operator, but that one is used much less often. The parallel to??may be more important, though. -
The inability to point to the location of the error correctly is very disappointing. It's something we can try to improve later, though—and I can think of at least two ways to do it besides the proposal's suggestion, one much deeper and the other much hackier.
But despite these small issues, I think this proposal is a great idea and, if it's accepted, I expect to use it pretty often.