Deprecate ! and make Never the bottom type

Evolution Pitches
2

@owenv created a post a while ago to discuss making Never a bottom type.

In the meantime, you can get the same effect if you do something like this:

func ??<T>(lhs: @autoclosure () -> T?, rhs: @autoclosure () -> Never) -> T {
	guard let _lhs = lhs() else {
		rhs()
	}

	return _lhs
}

let offset = "ABC".firstIndex(of: "D") ?? preconditionFailure() // failure

or just rewrite it as guard let offset = letters.firstIndex(of: fragment) else { preconditionFailure() } which is pretty close to the original.

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