It looks like 6 years ago I wasn't saying “that's a bad feature,” but ”this is how you do that in Swift as it exists today.” I probably wouldn't have supported adding a feature for it to Swift, though, because in the Swift of 6 years ago, it was purely a bit of nice syntactic sugar.
In Val, we realized that the distinction between projections and functions could help us solve the “property vs method” problem: if you need a projected semantics, you make it a property. If a result with independent lifetime is more appropriate, you make it a function/method. Then the question became how do you make a projection that has no receiver. We have freestanding functions; symmetry kind of demands that there are freestanding subscripts, like min. The case you see in that example is interesting if you consider noncopyable types, which would have to be consumed if you used a function to select the mininum. For a case that's interesting with “normal” types, consider this:
subscript min<T: Comparable>(_ x: inout T, _ y: inout T): inout T {
if y < x { yield y } else { yield x }
}
Now you can write something like this:
min[&x, &y] += 1 // increment the lesser of x and y
Of course in Val you'd define min as a bundle so one implementation could handle all the cases.