Rationale: This order restriction is arbitrary, but it's not harmful, and it eliminates the potential for stylistic debates.
I'm not sure about this for two reasons. Firstly, I believe it's unprecedented in the language to require that a semantically unordered list be placed in an arbitrary order. These concerns are currently, and probably rightfully, left to a linter to enforce. e.g. There's no enforced order for attributes like these:
class C {
@discardableResult @inlinable @objc func f() -> Int { 0 }
@objc @inlinable @discardableResult func g() -> Int { 0 }
// … etc
}
Secondly, there's no proposed general principle here, just “the order is async throws”, which makes it unclear if/how to apply it to other parts of the language (like the above example), hard to remember, and hard to extend in future. What do you do if there's a new word that can go in this position, like a new keyword or a user-defined token (e.g. I think this is where a general effects system would live)? Would the rule be “alphabetical order”? Does this apply to both the definition and the call site? What if the call site words alphabetise in a different order than the definition site words?
I would prefer that these semantically unordered lists of attributes/keywords remain unordered for simplicity.