ExtratcUnkeyContainerToArray.swift

import Foundation

struct S<T> {
	let elements: [T]
}

extension S: Decodable where T: Decodable {
	init(from decoder: Decoder) throws {
		let container = try decoder.unkeyedContainer()
		let s = sequence(state: container, next: {
			$0.isAtEnd ? nil : try? $0.decode(T.self)
		})
		self.init(elements: Array(s))
	}
}

let json = """
[1, 2, 3, 4, 5, 6, 7, 8, 9]
""".data(using: .utf8)!

let result = try JSONDecoder().decode(S<Int>.self, from: json)

print(result)