JSONの中身が配列の時はdecodeのTypeに[T].selfを渡せばいいっぽい

CodableForArrayJson.swift

let data = """
[
    {"id": 1, "name": "yutaro"},
    {"id": 2, "name": "yutailang"}
]
""".data(using: .utf8)!

struct User: Codable, CustomStringConvertible {
    let id: Int
    let name: String
    
    var description: String {
        return "User id: \(id) name:\(name)"
    }
}

do {
    let users = try JSONDecoder().decode([User].self, from: data)
    users.forEach({ print($0) })
} catch {
    print(error)
}