Allow lazy vars on immutable structs?

I can't access a lazy var on a struct, with an error about accessing a mutable getter:

struct TestModel: Decodable {
    let longitude: Double
    let latitude: Double

    lazy var coordinate: CLLocationCoordinate2D = {
        return CLLocationCoordinate2D(latitude: self.latitude, longitude: self.longitude)

let jsonString = "{ \"longitude\": 45.0, \"latitude\": 70.0 }"
let data = .utf8)!
let testModel = try? JSONDecoder().decode(TestModel.self, from: data)

print(testModel!.coordinate) // error: cannot use mutating getter on immutable value: 'testModel' is a 'let' constant

I understand that the first time accessing coordinate, you've updated the storage associated with the struct, however from a programmer's prospective I don't believe the lazy var should be considered to mutate the struct. Is there something more to this or is this a good evolution candidate?

See the recent conversation on this: Lazy initialization for constants.

Alternatively, is there a way to set coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude) at init, without having to implement init(from decoder: Decoder) from scratch?