Can anyone solve this problem with an extension for classes (non final) and return type from a prototype requirement on a static function?

OnTheNatureOfSelf.swift

import Foundation

public protocol BigData {
    static func cache(from path:String)->Self
}

extension String : BigData {
    public static func cache(from path: String) -> String {
        return (try? String(contentsOfFile: path)) ?? "Could not load"
    }
}

class MyDatabase {
    
}

extension MyDatabase : BigData {
    // static func cache(from path: String) -> BigData doesn't work, 'MyDatabase' doesn't conform to protocol "BigData"
    static func cache(from path: String) -> Self {
        // return MyDatabase() as Self doesn't work: 'Self' is only available in a protocol or as the result of a method in a class; did you mean 'MyDatabase'?
        return MyDatabase()         // Cannot convert return expression of type 'MyDatabase' to return type 'Self'
    }
}

// I don't want the consumer of this API to have to cast...
let stringCache     : String     = String.cache(from: "/SomeFile.db")
let databaseCache   : MyDatabase = MyDatabase.cache(from: "/SomeOtherFile.db")

You can use an associated type (thanks: https://twitter.com/XmasRights/status/995030099698348033) but then you can't pass around the protocol, only specific satisfactions of it https://gist.github.com/SwiftStudies/5a8e4d958ad526fbf82fc7038caabd3f/d62472907e37ce67bb99376fe90fd4607f740425