类的静态成员变量，为什么这样使用报错

[ch_majia]

import Foundation

print("Hello, World!")

public class wechat: NSObject{
    static var staticSelf:wechat?
    var a:Int = 1;

    override init() {
        super.init()
        wechat.staticSelf = self
    }
    public func loginSucc( ) -> Void {
        //error
        print(wechat.staticSelf.a);
        
        //ok
        if let controller = wechat.staticSelf {
            print(controller.a);
        }
        return
    }
}

为什么error那部分编译错误，下面的ok

[ch_majia]

//
//  main.swift
//  testCMD
//
//  Created by lxlx-cx on 2020/1/14.
//  Copyright © 2020 lxlx-cx. All rights reserved.
//

import Foundation

print("Hello, World!")

public class wechat: NSObject{
    static var staticSelf:wechat{
      set (selfInstance) {
          wechat.staticSelf = selfInstance
      }
      get {
          return wechat.staticSelf
      }
    }
    var a:Int = 1;

    override init() {
        super.init()
        wechat.staticSelf = self
    }
    public func loginSucc( ) -> Void {
        //error
        print(wechat.staticSelf.a);
        
        //ok
        /*
        if let controller = wechat.staticSelf {
            print(controller.a);
        }
        */
        return
    }
}

这样修改后，去掉可选类型也能编译了。