improve SingletonTrait compatibility

phpstan.neon

@@ -7,5 +7,6 @@ parameters:
     - '#Used constant OS_TYPE_(LINUX|WINDOWS) not found#'
     - '#Constant .* not found#'
     - '#PHPDoc tag @throws with type Psr\\Container\\ContainerExceptionInterface is not subtype of Throwable#'
+    - '#Unsafe usage of new static#'
   dynamicConstantNames:
     - SWOOLE_VERSION

src/ZM/Utils/SingletonTrait.php

@@ -1,30 +1,34 @@
 <?php
 
-/** @noinspection PhpUnused */
-
 declare(strict_types=1);
 
 namespace ZM\Utils;
 
 trait SingletonTrait
 {
+    /**
+     * @deprecated 将会于未来版本移除
+     *
+     * @var array
+     */
     protected static $cached = [];
 
     /**
-     * @var self
+     * @var null|static
      */
-    private static $instance;
+    protected static $instance;
 
     /**
-     * @return self
+     * 获取类实例
-     * @noinspection PhpMissingReturnTypeInspection
+     *
+     * @return static
      */
     public static function getInstance()
     {
-        if (self::$instance === null) {
+        if (is_null(static::$instance)) {
-            self::$instance = new self();
+            static::$instance = new static();
         }
 
-        return self::$instance;
+        return static::$instance;
     }
 }