Add generics for Promise.

src/Executor/Promise/Promise.php

@@ -9,6 +9,8 @@
 
 /**
  * Convenience wrapper for promises represented by Promise Adapter.
+ *
+ * @template T
  */
 class Promise
 {
@@ -30,6 +32,19 @@ public function __construct($adoptedPromise, PromiseAdapter $adapter)
         $this->adapter = $adapter;
     }
 
+    /**
+     * @template TFulfilled of mixed
+     * @template TRejected of mixed
+     *
+     * @param (callable(T): (Promise<TFulfilled>|TFulfilled))|null $onFulfilled
+     * @param (callable(mixed): (Promise<TRejected>|TRejected))|null $onRejected
+     *
+     * @return Promise<(
+     *   $onFulfilled is not null
+     *     ? ($onRejected is not null ? TFulfilled|TRejected : TFulfilled)
+     *     : ($onRejected is not null ? TRejected : T)
+     * )>
+     */
     public function then(?callable $onFulfilled = null, ?callable $onRejected = null): Promise
     {
         return $this->adapter->then($this, $onFulfilled, $onRejected);