<?php

namespace React\Promise;

interface CancellablePromiseInterface
{
    /**
     * @return void
     */
    public function cancel();
}

<?php

namespace React\Promise;

interface CancellablePromiseInterface
{
    public function cancel();
}

--- 
+++ 
@@ -4,5 +4,8 @@
 
 interface CancellablePromiseInterface
 {
+    /**
+     * @return void
+     */
     public function cancel();
 }
Add return tag to cancel()
