test.php

<?php
namespace Cz\Test\CompletionInClosure;

class Form
{
    public function validate(\Closure $then): void
    {
        $then();
    }
}

class Service
{
    public function process(): void
    {}
}

class Example
{
    /**
     * @var Form
     */
    private $form;

    /**
     * @var Service
     */
    private $service;

    public function __construct()
    {
        $this->form = new Form;
        $this->service = new Service;
    }

    public function run(): void
    {
        // I'm going to uncomment each line `$this->service->`
        // and try to invoke completion for `process()` method.

        $this->form->validate(
            function () {
                // $this->service->
                // ----------------^  autocompletion here will NOT work
            },
        );

        // $this->service->
        // ----------------^  autocompletion here will work
    }
}