Yaml: Escape leading At-Char

Some string-values want to have an At-Char in the beginning.
This would lead to a service
and an exception when this service does not exist.
To allow an At-Char for String-Values we consider "@@" as escaping
in the beginning of the string.

lib/ServiceDefinition.php

@@ -139,6 +139,9 @@ private function resolveParameter(Container $pimple, string $parameter)
         switch ($parameter[0]) {
             case '@':
                 $argument = substr($parameter, 1);
+                if ('@' === $argument[0]) {
+                    return $argument;
+                }
 
                 if (false === empty($this[Services::LAZY_ARGS])) {
                     return new LazyPimple($pimple, $argument);

opt/doc/Primitives/YamlExpressions/EscapingTest.php

@@ -0,0 +1,89 @@
+<?php
+/* vim: set expandtab tabstop=4 shiftwidth=4 softtabstop=4: */
+
+/**
+ * EscapingTest.php
+ *
+ * LICENSE: This source file is created by the company around M. Pretzlaw
+ * located in Germany also known as rmp-up. All its contents are proprietary
+ * and under german copyright law. Consider this file as closed source and/or
+ * without the permission to reuse or modify its contents.
+ * This license is available through the world-wide-web at the following URI:
+ * https://rmp-up.de/license-generic.txt . If you did not receive a copy
+ * of the license and are unable to obtain it through the web, please send a
+ * note to [email protected] so we can mail you a copy.
+ *
+ * @package   wp-di
+ * @copyright 2021 Pretzlaw
+ * @license   https://rmp-up.de/license-generic.txt
+ */
+
+declare(strict_types=1);
+
+namespace RmpUp\WpDi\Test\Primitives\YamlExpressions;
+
+use RmpUp\WpDi\Test\AbstractTestCase;
+use RmpUp\WpDi\Test\Mirror;
+
+/**
+ * Escaping
+ *
+ * Usually the "@" sign indicates a service as shown below.
+ * But when a string shall start with an @ it needs to be escaped using "@@":
+ *
+ * ```yaml
+ * parameters:
+ *   the_param: '@Parameter do not need such escaping@'
+ *
+ * services:
+ *   SomeThing: ~
+ *
+ *   SomeThingElse:
+ *     arguments:
+ *       - '@SomeThing'
+ *       - '@@twig-namespace/foo/bar.html.twig'
+ *       - '%the_param%'
+ * ```
+ *
+ * This only needs to be done in the beginning
+ * while any other @ character does not need to be escaped afterwards.
+ * We do this a bit inconsistent because the YAML should be close to the
+ * well known Symfony services YAML which has the same inconsistency:
+ * https://github.com/symfony/symfony/pull/7357/files#diff-942ccd0095dbe8010523f440b62cc19bc9b56357ea3a6f78b5a9e86faf56be38R293
+ * Parameters do not need escaping of the first At-Char.
+ *
+ * @copyright 2021 Pretzlaw (https://rmp-up.de)
+ */
+class EscapingTest extends AbstractTestCase
+{
+    protected function setUp()
+    {
+        parent::setUp();
+
+        $this->registerServices();
+    }
+
+    public function testDoubleAtCharHasBeenEscaped()
+    {
+        /** @var Mirror $service */
+        $service = $this->pimple[\SomeThingElse::class];
+
+        static::assertEquals('@twig-namespace/foo/bar.html.twig', $service->getConstructorArgs()[1]);
+    }
+
+    public function testSingleAtCharResolvesToService()
+    {
+        /** @var Mirror $service */
+        $service = $this->pimple[\SomeThingElse::class];
+
+        static::assertSame($this->pimple[\SomeThing::class], $service->getConstructorArgs()[0]);
+    }
+
+    public function testParameterStayedTheSame()
+    {
+        /** @var Mirror $service */
+        $service = $this->pimple[\SomeThingElse::class];
+
+        static::assertSame('@Parameter do not need such escaping@', $service->getConstructorArgs()[2]);
+    }
+}