Add end-to-end test that __() in error pages translates per URL locale

Renders a fixture template containing a {{ __('...') }} string through the
error controller and asserts it resolves in the locale recovered from the
URL (nl) and the default locale (en). The fixture path is registered on the
existing Twig filesystem loader so no test-only template ships in templates/.

Author: Vondry

tests/php/Controller/Frontend/ErrorControllerTest.php

@@ -18,6 +18,8 @@
 use Symfony\Contracts\Translation\TranslatorInterface;
 use Throwable;
 use Twig\Environment;
+use Twig\Loader\ChainLoader;
+use Twig\Loader\FilesystemLoader;
 
 class ErrorControllerTest extends DbAwareTestCase
 {
@@ -119,6 +121,34 @@ public function testErrorPageRecoversTranslatorLocaleFromPath(): void
         self::assertSame('nl', $translator->getLocale());
     }
 
+    public function testErrorPageTranslatesUnderscoreFunctionInRequestedLocale(): void
+    {
+        // End-to-end: a `{{ __('...') }}` string in the rendered error page must be
+        // translated in the locale recovered from the URL. `http_error.name` is
+        // "Error %status_code%" (en) / "Fout %status_code%" (nl).
+        $this->registerFixtureTemplatePath();
+        $this->setGeneralConfig('notfound', ['locale_probe.html.twig']);
+
+        $body = (string) $this->renderError(new NotFoundHttpException(), '/nl/this-page-does-not-exist')->getContent();
+
+        self::assertStringContainsString('LOCALE_PROBE:Fout 404', $body);
+        self::assertStringNotContainsString('Error 404', $body);
+    }
+
+    public function testErrorPageTranslatesUnderscoreFunctionInDefaultLocale(): void
+    {
+        // The counterpart of the test above: with no locale segment in the URL, the
+        // same `__()` string must fall back to the default locale (en) - and must not
+        // leak a previous request's locale onto the shared translator.
+        $this->registerFixtureTemplatePath();
+        $this->setGeneralConfig('notfound', ['locale_probe.html.twig']);
+
+        $body = (string) $this->renderError(new NotFoundHttpException(), '/this-page-does-not-exist')->getContent();
+
+        self::assertStringContainsString('LOCALE_PROBE:Error 404', $body);
+        self::assertStringNotContainsString('Fout 404', $body);
+    }
+
     public function testNotFoundPageWithNonLocalizedContentTypeDoesNotError(): void
     {
         // The default `notfound` points at `blocks/404-not-found`. The `blocks`
@@ -185,6 +215,31 @@ private function renderError(Throwable $exception, string $path): Response
         }
     }
 
+    /**
+     * Make the `Fixtures/` directory resolvable by name, so a fixture template can be
+     * pointed at via the `notfound` config. Mirrors how Bolt prepends paths to the
+     * existing filesystem loader (see TwigAwareController::setTwigLoader()) rather than
+     * replacing it, so the added path survives rendering.
+     */
+    private function registerFixtureTemplatePath(): void
+    {
+        /** @var Environment $twig */
+        $twig = self::getContainer()->get('twig');
+
+        $loader = $twig->getLoader();
+        $loaders = $loader instanceof ChainLoader ? $loader->getLoaders() : [$loader];
+
+        foreach ($loaders as $candidate) {
+            if ($candidate instanceof FilesystemLoader) {
+                $candidate->addPath(__DIR__ . '/Fixtures');
+
+                return;
+            }
+        }
+
+        self::fail('Could not find a FilesystemLoader to register the fixture template path on.');
+    }
+
     private function getPublishedPage(): Content
     {
         $page = $this->getEm()->getRepository(Content::class)

tests/php/Controller/Frontend/Fixtures/locale_probe.html.twig

@@ -0,0 +1,4 @@
+{# Test fixture for ErrorControllerTest: renders a `__()` string so the test can
+   assert error pages translate via the translator locale recovered from the URL.
+   `http_error.name` is "Error %status_code%" (en) / "Fout %status_code%" (nl). #}
+LOCALE_PROBE:{{ __('http_error.name', {'%status_code%': 404}) }}