#32619: Underscores not allowed in module names in \Magento\Framework\App\Utility\Files::accumulateThemeStaticFiles.

Author: engcom-Foxtrot

dev/tests/integration/testsuite/Magento/Framework/App/Utility/FilesTest.php

@@ -159,4 +159,16 @@ public function testReadLists()
         }
         $this->assertSame($expected, $actual);
     }
+
+    /**
+     * Tests if underscores are allowed in extended static files in theme.
+     *
+     * @magentoComponentsDir Magento/Framework/App/Utility/_files/design
+     * @magentoAppIsolation enabled
+     * @magentoDbIsolation enabled
+     */
+    public function testAllowUnderscoreInExtendedFiles()
+    {
+        $this->model->getStaticPreProcessingFiles('*.less');
+    }
 }

dev/tests/integration/testsuite/Magento/Framework/App/Utility/_files/design/frontend/Test/default/Module_Third_Party/web/css/source/_module.less

@@ -0,0 +1,9 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+use Magento\Framework\Component\ComponentRegistrar;
+
+ComponentRegistrar::register(ComponentRegistrar::THEME, 'frontend/FrameworkUtilityTest/default', __DIR__);

dev/tests/integration/testsuite/Magento/Framework/App/Utility/_files/design/frontend/Test/default/registration.php

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+-->
+<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
+    <title>Test Default</title>
+</theme>