Add polyfill for get_theme_file_uri()

Author: schlessera

features/bootstrap/support.php

@@ -216,3 +216,49 @@ function check_that_yaml_string_contains_yaml_string( $actual_yaml, $expected_ya
 	return compare_contents( $expected_value, $actual_value );
 }
 
+/*
+ * Add a polyfill for the get_theme_file_uri(), as it is required for the
+ * TwentyTwenty theme's starter content, and will fatal the site if you install
+ * WP 5.3 first (setting TwentyTwenty as the active theme) and then downgrade
+ * to a version of WP lower than 4.7.
+ *
+ * Note: This is a quick fix, and a cleaner solution would be to change the
+ * active theme on downgrading, if the current theme declares it is not
+ * supported.
+ *
+ * See: https://github.com/WordPress/twentytwenty/issues/973
+ */
+if ( ! function_exists( 'get_theme_file_uri' ) ) {
+	/**
+	 * Retrieves the URL of a file in the theme.
+	 *
+	 * Searches in the stylesheet directory before the template directory so themes
+	 * which inherit from a parent theme can just override one file.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @param string $file Optional. File to search for in the stylesheet directory.
+	 * @return string The URL of the file.
+	 */
+	function get_theme_file_uri( $file = '' ) {
+		$file = ltrim( $file, '/' );
+
+		if ( empty( $file ) ) {
+			$url = get_stylesheet_directory_uri();
+		} elseif ( file_exists( get_stylesheet_directory() . '/' . $file ) ) {
+			$url = get_stylesheet_directory_uri() . '/' . $file;
+		} else {
+			$url = get_template_directory_uri() . '/' . $file;
+		}
+
+		/**
+		 * Filters the URL to a file in the theme.
+		 *
+		 * @since 4.7.0
+		 *
+		 * @param string $url  The file URL.
+		 * @param string $file The requested file to search for.
+		 */
+		return apply_filters( 'theme_file_uri', $url, $file );
+	}
+}