Fix attachment page redirect for pretty permalink URLs

redirect_canonical() fails to redirect attachment pages accessed via
pretty permalinks (e.g. /my-image-jpg/) when wp_attachment_pages_enabled
is 0. get_query_var( 'attachment_id' ) is only populated for
?attachment_id=123 URLs, not slug-based URLs.

Falls back to get_queried_object_id() when the query var is empty.

Author: Claude

src/wp-includes/canonical.php

@@ -551,6 +551,9 @@ function redirect_canonical( $requested_url = null, $do_redirect = true ) {
 
 	if ( is_attachment() && ! get_option( 'wp_attachment_pages_enabled' ) ) {
 		$attachment_id        = get_query_var( 'attachment_id' );
+		if ( ! $attachment_id ) {
+			$attachment_id = get_queried_object_id();
+		}
 		$attachment_post      = get_post( $attachment_id );
 		$attachment_parent_id = $attachment_post ? $attachment_post->post_parent : 0;
 		$attachment_url       = wp_get_attachment_url( $attachment_id );

tests/phpunit/tests/canonical/attachmentRedirect.php

@@ -0,0 +1,65 @@
+<?php
+
+/**
+ * Tests for attachment page redirect when wp_attachment_pages_enabled is disabled.
+ *
+ * @group canonical
+ * @group rewrite
+ * @group query
+ */
+class Tests_Canonical_AttachmentRedirect extends WP_Canonical_UnitTestCase {
+
+	/**
+	 * Attachment post object.
+	 *
+	 * @var WP_Post
+	 */
+	public static $attachment;
+
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		// Create a fake attachment (no real file upload needed).
+		$attachment_id = $factory->post->create(
+			array(
+				'post_type'   => 'attachment',
+				'post_title'  => 'Test Image',
+				'post_name'   => 'test-image-jpg',
+				'post_status' => 'inherit',
+				'post_parent' => 0,
+			)
+		);
+
+		// Set a fake attachment URL via metadata.
+		update_post_meta( $attachment_id, '_wp_attached_file', '2025/01/test-image.jpg' );
+
+		self::$attachment = get_post( $attachment_id );
+	}
+
+	/**
+	 * Pretty permalink slug-based attachment URLs should redirect to the file URL
+	 * when wp_attachment_pages_enabled is 0.
+	 *
+	 * This is a regression test: get_query_var( 'attachment_id' ) is only populated
+	 * for ?attachment_id=123 URLs, not slug-based URLs. The fix falls back to
+	 * get_queried_object_id().
+	 */
+	public function test_pretty_permalink_attachment_redirects_when_pages_disabled() {
+		update_option( 'wp_attachment_pages_enabled', 0 );
+		$this->set_permalink_structure( '/%postname%/' );
+
+		$expected_url = wp_get_attachment_url( self::$attachment->ID );
+
+		$this->assertCanonical( '/test-image-jpg/', $expected_url );
+	}
+
+	/**
+	 * Query string ?attachment_id=ID should also redirect when pages are disabled.
+	 */
+	public function test_query_var_attachment_redirects_when_pages_disabled() {
+		update_option( 'wp_attachment_pages_enabled', 0 );
+		$this->set_permalink_structure( '/%postname%/' );
+
+		$expected_url = wp_get_attachment_url( self::$attachment->ID );
+
+		$this->assertCanonical( '/?attachment_id=' . self::$attachment->ID, $expected_url );
+	}
+}