Fix: static front page handling with custom query vars

src/wp-includes/class-wp-query.php

@@ -1054,7 +1054,22 @@ public function parse_query( $query = '' ) {
 
 			unset( $_query['embed'] );
 
-			if ( empty( $_query ) || ! array_diff( array_keys( $_query ), array( 'preview', 'page', 'paged', 'cpage' ) ) ) {
+			$_query_keys = array_keys( $_query );
+			if ( isset( $GLOBALS['wp'], $GLOBALS['wp_the_query'] )
+				&& $GLOBALS['wp_the_query'] === $this ) {
+				$_routing_query_vars = array();
+
+				if ( ! empty( $GLOBALS['wp']->matched_query ) ) {
+					parse_str( $GLOBALS['wp']->matched_query, $_routing_query_vars );
+				}
+
+				$_query_keys = array_intersect(
+					$_query_keys,
+					array_merge( WP::CORE_PUBLIC_QUERY_VARS, array_keys( $_routing_query_vars ) )
+				);
+			}
+
+			if ( empty( $_query_keys ) || ! array_diff( $_query_keys, array( 'preview', 'page', 'paged', 'cpage' ) ) ) {
 				$this->is_page         = true;
 				$this->is_home         = false;
 				$query_vars['page_id'] = get_option( 'page_on_front' );

src/wp-includes/class-wp.php

@@ -7,6 +7,17 @@
  */
 #[AllowDynamicProperties]
 class WP {
+	/**
+	 * Core public query variables.
+	 *
+	 * Long list of public query variables before plugins and themes can extend
+	 * them.
+	 *
+	 * @since x.x.x
+	 * @var string[]
+	 */
+	const CORE_PUBLIC_QUERY_VARS = array( 'm', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'feed', 'author_name', 'pagename', 'page_id', 'error', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots', 'favicon', 'taxonomy', 'term', 'cpage', 'post_type', 'embed' );
+
 	/**
 	 * Public query variables.
 	 *
@@ -15,7 +26,7 @@ class WP {
 	 * @since 2.0.0
 	 * @var string[]
 	 */
-	public $public_query_vars = array( 'm', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'feed', 'author_name', 'pagename', 'page_id', 'error', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots', 'favicon', 'taxonomy', 'term', 'cpage', 'post_type', 'embed' );
+	public $public_query_vars = self::CORE_PUBLIC_QUERY_VARS;
 
 	/**
 	 * Private query variables.

tests/phpunit/tests/query/conditionals.php

@@ -72,6 +72,148 @@ public function test_page_on_front() {
 		delete_option( 'page_for_posts' );
 	}
 
+	/**
+	 * A custom query var registered via the 'query_vars' filter must not prevent
+	 * the static front page from loading.
+	 *
+	 * @ticket 40521
+	 */
+	public function test_custom_query_var_via_filter_does_not_break_static_front_page() {
+		$page_on_front = self::factory()->post->create( array( 'post_type' => 'page' ) );
+		update_option( 'show_on_front', 'page' );
+		update_option( 'page_on_front', $page_on_front );
+
+		$filter = static function ( $vars ) {
+			$vars[] = 'custom_test_var';
+			return $vars;
+		};
+		add_filter( 'query_vars', $filter );
+
+		try {
+			$this->go_to( '/?custom_test_var=yep' );
+
+			$this->assertQueryTrue( 'is_front_page', 'is_page', 'is_singular' );
+		} finally {
+			remove_filter( 'query_vars', $filter );
+			update_option( 'show_on_front', 'posts' );
+			delete_option( 'page_on_front' );
+		}
+	}
+
+	/**
+	 * A custom query var registered via WP::add_query_var() must not prevent
+	 * the static front page from loading.
+	 *
+	 * @ticket 40521
+	 */
+	public function test_custom_query_var_via_add_query_var_does_not_break_static_front_page() {
+		global $wp;
+
+		$page_on_front = self::factory()->post->create( array( 'post_type' => 'page' ) );
+		update_option( 'show_on_front', 'page' );
+		update_option( 'page_on_front', $page_on_front );
+
+		// Register the var the way a plugin would outside the query_vars filter.
+		$wp->add_query_var( 'custom_test_var' );
+
+		try {
+			$this->go_to( '/?custom_test_var=yep' );
+
+			$this->assertQueryTrue( 'is_front_page', 'is_page', 'is_singular' );
+		} finally {
+			$wp->public_query_vars = array_diff( $wp->public_query_vars, array( 'custom_test_var' ) );
+			update_option( 'show_on_front', 'posts' );
+			delete_option( 'page_on_front' );
+		}
+	}
+
+	/**
+	 * A secondary WP_Query constructed with only a custom var must not be
+	 * coerced into querying page_on_front.
+	 *
+	 * @ticket 40521
+	 */
+	public function test_secondary_query_with_custom_var_not_coerced_to_front_page() {
+		$page_on_front = self::factory()->post->create( array( 'post_type' => 'page' ) );
+		update_option( 'show_on_front', 'page' );
+		update_option( 'page_on_front', $page_on_front );
+
+		$filter = static function ( $vars ) {
+			$vars[] = 'custom_test_var';
+			return $vars;
+		};
+		add_filter( 'query_vars', $filter );
+
+		try {
+			// Establish a proper main-request context.
+			$this->go_to( '/' );
+			$secondary = new WP_Query( array( 'custom_test_var' => 'yep' ) );
+
+			$this->assertFalse( $secondary->is_page(), 'Secondary query must not be flagged as is_page().' );
+			$this->assertFalse( $secondary->is_front_page(), 'Secondary query must not be flagged as is_front_page().' );
+			$this->assertNotEquals( $page_on_front, $secondary->get_queried_object_id(), 'Secondary query must not load page_on_front.' );
+		} finally {
+			remove_filter( 'query_vars', $filter );
+			update_option( 'show_on_front', 'posts' );
+			delete_option( 'page_on_front' );
+		}
+	}
+
+	/**
+	 * A custom query var from a matched rewrite rule must still prevent the
+	 * static front page correction.
+	 *
+	 * @ticket 40521
+	 */
+	public function test_custom_query_var_from_rewrite_rule_prevents_static_front_page_correction() {
+		$page_on_front = self::factory()->post->create( array( 'post_type' => 'page' ) );
+		update_option( 'show_on_front', 'page' );
+		update_option( 'page_on_front', $page_on_front );
+
+		$filter = static function ( $vars ) {
+			$vars[] = 'custom_route';
+			return $vars;
+		};
+		add_filter( 'query_vars', $filter );
+		add_rewrite_rule( '^custom-route/([^/]+)/?$', 'index.php?custom_route=$matches[1]', 'top' );
+		flush_rewrite_rules();
+
+		try {
+			$this->go_to( '/custom-route/matched/' );
+
+			$this->assertSame( 'matched', get_query_var( 'custom_route' ) );
+			$this->assertFalse( is_page(), 'Matched custom rewrite route must not be flagged as is_page().' );
+			$this->assertFalse( is_front_page(), 'Matched custom rewrite route must not be flagged as is_front_page().' );
+			$this->assertNotEquals( $page_on_front, get_queried_object_id(), 'Matched custom rewrite route must not load page_on_front.' );
+		} finally {
+			remove_filter( 'query_vars', $filter );
+			update_option( 'show_on_front', 'posts' );
+			delete_option( 'page_on_front' );
+			flush_rewrite_rules();
+		}
+	}
+
+	/**
+	 * Core routing vars such as 's' must still prevent the static front page
+	 * correction so that a search result is not replaced by the front page.
+	 *
+	 * @ticket 40521
+	 */
+	public function test_core_routing_var_prevents_static_front_page_correction() {
+		$page_on_front = self::factory()->post->create( array( 'post_type' => 'page' ) );
+		update_option( 'show_on_front', 'page' );
+		update_option( 'page_on_front', $page_on_front );
+
+		try {
+			$this->go_to( '/?s=test' );
+
+			$this->assertQueryTrue( 'is_search' );
+		} finally {
+			update_option( 'show_on_front', 'posts' );
+			delete_option( 'page_on_front' );
+		}
+	}
+
 	public function test_404() {
 		$this->go_to( '/notapage' );
 		$this->assertQueryTrue( 'is_404' );