Canonical: Validate NOT EXISTS tax_query does not error

src/wp-includes/canonical.php

@@ -331,7 +331,9 @@ function redirect_canonical( $requested_url = null, $do_redirect = true ) {
 			$term_count = 0;
 
 			foreach ( $wp_query->tax_query->queried_terms as $tax_query ) {
-				$term_count += count( $tax_query['terms'] );
+				if ( isset( $tax_query['terms'] ) && is_countable( $tax_query['terms'] ) ) {
+					$term_count += count( $tax_query['terms'] );
+				}
 			}
 
 			$obj = $wp_query->get_queried_object();

tests/phpunit/tests/canonical.php

@@ -375,4 +375,31 @@ public function test_utf8_query_keys_canonical() {
 
 		delete_option( 'page_on_front' );
 	}
+
+	/**
+	 * Ensure NOT EXISTS queries do not trigger not-countable or undefined array key errors.
+	 *
+	 * @ticket 55955
+	 */
+	public function test_feed_canonical_with_not_exists_query() {
+		// Set a NOT EXISTS tax_query on the global query.
+		$global_query        = $GLOBALS['wp_query'];
+		$GLOBALS['wp_query'] = new WP_Query(
+			array(
+				'post_type' => 'post',
+				'tax_query' => array(
+					array(
+						'taxonomy' => 'post_format',
+						'operator' => 'NOT EXISTS',
+					),
+				),
+			)
+		);
+
+		$url = redirect_canonical( get_term_feed_link( self::$terms['/category/parent/'] ), false );
+		$this->assertNull( $url );
+
+		// Restore original global.
+		$GLOBALS['wp_query'] = $global_query;
+	}
 }