wp-includes/class-wp-query.php:4565Determines whether the query is for an existing single post.
$postint|string|int[]|string[]optional''bool public function is_single( $post = '' ) { if ( ! $this->is_single ) { return false; } if ( empty( $post ) ) { return true; } $post_obj = $this->get_queried_object(); if ( ! $post_obj ) { return false; } $post = array_map( 'strval', (array) $post ); if ( in_array( (string) $post_obj->ID, $post, true ) ) { return true; } elseif ( in_array( $post_obj->post_title, $post, true ) ) { return true; } elseif ( in_array( $post_obj->post_name, $post, true ) ) { return true; } else { foreach ( $post as $postpath ) { if ( ! strpos( $postpath, '/' ) ) { continue; } $postpath_obj = get_page_by_path( $postpath, OBJECT, $post_obj->post_type ); if ( $postpath_obj && ( $postpath_obj->ID == $post_obj->ID ) ) { return true; } } } return false; }Introduced in 3.1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-query.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.