wp-includes/class-wp-query.php:4619Determines whether the query is for an existing single post of any post type (post, attachment, page, custom post types).
$post_typesstring|string[]optional''bool public function is_singular( $post_types = '' ) { if ( empty( $post_types ) || ! $this->is_singular ) { return (bool) $this->is_singular; } $post_obj = $this->get_queried_object(); if ( ! $post_obj ) { return false; } return in_array( $post_obj->post_type, (array) $post_types, true ); }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.