Source

File: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

protected function check_role_update( $user_id, $roles ) {
		global $wp_roles;

		foreach ( $roles as $role ) {

			if ( ! isset( $wp_roles->role_objects[ $role ] ) ) {
				return new WP_Error(
					'rest_user_invalid_role',
					/* translators: %s: Role key. */
					sprintf( __( 'The role %s does not exist.' ), $role ),
					array( 'status' => 400 )
				);
			}

			$potential_role = $wp_roles->role_objects[ $role ];

			/*
			 * Don't let anyone with 'edit_users' (admins) edit their own role to something without it.
			 * Multisite super admins can freely edit their blog roles -- they possess all caps.
			 */
			if ( ! ( is_multisite()
				&& current_user_can( 'manage_sites' ) )
				&& get_current_user_id() === $user_id
				&& ! $potential_role->has_cap( 'edit_users' )
			) {
				return new WP_Error(
					'rest_user_invalid_role',
					__( 'Sorry, you are not allowed to give users that role.' ),
					array( 'status' => rest_authorization_required_code() )
				);
			}

			// Include user admin functions to get access to get_editable_roles().
			require_once ABSPATH . 'wp-admin/includes/user.php';

			// The new role must be editable by the logged-in user.
			$editable_roles = get_editable_roles();

			if ( empty( $editable_roles[ $role ] ) ) {
				return new WP_Error(
					'rest_user_invalid_role',
					__( 'Sorry, you are not allowed to give users that role.' ),
					array( 'status' => 403 )
				);
			}
		}

		return true;
	}