Source

File: wp-includes/class-wp.php

public function handle_404() {
		global $wp_query;

		/**
		 * Filters whether to short-circuit default header status handling.
		 *
		 * Returning a non-false value from the filter will short-circuit the handling
		 * and return early.
		 *
		 * @since 4.5.0
		 *
		 * @param bool     $preempt  Whether to short-circuit default header status handling. Default false.
		 * @param WP_Query $wp_query WordPress Query object.
		 */
		if ( false !== apply_filters( 'pre_handle_404', false, $wp_query ) ) {
			return;
		}

		// If we've already issued a 404, bail.
		if ( is_404() ) {
			return;
		}

		$set_404 = true;

		// Never 404 for the admin, robots, or favicon.
		if ( is_admin() || is_robots() || is_favicon() ) {
			$set_404 = false;

			// If posts were found, check for paged content.
		} elseif ( $wp_query->posts ) {
			$content_found = true;

			if ( is_singular() ) {
				$post = isset( $wp_query->post ) ? $wp_query->post : null;

				// Only set X-Pingback for single posts that allow pings.
				if ( $post && pings_open( $post ) && ! headers_sent() ) {
					header( 'X-Pingback: ' . get_bloginfo( 'pingback_url', 'display' ) );
				}

				// Check for paged content that exceeds the max number of pages.
				$next = '<!--nextpage-->';
				if ( $post && ! empty( $this->query_vars['page'] ) ) {
					// Check if content is actually intended to be paged.
					if ( false !== strpos( $post->post_content, $next ) ) {
						$page          = trim( $this->query_vars['page'], '/' );
						$content_found = (int) $page <= ( substr_count( $post->post_content, $next ) + 1 );
					} else {
						$content_found = false;
					}
				}
			}

			// The posts page does not support the <!--nextpage--> pagination.
			if ( $wp_query->is_posts_page && ! empty( $this->query_vars['page'] ) ) {
				$content_found = false;
			}

			if ( $content_found ) {
				$set_404 = false;
			}

			// We will 404 for paged queries, as no posts were found.
		} elseif ( ! is_paged() ) {
			$author = get_query_var( 'author' );

			// Don't 404 for authors without posts as long as they matched an author on this site.
			if ( is_author() && is_numeric( $author ) && $author > 0 && is_user_member_of_blog( $author )
				// Don't 404 for these queries if they matched an object.
				|| ( is_tag() || is_category() || is_tax() || is_post_type_archive() ) && get_queried_object()
				// Don't 404 for these queries either.
				|| is_home() || is_search() || is_feed()
			) {
				$set_404 = false;
			}
		}

		if ( $set_404 ) {
			// Guess it's time to 404.
			$wp_query->set_404();
			status_header( 404 );
			nocache_headers();
		} else {
			status_header( 200 );
		}
	}