Rewrite Rules: Treat a request path of "0" as a real request

src/wp-includes/class-wp-query.php

@@ -1193,7 +1193,12 @@ public function parse_tax_query( &$query_vars ) {
 				continue; // Handled further down in the $query_vars['tag'] block.
 			}
 
-			if ( $t->query_var && ! empty( $query_vars[ $t->query_var ] ) ) {
+			if ( $t->query_var
+				&& isset( $query_vars[ $t->query_var ] )
+				&& ( is_array( $query_vars[ $t->query_var ] )
+					? ! empty( $query_vars[ $t->query_var ] )
+					: '' !== (string) $query_vars[ $t->query_var ] )
+			) {
 				$tax_query_defaults = array(
 					'taxonomy' => $taxonomy,
 					'field'    => 'slug',
@@ -3998,7 +4003,7 @@ public function get_queried_object() {
 
 				if ( $cat ) {
 					$term = get_term( $cat, 'category' );
-				} elseif ( $category_name ) {
+				} elseif ( '' !== (string) $category_name ) {
 					$term = get_term_by( 'slug', $category_name, 'category' );
 				}
 			} elseif ( $this->is_tag ) {

src/wp-includes/class-wp.php

@@ -218,7 +218,7 @@ public function parse_request( $extra_query_vars = '' ) {
 
 			// Look for matches.
 			$request_match = $requested_path;
-			if ( empty( $request_match ) ) {
+			if ( '' === $request_match ) {
 				// An empty request could only match against ^$ regex.
 				if ( isset( $rewrite['$'] ) ) {
 					$this->matched_rule = '$';

tests/phpunit/tests/rewrite/zeroPathSegment.php

@@ -0,0 +1,113 @@
+<?php
+
+/**
+ * Tests for requests whose path segment is the string "0".
+ *
+ * "0" is falsy in PHP, so code that guards on `empty()` or truthiness treats
+ * a legitimate "0" path segment as if no path had been requested at all.
+ *
+ * @group rewrite
+ * @group query
+ * @ticket 57858
+ */
+class Tests_Rewrite_ZeroPathSegment extends WP_UnitTestCase {
+
+	/**
+	 * Sets a permalink structure and rebuilds the rules with the taxonomy
+	 * rewrite tags registered.
+	 *
+	 * `WP_Rewrite::init()` resets the registered rewrite tags to the built-in
+	 * defaults, which do not include `%category%`. The taxonomies have to be
+	 * re-registered and the rules flushed again, otherwise `%category%` is
+	 * never substituted into the generated rules.
+	 *
+	 * @param string $structure Permalink structure to set.
+	 */
+	private function set_category_permalink_structure( $structure ) {
+		global $wp_rewrite;
+
+		$this->set_permalink_structure( $structure );
+		create_initial_taxonomies();
+		$wp_rewrite->flush_rules();
+	}
+
+	/**
+	 * A "0" path segment should not silently fall through to the front page.
+	 *
+	 * @ticket 57858
+	 */
+	public function test_zero_path_segment_should_404_when_no_matching_category_exists() {
+		$this->set_category_permalink_structure( '/%category%/%postname%/' );
+
+		self::factory()->post->create( array( 'post_status' => 'publish' ) );
+
+		$this->go_to( home_url( '/0/' ) );
+
+		$this->assertQueryTrue( 'is_404' );
+	}
+
+	/**
+	 * A deeper path ending in "0" should 404 rather than resolve to an archive.
+	 *
+	 * @ticket 57858
+	 */
+	public function test_nested_zero_path_segment_should_404() {
+		$this->set_category_permalink_structure( '/%category%/%postname%/' );
+
+		self::factory()->post->create( array( 'post_status' => 'publish' ) );
+
+		$this->go_to( home_url( '/wp-content/0/' ) );
+
+		$this->assertTrue( is_404() );
+		$this->assertFalse( is_home() );
+	}
+
+	/**
+	 * A category whose slug is "0" should still resolve to its archive.
+	 *
+	 * This guards against fixing the 404 by ignoring "0" outright.
+	 *
+	 * @ticket 57858
+	 */
+	public function test_category_with_zero_slug_should_resolve_to_its_archive() {
+		global $wpdb;
+
+		$this->set_category_permalink_structure( '/%category%/%postname%/' );
+
+		$category_id = self::factory()->category->create( array( 'name' => 'Zero' ) );
+
+		/*
+		 * The slug has to be forced directly, as `wp_insert_term()` discards a
+		 * slug of "0" and falls back to one derived from the name.
+		 */
+		$wpdb->update( $wpdb->terms, array( 'slug' => '0' ), array( 'term_id' => $category_id ) );
+		clean_term_cache( $category_id, 'category' );
+
+		self::factory()->post->create(
+			array(
+				'post_status'   => 'publish',
+				'post_category' => array( $category_id ),
+			)
+		);
+
+		$this->go_to( home_url( '/0/' ) );
+
+		$this->assertQueryTrue( 'is_archive', 'is_category' );
+		$this->assertSame( $category_id, get_queried_object_id() );
+	}
+
+	/**
+	 * The front page must keep working, since an empty request is also falsy.
+	 *
+	 * @ticket 57858
+	 */
+	public function test_empty_request_still_resolves_to_the_front_page() {
+		$this->set_category_permalink_structure( '/%category%/%postname%/' );
+
+		self::factory()->post->create( array( 'post_status' => 'publish' ) );
+
+		$this->go_to( home_url( '/' ) );
+
+		$this->assertQueryTrue( 'is_home', 'is_front_page' );
+	}
+}