Feeds: Guard against empty array passed to max() in get_feed_build_date()

src/wp-includes/feed.php

@@ -733,7 +733,9 @@ function get_feed_build_date( $format ) {
 		}
 
 		// Determine the maximum modified time.
-		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
+		if ( ! empty( $modified_times ) ) {
+			$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
+		}
 	}
 
 	if ( false === $datetime ) {

tests/phpunit/tests/date/getFeedBuildDate.php

@@ -40,6 +40,47 @@ public function test_should_return_correct_feed_build_date() {
 		$this->assertSame( '2018-07-23T03:13:23+00:00', get_feed_build_date( DATE_RFC3339 ) );
 	}
 
+	/**
+	 * Test that get_feed_build_date() does not throw a ValueError
+	 * when wp_list_pluck() returns an empty array because the posts
+	 * array contains non-object, non-array values.
+	 *
+	 * @ticket 59956
+	 */
+	public function test_should_not_error_when_modified_times_is_empty() {
+		global $wp_query;
+
+		$datetime     = new DateTimeImmutable( 'now', wp_timezone() );
+		$datetime_utc = $datetime->setTimezone( new DateTimeZone( 'UTC' ) );
+
+		// Create a real post so the fallback has something to return.
+		self::factory()->post->create(
+			array(
+				'post_date' => $datetime->format( 'Y-m-d H:i:s' ),
+			)
+		);
+
+		// Simulate a query where have_posts() is true but wp_list_pluck()
+		// returns an empty array because the posts array contains scalar
+		// values (neither objects nor arrays). This triggers the _doing_it_wrong
+		// notice in WP_List_Util::pluck() and produces an empty result.
+		$wp_query = new WP_Query();
+
+		$wp_query->posts      = array( 1 );
+		$wp_query->post_count = 1;
+
+		$this->setExpectedIncorrectUsage( 'WP_List_Util::pluck' );
+
+		$result = get_feed_build_date( DATE_RFC3339 );
+
+		$this->assertEqualsWithDelta(
+			strtotime( $datetime_utc->format( DATE_RFC3339 ) ),
+			strtotime( $result ),
+			2,
+			'Should fall back to last post modified when modified_times is empty.'
+		);
+	}
+
 	/**
 	 * Test that get_feed_build_date() works with invalid post dates.
 	 *