Merge branch '6.0' into 6.1

* 6.0:
  Fix return type patching for list and class-string pseudo types
  [HttpKernel] [HttpCache] Don't throw on 304 Not Modified

Author: derrabus

DebugClassLoader.php

@@ -73,6 +73,7 @@ class DebugClassLoader
         'static' => 'static',
         '$this' => 'static',
         'list' => 'array',
+        'class-string' => 'string',
     ];
 
     private const BUILTIN_RETURN_TYPES = [