readonly does not have write access, therefore is safe as long as the…

src/Psalm/Internal/Analyzer/ClassAnalyzer.php

@@ -802,7 +802,20 @@ public static function addContextProperties(
                         $codebase,
                     );
 
-                    if ($property_storage->location
+                    if ($guide_property_storage->readonly
+                        && UnionTypeComparator::isContainedBy(
+                            $codebase,
+                            $property_type,
+                            $guide_property_type,
+                            false,
+                            false,
+                            null,
+                            false,
+                            false,
+                        )) {
+                        // if the original property is readonly, it cannot be written
+                        // therefore invariance is not a problem, if the parent type contains the child type
+                    } elseif ($property_storage->location
                         && !$property_type->equals($guide_property_type, false)
                         && $guide_class_storage->user_defined
                     ) {

tests/PropertyTypeInvarianceTest.php

@@ -231,6 +231,23 @@ class Baz extends Bar {
                         public $d;
                     }',
             ],
+            'allowReadonly' => [
+                'code' => '<?php
+                    class ParentClass
+                    {
+                        /**
+                         * @readonly
+                         * @var null|string
+                         */
+                        protected $mightExist;
+                    }
+
+                    class ChildClass extends ParentClass
+                    {
+                        /** @var string */
+                        protected $mightExist = "";
+                    }',
+            ],
         ];
     }