Fix #3870 - mark properties as reference-free only for externally-immutable classes

src/Psalm/Internal/Analyzer/FunctionLikeAnalyzer.php

@@ -216,9 +216,7 @@ abstract class FunctionLikeAnalyzer extends SourceAnalyzer
 
                 $context->vars_in_scope['$this'] = new Type\Union([$this_object_type]);
 
-                if ($storage->external_mutation_free
+                if ($appearing_class_storage->external_mutation_free) {
-                    && !$storage->mutation_free_inferred
-                ) {
                     $context->vars_in_scope['$this']->reference_free = true;
 
                     if ($this->function->name->name !== '__construct') {

tests/ImmutableAnnotationTest.php

@@ -699,6 +699,32 @@ class ImmutableAnnotationTest extends TestCase
                     }',
                 'error_message' => 'ImpurePropertyAssignment',
             ],
+            'mutationInPropertyAssignment' => [
+                '<?php
+                    class D {
+                        private string $s;
+
+                        public function __construct(string $s) {
+                            $this->s = $s;
+                        }
+
+                        /**
+                         * @psalm-mutation-free
+                         */
+                        public function getShort() : string {
+                            return substr($this->s, 0, 5);
+                        }
+
+                        /**
+                         * @psalm-mutation-free
+                         */
+                        public function getShortMutating() : string {
+                            $this->s .= "hello";
+                            return substr($this->s, 0, 5);
+                        }
+                    }',
+                'error_message' => 'ImpurePropertyAssignment',
+            ],
         ];
     }
 }