Refine isset checks

src/Psalm/Internal/Analyzer/Statements/Block/IfAnalyzer.php

@@ -96,16 +96,7 @@ class IfAnalyzer
         $mixed_var_ids = [];
 
         foreach ($if_context->vars_in_scope as $var_id => $type) {
-            if ($type->hasMixed()) {
+            if ($type->hasMixed() && isset($context->vars_in_scope[$var_id])) {
-                foreach ($if_context->vars_in_scope as $alt_var_id => $alt_type) {
-                    if ($alt_var_id !== $alt_type
-                        && preg_match('/^' . preg_quote($alt_var_id, '/') . '(\[|-)/', $var_id)
-                        && !$alt_type->hasMixed()
-                    ) {
-                        continue 2;
-                    }
-                }
-
                 $mixed_var_ids[] = $var_id;
             }
         }

tests/AssertTest.php

@@ -358,6 +358,27 @@ class AssertTest extends TestCase
                         if ($c && rand(0, 1)) {}
                     }'
             ],
+            'assertMixedOffsetExists' => [
+                '<?php
+                    class A {
+                        /** @var mixed */
+                        private $arr;
+
+                        /**
+                         * @psalm-suppress MixedArrayAccess
+                         * @psalm-suppress MixedReturnStatement
+                         * @psalm-suppress MixedInferredReturnType
+                         */
+                        public function foo() : stdClass {
+                            if (isset($this->arr[0])) {
+                                return $this->arr[0];
+                            }
+
+                            $this->arr[0] = new stdClass;
+                            return $this->arr[0];
+                        }
+                    }'
+            ],
         ];
     }
 }