Prevent isset on uknown property in pure function

src/Psalm/Internal/Analyzer/Statements/Expression/Fetch/InstancePropertyFetchAnalyzer.php

@@ -789,6 +789,26 @@ class InstancePropertyFetchAnalyzer
                     $property_id = $context->self . '::$' . $prop_name;
                 } else {
                     if ($context->inside_isset || $context->collect_initializations) {
+                        $project_analyzer = $statements_analyzer->getProjectAnalyzer();
+
+                        if ($context->pure) {
+                            if (IssueBuffer::accepts(
+                                new ImpurePropertyFetch(
+                                    'Cannot access a property on a mutable object from a pure context',
+                                    new CodeLocation($statements_analyzer, $stmt)
+                                ),
+                                $statements_analyzer->getSuppressedIssues()
+                            )) {
+                                // fall through
+                            }
+                        } elseif ($codebase->alter_code
+                            && isset($project_analyzer->getIssuesToFix()['MissingPureAnnotation'])
+                            && $statements_analyzer->getSource()
+                                instanceof \Psalm\Internal\Analyzer\FunctionLikeAnalyzer
+                        ) {
+                            $statements_analyzer->getSource()->inferred_impure = true;
+                        }
+
                         return true;
                     }
 

tests/PureAnnotationTest.php

@@ -670,6 +670,50 @@ class PureAnnotationTest extends TestCase
                             return true;
                         }
 
+                        return false;
+                    }',
+                'error_message' => 'ImpurePropertyFetch',
+            ],
+            'preventIssetOnMutableClassKnownProperty' => [
+                '<?php
+                    namespace Bar;
+
+                    class A {
+                        public ?int $a;
+
+                        public function __construct(?int $a) {
+                            $this->a = $a;
+                        }
+                    }
+
+                    /** @psalm-pure */
+                    function filterOdd(A $a) : bool {
+                        if (isset($a->a)) {
+                            return true;
+                        }
+
+                        return false;
+                    }',
+                'error_message' => 'ImpurePropertyFetch',
+            ],
+            'preventIssetOnMutableClassUnknownProperty' => [
+                '<?php
+                    namespace Bar;
+
+                    class A {
+                        public ?int $a;
+
+                        public function __construct(?int $a) {
+                            $this->a = $a;
+                        }
+                    }
+
+                    /** @psalm-pure */
+                    function filterOdd(A $a) : bool {
+                        if (isset($a->b)) {
+                            return true;
+                        }
+
                         return false;
                     }',
                 'error_message' => 'ImpurePropertyFetch',