Fix #4429 - any class with a __toString method in PHP8 assumed to implement Stringable

src/Psalm/Internal/PhpVisitor/ReflectorVisitor.php

@@ -234,6 +234,16 @@ class ReflectorVisitor extends PhpParser\NodeVisitorAbstract implements FileSour
 
             $this->functionlike_node_scanners[] = $functionlike_node_scanner;
 
+            if ($classlike_storage
+                && !$classlike_storage->is_interface
+                && $this->codebase->php_major_version >= 8
+                && $node instanceof PhpParser\Node\Stmt\ClassMethod
+                && strtolower($node->name->name) === '__tostring'
+            ) {
+                $classlike_storage->class_implements['stringable'] = 'Stringable';
+                $this->codebase->scanner->queueClassLikeForScanning('Stringable');
+            }
+
             if (!$this->scan_deep) {
                 return PhpParser\NodeTraverser::DONT_TRAVERSE_CHILDREN;
             }

tests/ToStringTest.php

@@ -169,6 +169,21 @@ class ToStringTest extends TestCase
                 [],
                 '8.0'
             ],
+            'implicitStringable' => [
+                '<?php
+                    function foo(Stringable $s): void {}
+
+                    class Bar {
+                        public function __toString() {
+                            return "foo";
+                        }
+                    }
+
+                    foo(new Bar());',
+                [],
+                [],
+                '8.0',
+            ],
         ];
     }
 
@@ -361,6 +376,21 @@ class ToStringTest extends TestCase
                     fora((string) $address);',
                 'error_message' => 'UndefinedGlobalVariable',
             ],
+            'implicitStringableDisallowed' => [
+                '<?php
+                    function foo(Stringable $s): void {}
+
+                    class Bar {
+                        public function __toString() {
+                            return "foo";
+                        }
+                    }
+
+                    foo(new Bar());',
+                'error_message' => 'InvalidArgument',
+                [],
+                '7.4',
+            ],
         ];
     }
 }