From d532133325bd37986277b0876826f4e84b415046 Mon Sep 17 00:00:00 2001
From: Matt Brown <github@muglug.com>
Date: Sun, 4 Oct 2020 00:17:16 -0400
Subject: [PATCH] Add support for Stringable

---
 .../Type/Comparator/AtomicTypeComparator.php  | 36 +++++++++++++------
 tests/ToStringTest.php                        | 23 +++++++++++-
 2 files changed, 47 insertions(+), 12 deletions(-)

diff --git a/src/Psalm/Internal/Type/Comparator/AtomicTypeComparator.php b/src/Psalm/Internal/Type/Comparator/AtomicTypeComparator.php
index 58dea4ad2..aea970a40 100644
--- a/src/Psalm/Internal/Type/Comparator/AtomicTypeComparator.php
+++ b/src/Psalm/Internal/Type/Comparator/AtomicTypeComparator.php
@@ -495,19 +495,33 @@ class AtomicTypeComparator
         if ($container_type_part instanceof TString || $container_type_part instanceof TScalar) {
             if ($input_type_part instanceof TNamedObject) {
                 // check whether the object has a __toString method
-                if ($codebase->classOrInterfaceExists($input_type_part->value)
-                    && $codebase->methods->methodExists(
-                        new \Psalm\Internal\MethodIdentifier(
-                            $input_type_part->value,
-                            '__tostring'
-                        )
-                    )
-                ) {
-                    if ($atomic_comparison_result) {
-                        $atomic_comparison_result->to_string_cast = true;
+                if ($codebase->classOrInterfaceExists($input_type_part->value)) {
+                    if ($codebase->php_major_version >= 8
+                        && ($input_type_part->value === 'Stringable'
+                            || ($codebase->classlikes->classExists($input_type_part->value)
+                                && $codebase->classlikes->classImplements($input_type_part->value, 'Stringable'))
+                            || $codebase->classlikes->interfaceExtends($input_type_part->value, 'Stringable'))
+                    ) {
+                        if ($atomic_comparison_result) {
+                            $atomic_comparison_result->to_string_cast = true;
+                        }
+
+                        return true;
                     }
 
-                    return true;
+                    if ($codebase->methods->methodExists(
+                            new \Psalm\Internal\MethodIdentifier(
+                                $input_type_part->value,
+                                '__tostring'
+                            )
+                        )
+                    ) {
+                        if ($atomic_comparison_result) {
+                            $atomic_comparison_result->to_string_cast = true;
+                        }
+
+                        return true;
+                    }
                 }
 
                 // PHP 5.6 doesn't support this natively, so this introduces a bug *just* when checking PHP 5.6 code
diff --git a/tests/ToStringTest.php b/tests/ToStringTest.php
index 5eb3fd8b2..0247f5a0a 100644
--- a/tests/ToStringTest.php
+++ b/tests/ToStringTest.php
@@ -145,8 +145,29 @@ class ToStringTest extends TestCase
                     if (is_callable([$object, \'__toString\'])) {
                         $a = (string) $object;
                         echo $a;
+                    }'
+            ],
+            'stringableInterface' => [
+                '<?php
+                    interface Foo extends Stringable {}
+
+                    function takesString(string $s) : void {}
+
+                    function takesFoo(Foo $foo) : void {
+                        /** @psalm-suppress ImplicitToStringCast */
+                        takesString($foo);
                     }
-                    '
+
+                    class FooImplementer implements Foo {
+                        public function __toString() : string {
+                            return "hello";
+                        }
+                    }
+
+                    takesFoo(new FooImplementer());',
+                [],
+                [],
+                '8.0'
             ],
         ];
     }