Restrict type of implode to prevent Array-to-string conversion;

src/Psalm/Internal/Type/TypeTokenizer.php

@@ -43,6 +43,8 @@ class TypeTokenizer
         'class-string' => true,
         'callable-string' => true,
         'callable-array' => true,
+        'callable-object' => true,
+        'stringable-object' => true,
         'pure-callable' => true,
         'pure-Closure' => true,
         'trait-string' => true,

src/Psalm/Type/Atomic.php

@@ -244,6 +244,9 @@ abstract class Atomic implements TypeNode
             case 'callable-object':
                 return new TCallableObject();
 
+            case 'stringable-object':
+                return new Type\Atomic\TObjectWithProperties([], ['__tostring' => 'string']);
+
             case 'class-string':
             case 'interface-string':
                 return new TClassString();

stubs/CoreGenericFunctions.phpstub

@@ -567,6 +567,7 @@ function rtrim(string $str, string $character_mask = " \t\n\r\0\x0B") : string {
  * @psalm-pure
  *
  * @param string|array $glue
+ * @param array<string|float|int|stringable-object> $pieces
  *
  * @return (
  *     $glue is non-empty-string

tests/ToStringTest.php

@@ -193,6 +193,16 @@ class ToStringTest extends TestCase
                     }
                 '
             ],
+            'toStringToImplode' => [
+                '<?php
+                    class Bar {
+                        public function __toString() {
+                            return "foo";
+                        }
+                    }
+
+                    echo implode(":", [new Bar()]);',
+            ],
         ];
     }
 

tests/UnusedVariableTest.php

@@ -1531,6 +1531,7 @@ class UnusedVariableTest extends TestCase
                         takes_ref($a[$e]);
                     }
 
+                    /** @param array<string|int> $p */
                     function takes_ref(array &$p): void {
                         echo implode(",", $p);
                     }'
@@ -2381,6 +2382,7 @@ class UnusedVariableTest extends TestCase
             ],
             'varDocblockVariableIsUsedByRef' => [
                 '<?php
+                    /** @param array<string|int> $arr */
                     function foo(array $arr) : string {
                         /** @var string $val */
                         foreach ($arr as &$val) {