Fix #266 - do better analysis of array_rand

And also have better fallback when accessing string key on object-like array

src/Psalm/Checker/Statements/Expression/FetchChecker.php

@@ -1130,6 +1130,8 @@ class FetchChecker
                             )) {
                                 return false;
                             }
+                        } elseif ($key_type->hasString()) {
+                            $stmt->inferredType = $type->getGenericTypeParam();
                         }
                     }
                     continue;

src/Psalm/Stubs/CoreGenericFunctions.php

@@ -107,3 +107,11 @@ function array_pop(array $arr) {}
  * @return array<TKey, TValue>
  */
 function array_reverse(array $arr) {}
+
+/**
+ * @template TKey
+ *
+ * @param array<TKey, mixed> $arr
+ * @return TKey
+ */
+function array_rand(array $arr) {}

tests/FunctionCallTest.php

@@ -281,6 +281,18 @@ class FunctionCallTest extends TestCase
                     fooFoo($a->foo);
                     fooFoo($b);',
             ],
+            'arrayRand' => [
+                '<?php
+                    $vars = ["x" => "a", "y" => "b"];
+                    $c = array_rand($vars);
+                    $d = $vars[$c];',
+
+                'assertions' => [
+                    '$vars' => 'array{x:string, y:string}',
+                    '$c' => 'string',
+                    '$d' => 'string',
+                ],
+            ],
         ];
     }