Fix reconciliation of mixed to Traversable

src/Psalm/Internal/Type/AssertionReconciler.php

@@ -1298,7 +1298,7 @@ class AssertionReconciler extends \Psalm\Type\Reconciler
         $existing_var_atomic_types = $existing_var_type->getTypes();
 
         if ($existing_var_type->hasMixed() || $existing_var_type->hasTemplate()) {
-            return new Type\Union([new Type\Atomic\TIterable]);
+            return new Type\Union([new Type\Atomic\TNamedObject('Traversable')]);
         }
 
         $traversable_types = [];

tests/TypeReconciliationTest.php

@@ -1458,6 +1458,16 @@ class TypeReconciliationTest extends TestCase
                         if (isset($e->bar)) {}
                     }',
             ],
+            'assertArrayOffsetToTraversable' => [
+                '<?php
+                    function render(array $data): ?Traversable {
+                        if ($data["o"] instanceof Traversable) {
+                            return $data["o"];
+                        }
+
+                        return null;
+                    }'
+            ],
         ];
     }