Fix #1826 - don’t allow mixed unions when combining with object-like array

src/Psalm/Internal/Type/TypeCombination.php

@@ -371,6 +371,8 @@ class TypeCombination
                     $overwrite_empty_array,
                     $allow_mixed_union
                 );
+
+                if (!$generic_type_params[1]->isMixed()) {
                     $generic_type_params[1] = Type::combineUnionTypes(
                         $generic_type_params[1],
                         $objectlike_generic_type,
@@ -379,6 +381,7 @@ class TypeCombination
                         $allow_mixed_union
                     );
                 }
+            }
 
             if ($combination->array_always_filled
                 || ($combination->array_sometimes_filled && $overwrite_empty_array)

tests/ArrayAccessTest.php

@@ -672,17 +672,6 @@ class ArrayAccessTest extends TestCase
                     }',
                 'error_message' => 'PossiblyInvalidArrayOffset',
             ],
-            'possiblyInvalidMixedUnionArrayOffset' => [
-                '<?php
-                    function foo(?array $index): void {
-                        if (!$index) {
-                            $index = ["foo", []];
-                        }
-                        $index[1][] = "bar";
-                    }',
-                'error_message' => 'PossiblyInvalidArrayOffset',
-                'error_level' => ['MixedArrayAssignment'],
-            ],
             'arrayAccessOnIterable' => [
                 '<?php
                     function foo(iterable $i) {

tests/TypeCombinationTest.php

@@ -379,6 +379,13 @@ class TypeCombinationTest extends TestCase
                     'string',
                 ],
             ],
+            'combineMixedArrayWithObjectLike' => [
+                'array<array-key, mixed>',
+                [
+                    'array{a: int}',
+                    'array',
+                ],
+            ],
             'traversableAorB' => [
                 'Traversable<mixed, A|B>',
                 [