Non-sequential arrays are forbidden.

src/Util.php

@@ -14,4 +14,12 @@ public static function normalizeIndex(int $index, int $containerLength, bool $th
         }
         return $index < 0 ? $containerLength + $index : $index;
     }
+
+    public static function isArraySequential(array $source): bool
+    {
+        if (!function_exists('array_is_list')) {
+            return array_keys($source) === range(0, count($source) - 1);
+        }
+        return array_is_list($source);
+    }
 }

src/Views/ArrayView.php

@@ -9,6 +9,7 @@
 use Smoren\ArrayView\Exceptions\LengthError;
 use Smoren\ArrayView\Exceptions\NotSupportedError;
 use Smoren\ArrayView\Exceptions\ReadonlyError;
+use Smoren\ArrayView\Exceptions\ValueError;
 use Smoren\ArrayView\Interfaces\ArraySelectorInterface;
 use Smoren\ArrayView\Interfaces\ArrayViewInterface;
 use Smoren\ArrayView\Selectors\MaskSelector;
@@ -57,6 +58,10 @@ public static function toView(&$source, ?bool $readonly = null): ArrayView
      */
     public function __construct(&$source, ?bool $readonly = null)
     {
+        if (is_array($source) && !Util::isArraySequential($source)) {
+            throw new ValueError('Cannot create view for non-sequential array.');
+        }
+
         $this->source = &$source;
         $this->readonly = $readonly ?? (($source instanceof ArrayViewInterface) ? $source->isReadonly() : false);
         $this->parentView = ($source instanceof ArrayViewInterface) ? $source : null;

tests/unit/ArrayView/NonSequentialErrorTest.php

@@ -0,0 +1,60 @@
+<?php
+
+namespace Smoren\ArrayView\Tests\Unit\ArrayView;
+
+use Smoren\ArrayView\Exceptions\ValueError;
+use Smoren\ArrayView\Views\ArrayView;
+
+class NonSequentialErrorTest extends \Codeception\Test\Unit
+{
+    /**
+     * @dataProvider dataProviderForNonSequentialError
+     */
+    public function testNonSequentialError(callable $arrayGetter)
+    {
+        $nonSequentialArray = $arrayGetter();
+        $this->expectException(ValueError::class);
+        ArrayView::toView($nonSequentialArray);
+    }
+
+    public function dataProviderForNonSequentialError(): array
+    {
+        return [
+            [fn () => ['test' => 1]],
+            [fn () => [1 => 1]],
+            [fn () => [0 => 1, 2 => 2]],
+            [fn () => [0 => 1, -1 => 2]],
+            [fn () => [0 => 1, 'a' => 2]],
+            [static function () {
+                $array = [1, 2, 3, 4, 5];
+                unset($array[0]);
+                return $array;
+            }],
+            [static function () {
+                $array = [1, 2, 3, 4, 5];
+                unset($array[1]);
+                return $array;
+            }],
+            [static function () {
+                $array = [1, 2, 3, 4, 5];
+                $array[6] = 111;
+                return $array;
+            }],
+            [static function () {
+                $array = [1, 2, 3, 4, 5];
+                $array[-1] = 111;
+                return $array;
+            }],
+            [static function () {
+                $array = [1, 2, 3, 4, 5];
+                $array[-2] = 111;
+                return $array;
+            }],
+            [static function () {
+                $array = [1, 2, 3, 4, 5];
+                $array['test'] = 111;
+                return $array;
+            }],
+        ];
+    }
+}