Fix array auto-index for negative keys on PHP 8.3+

- PHP 8.3 fixed an incomplete RFC implementation where $a[] after negative keys should use max_key+1, not 0
- Updated ConstantArrayTypeBuilder to account for this when computing nextAutoIndexes
- Added regression test in tests/PHPStan/Analyser/nsrt/bug-10862.php

Closes phpstan/phpstan#10862

Author: VincentLanglet

src/Type/Constant/ConstantArrayTypeBuilder.php

@@ -2,6 +2,7 @@
 
 namespace PHPStan\Type\Constant;
 
+use PHPStan\Reflection\PhpVersionStaticAccessor;
 use PHPStan\ShouldNotHappenException;
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\Accessory\AccessoryArrayListType;
@@ -209,7 +210,10 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 						$this->isList = TrinaryLogic::createNo();
 					}
 
-					if ($offsetValue >= $max) {
+					if (
+						$offsetValue >= $max
+						|| ($offsetValue < 0 && $max === 0 && PhpVersionStaticAccessor::getInstance()->getVersionId() >= 80300)
+					) {
 						/** @var int|float $newAutoIndex */
 						$newAutoIndex = $offsetValue + 1;
 						if (is_float($newAutoIndex)) {

tests/PHPStan/Analyser/nsrt/bug-10862.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug10862;
+
+use function PHPStan\Testing\assertType;
+
+function () {
+	$a = [];
+	$a[-4] = 1;
+	$a[] = 2;
+
+	assertType('array{-4: 1, -3: 2}', $a); // PHP 8.3+: next key after -4 is -3
+	assertType('array{-4, -3}', array_keys($a));
+};
+
+function () {
+	$a = [];
+	$a[-1] = 'x';
+	$a[] = 'y';
+
+	assertType("array{-1: 'x', 0: 'y'}", $a); // PHP 8.3+: next key after -1 is 0
+	assertType('array{-1, 0}', array_keys($a));
+};
+
+function () {
+	$a = [];
+	$a[-10] = 'a';
+	$a[-5] = 'b';
+	$a[] = 'c';
+
+	assertType("array{-10: 'a', -5: 'b', -4: 'c'}", $a); // PHP 8.3+: next key after max(-10,-5)=-5 is -4
+	assertType('array{-10, -5, -4}', array_keys($a));
+};
+
+function () {
+	$a = [];
+	$a[-3] = 'a';
+	$a[5] = 'b';
+	$a[] = 'c';
+
+	assertType("array{-3: 'a', 5: 'b', 6: 'c'}", $a); // positive key dominates
+	assertType('array{-3, 5, 6}', array_keys($a));
+};