Use toArrayKey() for proper array key coercion and add tests for multiple scalar keys

Array keys in PHP undergo type coercion (e.g. string "2" becomes int 2),
so use toArrayKey() before checking key values for implicit index tracking.
Also handle multiple-scalar keys correctly: string-only keys don't affect
implicit int indices.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Analyser/ExprHandler/AssignHandler.php

@@ -946,15 +946,18 @@ private function processArrayByRefItems(MutatingScope $scope, string $rootVarNam
 		$implicitIndex = 0;
 		foreach ($arrayExpr->items as $arrayItem) {
 			if ($arrayItem->key !== null && $implicitIndex !== null) {
-				$keyValues = $scope->getType($arrayItem->key)->getConstantScalarValues();
+				$keyType = $scope->getType($arrayItem->key)->toArrayKey();
+				$keyValues = $keyType->getConstantScalarValues();
 				if (count($keyValues) === 1) {
 					$keyValue = $keyValues[0];
 					if (is_int($keyValue) && $keyValue >= $implicitIndex) {
 						$implicitIndex = $keyValue + 1;
 					}
+				} elseif ($keyType->isInteger()->no()) {
+					// All possible key values are strings, so they don't affect implicit int indices
 				} else {
-					// Non-constant key makes subsequent implicit indices unpredictable,
-					// so we stop tracking implicit indices for the rest of the array
+					// Key could be an integer but we don't know which one,
+					// so subsequent implicit indices are unpredictable
 					$implicitIndex = null;
 				}
 			}

tests/PHPStan/Analyser/nsrt/bug-14333.php

@@ -85,6 +85,45 @@ function testMultipleScalarKeyValues(bool $key): void
 	assertType('1|2|3', $a);
 }
 
+/** @param 'a'|'b' $key */
+function testMultipleScalarKey(string $key): void
+{
+	$a = 1;
+
+	$b = [$key => 'x', &$a];
+	assertType('1', $a);
+
+	// $key has multiple possible values but both are strings,
+	// so the implicit index for &$a is still 0
+	$b[0] = 2;
+	assertType('2', $a);
+}
+
+/** @param 0|1 $key */
+function testMultipleIntScalarKey(int $key): void
+{
+	$a = 1;
+
+	$b = [$key => 'x', &$a];
+	assertType('1', $a);
+
+	// $key could be 0 or 1, so implicit index could be 1 or 2 — unpredictable
+	$b[0] = 2;
+	assertType('1', $a);
+}
+
+function testStringNumericKey(): void
+{
+	$a = 1;
+
+	// PHP coerces string "2" to int 2 as array key, so next implicit index is 3
+	$b = ['2' => 'x', &$a];
+	assertType('1', $a);
+
+	$b[3] = 2;
+	assertType('2', $a);
+}
+
 function foo(array &$a): void {}
 
 function testFunctionCall() {