infer non-empty-list/array after isset($arr[$i])

Author: staabm

src/Analyser/TypeSpecifier.php

@@ -1065,6 +1065,18 @@ public function specifyTypesInCondition(
 					&& !$scope->getType($var->var) instanceof MixedType
 				) {
 					$dimType = $scope->getType($var->dim);
+					$varType = $scope->getType($var->var);
+
+					if ($varType->isArray()->yes()) {
+						$types = $types->unionWith(
+							$this->create(
+								$var->var,
+								new NonEmptyArrayType(),
+								$context,
+								$scope,
+							)->setRootExpr($expr),
+						);
+					}
 
 					if ($dimType instanceof ConstantIntegerType || $dimType instanceof ConstantStringType) {
 						$types = $types->unionWith(

tests/PHPStan/Analyser/nsrt/bug-13674.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Bug13674a;
+
+use function assert;
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @param array<int> $arrayA
+	 * @param list<int> $listA
+	 */
+	public function sayHello($arrayA, $listA, int $i): void
+	{
+		if (isset($arrayA[$i])) {
+			assertType('non-empty-array<int>', $arrayA);
+		} else {
+			assertType('array<int>', $arrayA);
+		}
+		assertType('array<int>', $arrayA);
+
+		if (isset($listA[$i])) {
+			assertType('non-empty-list<int>', $listA);
+		} else {
+			assertType('list<int>', $listA);
+		}
+		assertType('list<int>', $listA);
+	}
+}