Fix phpstan/phpstan#13773: for loop index inference should only work for lists

- Added isList() check in inferForLoopExpressions() before assigning $array[$i] type
- The inference that $array[$i] exists in `for ($i=0; $i<count($array); $i++)` is
  only valid when $array is a list (sequential integer keys starting from 0)
- New regression test in tests/PHPStan/Rules/Arrays/data/bug-13773.php

Author: phpstan-bot

src/Analyser/NodeScopeResolver.php

@@ -4545,11 +4545,15 @@ private function inferForLoopExpressions(For_ $stmt, Expr $lastCondExpr, Mutatin
 				&& $stmt->init[0]->var->name === $lastCondExpr->left->name
 			) {
 				$arrayArg = $lastCondExpr->right->getArgs()[0]->value;
-				$bodyScope = $bodyScope->assignExpression(
-					new ArrayDimFetch($lastCondExpr->right->getArgs()[0]->value, $lastCondExpr->left),
-					$bodyScope->getType($arrayArg)->getIterableValueType(),
-					$bodyScope->getNativeType($arrayArg)->getIterableValueType(),
-				);
+				$arrayType = $bodyScope->getType($arrayArg);
+				$arrayNativeType = $bodyScope->getNativeType($arrayArg);
+				if ($arrayType->isList()->yes() || $arrayNativeType->isList()->yes()) {
+					$bodyScope = $bodyScope->assignExpression(
+						new ArrayDimFetch($lastCondExpr->right->getArgs()[0]->value, $lastCondExpr->left),
+						$arrayType->getIterableValueType(),
+						$arrayNativeType->getIterableValueType(),
+					);
+				}
 			}
 
 			// count($items) > $i
@@ -4566,11 +4570,15 @@ private function inferForLoopExpressions(For_ $stmt, Expr $lastCondExpr, Mutatin
 				&& $stmt->init[0]->var->name === $lastCondExpr->right->name
 			) {
 				$arrayArg = $lastCondExpr->left->getArgs()[0]->value;
-				$bodyScope = $bodyScope->assignExpression(
-					new ArrayDimFetch($lastCondExpr->left->getArgs()[0]->value, $lastCondExpr->right),
-					$bodyScope->getType($arrayArg)->getIterableValueType(),
-					$bodyScope->getNativeType($arrayArg)->getIterableValueType(),
-				);
+				$arrayType = $bodyScope->getType($arrayArg);
+				$arrayNativeType = $bodyScope->getNativeType($arrayArg);
+				if ($arrayType->isList()->yes() || $arrayNativeType->isList()->yes()) {
+					$bodyScope = $bodyScope->assignExpression(
+						new ArrayDimFetch($lastCondExpr->left->getArgs()[0]->value, $lastCondExpr->right),
+						$arrayType->getIterableValueType(),
+						$arrayNativeType->getIterableValueType(),
+					);
+				}
 			}
 		}
 

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1240,4 +1240,20 @@ public function testBug14234(): void
 		]);
 	}
 
+	public function testBug13773(): void
+	{
+		$this->reportPossiblyNonexistentGeneralArrayOffset = true;
+
+		$this->analyse([__DIR__ . '/data/bug-13773.php'], [
+			[
+				'Offset int<0, max> might not exist on non-empty-array<int, string>.',
+				14,
+			],
+			[
+				'Offset int<0, max> might not exist on non-empty-array<int, string>.',
+				38,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-13773.php

@@ -0,0 +1,40 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13773;
+
+/** @return array<int, string> */
+function getArray(): array {
+	return [100 => "hey"];
+}
+
+function testNonListArray(): void
+{
+	$array = getArray();
+	for ($i = 0; $i < count($array); $i++) {
+		$a = $array[$i];
+	}
+}
+
+/** @param list<string> $list */
+function testList(array $list): void
+{
+	for ($i = 0; $i < count($list); $i++) {
+		$a = $list[$i];
+	}
+}
+
+/** @param list<string> $list */
+function testListReversed(array $list): void
+{
+	for ($i = 0; count($list) > $i; ++$i) {
+		$a = $list[$i];
+	}
+}
+
+/** @param array<int, string> $array */
+function testNonListReversed(array $array): void
+{
+	for ($i = 0; count($array) > $i; ++$i) {
+		$a = $array[$i];
+	}
+}