Fix non-existent offset when guarding

Author: SplotyCode

src/Analyser/TypeSpecifier.php

@@ -291,6 +291,25 @@ public function specifyTypesInCondition(
 				if ($specifiedTypes !== null) {
 					$result = $result->unionWith($specifiedTypes);
 				}
+				if (
+					$context->true()
+					&& $expr instanceof Node\Expr\BinaryOp\Smaller
+				    && $argType->isList()->yes()
+					&& IntegerRangeType::fromInterval(0, null)->isSuperTypeOf($leftType)->yes()
+				) {
+					$dimFetch = new ArrayDimFetch(
+						$expr->right->getArgs()[0]->value,
+						$expr->left,
+					);
+					$result = $result->unionWith(
+						$this->create(
+							$dimFetch,
+							$argType->getIterableValueType(),
+							TypeSpecifierContext::createTrue(),
+							$scope,
+						)->setRootExpr($expr)
+					);
+				}
 
 				if (
 					$context->true() && (IntegerRangeType::createAllGreaterThanOrEqualTo(1 - $offset)->isSuperTypeOf($leftType)->yes())

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -700,6 +700,10 @@ public static function dataReportPossiblyNonexistentArrayOffset(): iterable
 				"Offset 'foo' might not exist on array.",
 				9,
 			],
+			[
+				'Offset int might not exist on list<int>.',
+				77
+			],
 		]];
 		yield [true, true, [
 			[
@@ -710,6 +714,10 @@ public static function dataReportPossiblyNonexistentArrayOffset(): iterable
 				'Offset string might not exist on array{foo: 1}.',
 				20,
 			],
+			[
+				'Offset int might not exist on list<int>.',
+				77
+			],
 		]];
 	}
 

tests/PHPStan/Rules/Arrays/data/report-possibly-nonexistent-array-offset.php

@@ -57,4 +57,25 @@ public function nonEmpty(array $a): void
 		echo $a[0];
 	}
 
+	/**
+	 * @param list<int> $array
+	 * @param positive-int $index
+	 */
+	public function guard(array $array, int $index) {
+		if ($index < count($array)) {
+			return $array[$index];
+		}
+		return null;
+	}
+
+
+	/**
+	 * @param list<int> $array
+	 */
+	public function guardNotSafe(array $array, int $index) {
+		if ($index < count($array)) {
+			return $array[$index];
+		}
+		return null;
+	}
 }