Fix phpstan/phpstan#13770: Nonexistent offset detection on lists (if ($index < count($array)))

src/Analyser/TypeSpecifier.php

@@ -333,6 +333,22 @@ public function specifyTypesInCondition(
 						);
 					}
 				}
+
+				// infer $list[$index] after $index < count($list)
+				if (
+					$context->true()
+					&& !$orEqual
+					// constant offsets are handled via HasOffsetType/HasOffsetValueType
+					&& !$leftType instanceof ConstantIntegerType
+					&& $argType->isList()->yes()
+					&& IntegerRangeType::fromInterval(0, null)->isSuperTypeOf($leftType)->yes()
+				) {
+					$arrayArg = $expr->right->getArgs()[0]->value;
+					$dimFetch = new ArrayDimFetch($arrayArg, $expr->left);
+					$result = $result->unionWith(
+						$this->create($dimFetch, $argType->getIterableValueType(), TypeSpecifierContext::createTrue(), $scope)->setRootExpr($expr),
+					);
+				}
 			}
 
 			if (

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1155,4 +1155,32 @@ public function testBug11276(): void
 		$this->analyse([__DIR__ . '/data/bug-11276.php'], []);
 	}
 
+	public function testBug13770(): void
+	{
+		$this->reportPossiblyNonexistentGeneralArrayOffset = true;
+
+		$this->analyse([__DIR__ . '/data/bug-13770.php'], [
+			[
+				'Offset int<1, max> might not exist on non-empty-list<int>.',
+				53,
+			],
+			[
+				'Offset int might not exist on list<int>.',
+				66,
+			],
+			[
+				'Offset int might not exist on list<int>.',
+				91,
+			],
+			[
+				'Offset int might not exist on array.',
+				100,
+			],
+			[
+				'Offset -1|3|6|10 might not exist on list<int>.',
+				126,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-13770.php

@@ -0,0 +1,131 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13770;
+
+class HelloWorld
+{
+	/**
+	 * @param list<int> $array
+	 * @param positive-int $index
+	 */
+	public function positiveIntLessThanCount(array $array, int $index): int
+	{
+		if ($index < count($array)) {
+			return $array[$index]; // should not report
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param positive-int $index
+	 */
+	public function positiveIntLessThanCountInversed(array $array, int $index): int
+	{
+		if (count($array) > $index) {
+			return $array[$index]; // should not report
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param int<0, max> $index
+	 */
+	public function nonNegativeIntLessThanCount(array $array, int $index): int
+	{
+		if ($index < count($array)) {
+			return $array[$index]; // should not report
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param positive-int $index
+	 */
+	public function positiveIntLessThanOrEqualCount(array $array, int $index): int
+	{
+		if ($index <= count($array)) {
+			return $array[$index]; // SHOULD still report - off by one
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param int $index
+	 */
+	public function anyIntLessThanCount(array $array, int $index): int
+	{
+		if ($index < count($array)) {
+			return $array[$index]; // SHOULD still report - could be negative
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param int<0, max> $index
+	 */
+	public function positiveIntOnNormalCountMode(array $array, int $index): int
+	{
+		if ($index < count($array, COUNT_NORMAL)) {
+			return $array[$index]; // should not error
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 */
+	public function anyIntOnUnknownCountMode(array $array, int $index, $countMode): int
+	{
+		if ($index < count($array, $countMode)) {
+			return $array[$index]; // SHOULD still report - could be negative
+		}
+
+		return 0;
+	}
+
+	public function anyIntOnRecursiveCount(array $array, int $index): int
+	{
+		if ($index < count($array, COUNT_RECURSIVE)) {
+			return $array[$index]; // SHOULD still report - could be negative
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param 3|6|10 $index
+	 */
+	public function constantPositiveIntLessThanCount(array $array, int $index): int
+	{
+		if ($index < count($array)) {
+			return $array[$index]; // should not report
+		}
+
+		return 0;
+	}
+
+	/**
+	 * @param list<int> $array
+	 * @param -1|3|6|10 $index
+	 */
+	public function constantMaybeNegativeIntLessThanCount(array $array, int $index): int
+	{
+		if ($index < count($array)) {
+			return $array[$index]; // SHOULD still report - could be negative
+		}
+
+		return 0;
+	}
+}