Fix bogus write-only error when appending to ArrayAccess property

- In ClassStatementsGatherer, also record a PropertyRead when a PropertyAssignNode
  is for an ArrayAccess property via array dim fetch (e.g. $this->array[] = $s),
  because this is semantically $this->array->offsetSet() which reads the property
- New regression test in tests/PHPStan/Rules/DeadCode/data/bug-6777.php
- Updated existing WriteToCollection test expectations

Closes phpstan/phpstan#6777

Author: github-actions[bot]

src/Node/ClassStatementsGatherer.php

@@ -13,11 +13,14 @@
 use PhpParser\Node\Identifier;
 use PHPStan\Analyser\Scope;
 use PHPStan\Node\Constant\ClassConstantFetch;
+use PHPStan\Node\Expr\SetExistingOffsetValueTypeExpr;
+use PHPStan\Node\Expr\SetOffsetValueTypeExpr;
 use PHPStan\Node\Property\PropertyAssign;
 use PHPStan\Node\Property\PropertyRead;
 use PHPStan\Node\Property\PropertyWrite;
 use PHPStan\Reflection\ClassReflection;
 use PHPStan\ShouldNotHappenException;
+use PHPStan\Type\ObjectType;
 use PHPStan\Type\TypeUtils;
 use ReflectionProperty;
 use function count;
@@ -200,7 +203,18 @@ private function gatherNodes(Node $node, Scope $scope): void
 			return;
 		}
 		if ($node instanceof PropertyAssignNode) {
-			$this->propertyUsages[] = new PropertyWrite($node->getPropertyFetch(), $scope, false);
+			$propertyFetch = $node->getPropertyFetch();
+			$assignedExpr = $node->getAssignedExpr();
+			if ($assignedExpr instanceof SetOffsetValueTypeExpr || $assignedExpr instanceof SetExistingOffsetValueTypeExpr) {
+				$propertyType = $scope->getType($propertyFetch);
+				if (
+					!$propertyType->isArray()->yes()
+					&& (new ObjectType(\ArrayAccess::class))->isSuperTypeOf($propertyType)->yes()
+				) {
+					$this->propertyUsages[] = new PropertyRead($propertyFetch, $scope);
+				}
+			}
+			$this->propertyUsages[] = new PropertyWrite($propertyFetch, $scope, false);
 			$this->propertyAssigns[] = new PropertyAssign($node, $scope);
 			return;
 		}

tests/PHPStan/Rules/DeadCode/UnusedPrivatePropertyRuleTest.php

@@ -130,16 +130,6 @@ public function testRule(): void
 				191,
 				$tip,
 			],
-			[
-				'Property UnusedPrivateProperty\WriteToCollection::$collection1 is never read, only written.',
-				221,
-				$tip,
-			],
-			[
-				'Property UnusedPrivateProperty\WriteToCollection::$collection2 is never read, only written.',
-				224,
-				$tip,
-			],
 		]);
 		$this->analyse([__DIR__ . '/data/TestExtension.php'], [
 			[
@@ -410,4 +400,13 @@ public function testBug9213(): void
 		$this->analyse([__DIR__ . '/data/bug-9213.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.0')]
+	public function testBug6777(): void
+	{
+		$this->alwaysWrittenTags = [];
+		$this->alwaysReadTags = [];
+
+		$this->analyse([__DIR__ . '/data/bug-6777.php'], []);
+	}
+
 }

tests/PHPStan/Rules/DeadCode/data/bug-6777.php

@@ -0,0 +1,15 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug6777;
+
+class HelloWorld
+{
+	/** @param \ArrayObject<int, string> $array */
+	public function __construct(private \ArrayObject $array){}
+
+	public function send(string $s) : void{
+		$this->array[] = $s;
+	}
+}