Fix one more false positive about unused private property

src/Reflection/ClassReflection.php

@@ -977,7 +977,7 @@ public function hasTraitUse(string $traitName): bool
 	private function getTraitNames(): array
 	{
 		$class = $this->reflection;
-		$traitNames = $class->getTraitNames();
+		$traitNames = array_map(static fn (ReflectionClass $class) => $class->getName(), $this->collectTraits($class));
 		while ($class->getParentClass() !== false) {
 			$traitNames = array_values(array_unique(array_merge($traitNames, $class->getParentClass()->getTraitNames())));
 			$class = $class->getParentClass();

tests/PHPStan/Rules/DeadCode/UnusedPrivatePropertyRuleTest.php

@@ -272,4 +272,11 @@ public function testBug8204(): void
 		$this->analyse([__DIR__ . '/data/bug-8204.php'], []);
 	}
 
+	public function testBug8850(): void
+	{
+		$this->alwaysWrittenTags = [];
+		$this->alwaysReadTags = [];
+		$this->analyse([__DIR__ . '/data/bug-8850.php'], []);
+	}
+
 }

tests/PHPStan/Rules/DeadCode/data/bug-8850.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace Bug8850;
+
+class Security
+{
+	public function getUser(): string
+	{
+		return 'foo';
+	}
+}
+
+final class UserInSessionInRoleEndpointExtension
+{
+	use QueryBuilderHelperTrait;
+
+	/** @var Security */
+	private $security;
+
+	public function __construct(
+		Security $security
+	) {
+		$this->security = $security;
+	}
+}
+
+trait QueryBuilderHelperTrait
+{
+	use OrganisationExtensionHelperTrait;
+}
+
+trait OrganisationExtensionHelperTrait
+{
+	use UserHelperTrait;
+
+	public function getOrganisationIds(): void
+	{
+		$user = $this->getUser();
+	}
+}
+
+trait UserHelperTrait
+{
+	public function getUser(): string
+	{
+		$user = $this->security->getUser();
+
+		return 'foo';
+	}
+}

tests/PHPStan/Type/ObjectTypeTest.php

@@ -4,6 +4,8 @@
 
 use ArrayAccess;
 use ArrayObject;
+use Bug8850\UserHelperTrait;
+use Bug8850\UserInSessionInRoleEndpointExtension;
 use Closure;
 use Countable;
 use DateInterval;
@@ -432,6 +434,11 @@ public function dataIsSuperTypeOf(): array
 				new ObjectType(DateTime::class),
 				TrinaryLogic::createNo(),
 			],
+			61 => [
+				new ObjectType(UserInSessionInRoleEndpointExtension::class),
+				new ThisType($reflectionProvider->getClass(UserInSessionInRoleEndpointExtension::class), null, $reflectionProvider->getClass(UserHelperTrait::class)),
+				TrinaryLogic::createMaybe(),
+			],
 		];
 	}