Do not report a private method overriding a private trait method as unused

src/Rules/DeadCode/UnusedPrivateMethodRule.php

@@ -9,6 +9,7 @@
 use PHPStan\DependencyInjection\ExtensionsCollection;
 use PHPStan\DependencyInjection\RegisteredRule;
 use PHPStan\Node\ClassMethodsNode;
+use PHPStan\Reflection\ClassReflection;
 use PHPStan\Reflection\MethodReflection;
 use PHPStan\Rules\Methods\AlwaysUsedMethodExtension;
 use PHPStan\Rules\Rule;
@@ -70,6 +71,10 @@ public function processNode(Node $node, Scope $scope): array
 				continue;
 			}
 
+			if ($this->isOverridingPrivateTraitMethod($classReflection, $methodName)) {
+				continue;
+			}
+
 			$methodReflection = $classReflection->getNativeMethod($methodName);
 			foreach ($this->extensions->getAll() as $extension) {
 				if ($extension->isAlwaysUsed($methodReflection)) {
@@ -203,4 +208,26 @@ public function processNode(Node $node, Scope $scope): array
 		return $errors;
 	}
 
+	/**
+	 * A private method overriding a private method of a used trait is called from
+	 * the trait's own methods. Those call sites are invisible when the trait is not
+	 * part of the analysed files.
+	 */
+	private function isOverridingPrivateTraitMethod(ClassReflection $classReflection, string $methodName): bool
+	{
+		foreach ($classReflection->getTraits() as $trait) {
+			if (!$trait->hasNativeMethod($methodName)) {
+				continue;
+			}
+
+			if (!$trait->getNativeMethod($methodName)->isPrivate()) {
+				continue;
+			}
+
+			return true;
+		}
+
+		return false;
+	}
+
 }

tests/PHPStan/Rules/DeadCode/UnusedPrivateMethodRuleTest.php

@@ -134,6 +134,16 @@ public function testBug11802(): void
 		]);
 	}
 
+	public function testBug12201(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-12201.php'], [
+			[
+				'Method Bug12201\AnotherKernel::doNothing() is unused.',
+				24,
+			],
+		]);
+	}
+
 	public function testBug14880(): void
 	{
 		$this->analyse([__DIR__ . '/data/bug-14880.php'], []);

tests/PHPStan/Rules/DeadCode/data/bug-12201-traits.php

@@ -0,0 +1,27 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12201;
+
+trait KernelTrait
+{
+	/**
+	 * @return string[]
+	 */
+	private function getAllowedEnvs(): array
+	{
+		return [];
+	}
+
+	/**
+	 * @return string[]
+	 */
+	protected function getKernelParameters(): array
+	{
+		return $this->getAllowedEnvs();
+	}
+}
+
+trait MicroKernelTrait
+{
+	use KernelTrait;
+}

tests/PHPStan/Rules/DeadCode/data/bug-12201.php

@@ -0,0 +1,27 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12201;
+
+// The traits live in bug-12201-traits.php which is not analysed on purpose:
+// it stands for a dependency living outside of the analysed paths.
+class AppKernel
+{
+	use MicroKernelTrait;
+
+	/**
+	 * @return list<string>
+	 */
+	private function getAllowedEnvs(): array
+	{
+		return ['prod', 'dev', 'test'];
+	}
+}
+
+class AnotherKernel
+{
+	use MicroKernelTrait;
+
+	private function doNothing(): void
+	{
+	}
+}