Fix phpstan/phpstan#6139: False positive for covariant generic parameter

src/Rules/Generics/VarianceCheck.php

@@ -9,6 +9,8 @@
 use PHPStan\Type\Generic\TemplateType;
 use PHPStan\Type\Generic\TemplateTypeVariance;
 use PHPStan\Type\Type;
+use function count;
+use function in_array;
 use function sprintf;
 
 #[AutowiredService]
@@ -85,13 +87,19 @@ public function check(TemplateTypeVariance $positionVariance, Type $type, string
 	{
 		$errors = [];
 
+		$skipTemplates = $this->findCovariantTemplatesInCovariantGeneric($positionVariance, $type);
+
 		foreach ($type->getReferencedTemplateTypes($positionVariance) as $reference) {
 			$referredType = $reference->getType();
 			if (($referredType->getScope()->getFunctionName() !== null && !$referredType->getVariance()->invariant())
 				|| $this->isTemplateTypeVarianceValid($reference->getPositionVariance(), $referredType)) {
 				continue;
 			}
 
+			if (in_array($referredType, $skipTemplates, true)) {
+				continue;
+			}
+
 			$errors[] = RuleErrorBuilder::message(sprintf(
 				'Template type %s is declared as %s, but occurs in %s position %s.',
 				$referredType->getName(),
@@ -104,6 +112,45 @@ public function check(TemplateTypeVariance $positionVariance, Type $type, string
 		return $errors;
 	}
 
+	/**
+	 * When a covariant generic type (e.g. Element<T> where Element has @template-covariant)
+	 * is used as a parameter type, covariant template type arguments should not be flagged.
+	 * The covariant generic only produces values of type T, so using it in a parameter
+	 * position does not create a true contravariant use of T.
+	 *
+	 * @return list<TemplateType>
+	 */
+	private function findCovariantTemplatesInCovariantGeneric(TemplateTypeVariance $positionVariance, Type $type): array
+	{
+		if (!$positionVariance->contravariant()) {
+			return [];
+		}
+
+		$classReflections = $type->getObjectClassReflections();
+		if (count($classReflections) !== 1) {
+			return [];
+		}
+
+		$classReflection = $classReflections[0];
+		$templateTypeMap = $classReflection->getTemplateTypeMap();
+		$skipTemplates = [];
+
+		foreach ($templateTypeMap->getTypes() as $templateName => $templateType) {
+			if (!$templateType instanceof TemplateType || !$templateType->getVariance()->covariant()) {
+				continue;
+			}
+
+			$resolvedType = $type->getTemplateType($classReflection->getName(), $templateName);
+			if (!($resolvedType instanceof TemplateType) || !$resolvedType->getVariance()->covariant()) {
+				continue;
+			}
+
+			$skipTemplates[] = $resolvedType;
+		}
+
+		return $skipTemplates;
+	}
+
 	private function isTemplateTypeVarianceValid(TemplateTypeVariance $positionVariance, TemplateType $type): bool
 	{
 		return $positionVariance->validPosition($type->getVariance());

tests/PHPStan/Rules/Generics/MethodSignatureVarianceRuleTest.php

@@ -47,10 +47,6 @@ public function testRule(): void
 				'Template type X is declared as covariant, but occurs in invariant position in parameter e of method MethodSignatureVariance\Covariant\C::a().',
 				35,
 			],
-			[
-				'Template type X is declared as covariant, but occurs in contravariant position in parameter f of method MethodSignatureVariance\Covariant\C::a().',
-				35,
-			],
 			[
 				'Template type X is declared as covariant, but occurs in contravariant position in parameter h of method MethodSignatureVariance\Covariant\C::a().',
 				35,
@@ -187,10 +183,6 @@ public function testRule(): void
 				'Template type X is declared as covariant, but occurs in contravariant position in parameter a of method MethodSignatureVariance\StaticMethod\B::a().',
 				43,
 			],
-			[
-				'Template type X is declared as covariant, but occurs in contravariant position in parameter c of method MethodSignatureVariance\StaticMethod\B::a().',
-				43,
-			],
 			[
 				'Template type X is declared as covariant, but occurs in contravariant position in return type of method MethodSignatureVariance\StaticMethod\B::c().',
 				49,
@@ -235,6 +227,11 @@ public function testPr2465(): void
 		]);
 	}
 
+	public function testBug6139(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-6139.php'], []);
+	}
+
 	#[RequiresPhp('>= 8.0')]
 	public function testBug10609(): void
 	{

tests/PHPStan/Rules/Generics/data/bug-6139.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace Bug6139;
+
+/**
+ * @template-covariant T
+ */
+interface Element {
+	/**
+	 * @return T
+	 */
+	public function render(): mixed;
+}
+
+/**
+ * @template-covariant T
+ */
+interface FormatLoader
+{
+	/**
+	 * @param Element<T> $element
+	 */
+	public function addElement(Element $element): void;
+}