Fix phpstan/phpstan#14413: Narrowing generic union via ::class comparison discards type parameters

src/Analyser/TypeSpecifier.php

@@ -2844,7 +2844,7 @@
 			if ($rightType instanceof ConstantStringType && $this->reflectionProvider->hasClass($rightType->getValue())) {
 				return $this->create(
 					$unwrappedLeftExpr->getArgs()[0]->value,
-					new ObjectType($rightType->getValue(), classReflection: $this->reflectionProvider->getClass($rightType->getValue())->asFinal()),
+					$this->determineExactClassType($scope, $unwrappedLeftExpr->getArgs()[0]->value, $rightType->getValue()),
 					$context,
 					$scope,
 				)->unionWith($this->create($leftExpr, $rightType, $context, $scope))->setRootExpr($expr);
@@ -2969,7 +2969,7 @@
 			if ($this->reflectionProvider->hasClass($rightType->getValue())) {
 				return $this->create(
 					$unwrappedLeftExpr->class,
-					new ObjectType($rightType->getValue(), classReflection: $this->reflectionProvider->getClass($rightType->getValue())->asFinal()),
+					$this->determineExactClassType($scope, $unwrappedLeftExpr->class, $rightType->getValue()),
 					$context,
 					$scope,
 				)->unionWith($this->create($leftExpr, $rightType, $context, $scope))->setRootExpr($expr);
@@ -3000,7 +3000,7 @@
 			if ($this->reflectionProvider->hasClass($leftType->getValue())) {
 				return $this->create(
 					$unwrappedRightExpr->class,
-					new ObjectType($leftType->getValue(), classReflection: $this->reflectionProvider->getClass($leftType->getValue())->asFinal()),
+					$this->determineExactClassType($scope, $unwrappedRightExpr->class, $leftType->getValue()),
 					$context,
 					$scope,
 				)->unionWith($this->create($rightExpr, $leftType, $context, $scope)->setRootExpr($expr));
@@ -3123,4 +3123,20 @@
 		return (new SpecifiedTypes([], []))->setRootExpr($expr);
 	}
 
+	private function determineExactClassType(Scope $scope, Expr $exprNode, string $className): Type
+	{
+		$exprType = $scope->getType($exprNode);
+		$classReflection = $this->reflectionProvider->getClass($className)->asFinal();
+		$asFinalType = new ObjectType($className, classReflection: $classReflection);
+
+		$plainType = new ObjectType($className);
+		$narrowed = TypeCombinator::intersect($exprType, $plainType);
+
+		if ($plainType->isSuperTypeOf($narrowed)->yes() && !$narrowed->isSuperTypeOf($plainType)->yes()) {
+			return $narrowed;
+		}
+
+		return $asFinalType;
+	}
+
 }

tests/PHPStan/Analyser/nsrt/bug-14413.php

@@ -0,0 +1,76 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug14413;
+
+use function PHPStan\Testing\assertType;
+
+/** @template T */
+abstract class Animal {
+	/** @return T */
+	abstract public function value(): mixed;
+}
+
+/**
+ * @template T
+ * @extends Animal<T>
+ */
+class Cat extends Animal {
+	/** @param T $val */
+	public function __construct(private mixed $val) {}
+	/** @return T */
+	public function value(): mixed { return $this->val; }
+}
+
+/**
+ * @template T
+ * @extends Animal<T>
+ */
+class Dog extends Animal {
+	/** @return never */
+	public function value(): never { throw new \RuntimeException(); }
+}
+
+/** @param Cat<string>|Dog<string> $a */
+function unionMatchPreservesGeneric(Animal $a): void {
+	match ($a::class) {
+		Cat::class => assertType('string', $a->value()),
+		Dog::class => assertType('never', $a->value()),
+	};
+}
+
+/** @param Cat<int>|Dog<int> $a */
+function ifElseClassPreservesGeneric(Animal $a): void {
+	if ($a::class === Cat::class) {
+		assertType('Bug14413\Cat<int>', $a);
+		assertType('int', $a->value());
+	} else {
+		assertType('int', $a->value());
+	}
+}
+
+/** @param Cat<float>|Dog<float> $a */
+function mirrorCasePreservesGeneric(Animal $a): void {
+	if (Cat::class === $a::class) {
+		assertType('float', $a->value());
+	}
+}
+
+/** @param Cat<array<string>>|Dog<array<string>> $a */
+function matchWithMethodCall(Animal $a): void {
+	$result = match ($a::class) {
+		Cat::class => $a->value(),
+		Dog::class => [],
+	};
+	assertType('array<string>', $result);
+}
+
+/** @param Cat<string>|Dog<string> $a */
+function nonMatchingClass(Animal $a): void {
+	if ($a::class === \stdClass::class) {
+		assertType('*NEVER*', $a);
+	} else {
+		assertType('Bug14413\Cat<string>|Bug14413\Dog<string>', $a);
+	}
+}