Fix incorrect type inference for @phpstan-assert-if-true on $this with union types

src/Reflection/Type/UnionTypeMethodReflection.php

@@ -177,7 +177,13 @@ public function getDocComment(): ?string
 
 	public function getAsserts(): Assertions
 	{
-		return Assertions::createEmpty();
+		$mergedAssertions = Assertions::createEmpty();
+
+		foreach ($this->methods as $method) {
+			$mergedAssertions = $mergedAssertions->intersectWith($method->getAsserts());
+		}
+
+		return $mergedAssertions;
 	}
 
 	public function acceptsNamedArguments(): TrinaryLogic

tests/PHPStan/Rules/Methods/AssertIfTrueOnThisTest.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Methods;
+
+use PHPStan\Testing\TypeInferenceTestCase;
+
+class AssertIfTrueOnThisTest extends TypeInferenceTestCase
+{
+
+	public static function dataFileAsserts(): iterable
+	{
+		yield from self::gatherAssertTypes(__DIR__ . '/data/bug-13358.php');
+	}
+
+	/**
+	 * @dataProvider dataFileAsserts
+	 * @param mixed ...$args
+	 */
+	public function testFileAsserts(string $assertType, string $file, ...$args): void
+	{
+		$this->assertFileAsserts($assertType, $file, ...$args);
+	}
+
+}

tests/PHPStan/Rules/Methods/data/bug-13358.php

@@ -0,0 +1,40 @@
+<?php declare(strict_types=1);
+
+namespace Bug13358;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @phpstan-sealed SystemActor|AnonymousVisitorActor
+ */
+abstract class Actor
+{
+    /**
+     * @phpstan-assert-if-true SystemActor $this
+     */
+    public function isSystem(): bool
+    {
+        return $this instanceof SystemActor;
+    }
+
+    /**
+     * @phpstan-assert-if-true AnonymousVisitorActor $this
+     */
+    public function isAnonymousVisitor(): bool
+    {
+        return $this instanceof AnonymousVisitorActor;
+    }
+}
+
+class SystemActor extends Actor {}
+class AnonymousVisitorActor extends Actor {}
+
+function test(AnonymousVisitorActor|SystemActor $actor): void {
+    assertType('Bug13358\AnonymousVisitorActor|Bug13358\SystemActor', $actor);
+
+    if ($actor->isSystem()) {
+        assertType('Bug13358\SystemActor', $actor);
+    } else {
+        assertType('Bug13358\AnonymousVisitorActor', $actor);
+    }
+}