Use actual reproducer from issue #8048 and add NSRT test

The previous test didn't match the playground reproducer — it used
@return T|null instead of the conditional return type that triggers
the bug. Replace with the actual reproducer: a conditional return type
($responseType is class-string<T> ? T : null) called with null.

Add NSRT test verifying that request(null) resolves to null and
request(CustomResponse::class) resolves to CustomResponse.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-8048.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types=1);
+
+namespace Bug8048Nsrt;
+
+use function PHPStan\Testing\assertType;
+
+interface CustomResponseInterface {}
+
+class CustomResponse implements CustomResponseInterface {}
+
+class ApiService
+{
+	/**
+	 * @template T of CustomResponseInterface
+	 *
+	 * @param class-string<T>|null $responseType
+	 *
+	 * @return ($responseType is class-string<T> ? T : null)
+	 */
+	public function request(?string $responseType = null): ?CustomResponseInterface
+	{
+		if ($responseType === null) {
+			return null;
+		}
+
+		return new CustomResponse();
+	}
+}
+
+function (): void {
+	assertType('null', (new ApiService())->request(null));
+	assertType('Bug8048Nsrt\CustomResponse', (new ApiService())->request(CustomResponse::class));
+};

tests/PHPStan/Rules/Methods/data/bug-8048.php

@@ -1,33 +1,31 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Bug8048;
 
-interface CustomResponseInterface
-{
-}
+interface CustomResponseInterface {}
 
-final class CustomResponse implements CustomResponseInterface
-{
-}
+class CustomResponse implements CustomResponseInterface {}
 
-final class ApiService
+class ApiService
 {
 	/**
 	 * @template T of CustomResponseInterface
-	 * @param class-string<T> $class
-	 * @return T|null
+	 *
+	 * @param class-string<T>|null $responseType
+	 *
+	 * @return ($responseType is class-string<T> ? T : null)
 	 */
-	public function request(string $class): ?CustomResponseInterface
+	public function request(?string $responseType = null): ?CustomResponseInterface
 	{
+		if ($responseType === null) {
+			return null;
+		}
+
 		return new CustomResponse();
 	}
 }
 
-final class Consumer
-{
-	public function test(): void
-	{
-		$apiService = new ApiService();
-		$result = $apiService->request(CustomResponse::class);
-	}
-}
+function (): void {
+	(new ApiService())->request(null);
+	(new ApiService())->request(CustomResponse::class);
+};