wouldn't array_merge be more appropriate?

I wish it work, here I put more details #54901 (comment)

Shouldn't this be spotted by fabbot? 🤔

What about the situation when the argument is UploadedFile and the $payload is an array of files? I think this would still result in a 500?

Interesting 🤔
Is it possible to have UploadedFile as part of #[MapRequestPayload] $payload? (#[MapQuery] we can exclude, I guess)
Could you please share an example of what you mean, so I can elaborate more on this ?

Should we check if it is variadic as well as if the type is an array here, similar to the check in the mapUploadedFile method?

Basically $arguments here are original arguments of an action.
In example below $arguments = [$request, $payload, $files];

public function action(
 Request $request,
 #[MapRequestPayload] Payload $payload,
 #[MapUploadedFile] UploadedFile ...$files]
) {
   ...
}

And the magic here is to expand variadic $files into several items of $arguments.
I hope PHP won't change mechanics of how variadic works and it will be always only 1 possible variadic argument in function signature 🤞

Also, there is already an ability to use #[MapUploadedFile] as bellow. And it works and covered by UploadedFileValueResolverTest::testMultipleFilesArray test case.

public function action(
 Request $request,
 #[MapRequestPayload] Payload $payload,
 /** @var UploadedFile[] */
 #[MapUploadedFile] array $files]
) {
   ...
}

I have tested variant

final class TestPayloadRequest
{
    public UploadedFile $file;
}

final class IndexController extends AbstractController
{
    #[Route(path: "/test-upload", methods: "POST")]
    public function testUpload(
        #[MapRequestPayload] TestPayloadRequest $file,
        #[MapUploadedFile] UploadedFile ...$files,
    ): Response {
        return new Response();
    }
}

with request like

POST /test-upload HTTP/1.1
Cookie: XDEBUG_SESSION=PHPSTORM;
Content-Type: multipart/form-data; charset=utf-8; boundary=__X_PAW_BOUNDARY__

--__X_PAW_BOUNDARY__
Content-Disposition: form-data; name="files[]"; filename="test.yaml"
Content-Type: application/x-yaml

info: test
--__X_PAW_BOUNDARY__
Content-Disposition: form-data; name="file"; filename="test.yaml"
Content-Type: application/x-yaml

info: test
--__X_PAW_BOUNDARY__
Content-Disposition: form-data; name="files[]"; filename="test.yaml"
Content-Type: application/x-yaml

info: test

Shortly: current implementation is not designed to work like this, because data for #[MapRequestPayload]
is fetched like this :

https://github.com/melya/symfony/blob/fix-map-uploaded-file-in-request-payload-resolver2/src/Symfony/Component/HttpKernel/Controller/ArgumentResolver/RequestPayloadValueResolver.php?plain=1#L212-L232