Symfony version(s) affected: 4.x, 5.x

Description

RejectedPromise returned here is Http\Promise\RejectedPromise, but not the full implementation like Symfony\Component\HttpClient\Response\HttplugPromise (as in docblock @return)
This can cause problem. For example, promises returned by then functions will be treated as plain value.

The code:
https://github.com/php-http/promise/blob/02ee67f2346b4eec3861afec741c656f5ff717ab/src/RejectedPromise.php

How Guzzle promise settle promises:
https://github.com/guzzle/promises/blob/bbf3b200bc83c1e9298580a9f99b9be248543467/src/RejectedPromise.php#L40
https://github.com/guzzle/promises/blob/bbf3b200bc83c1e9298580a9f99b9be248543467/src/Promise.php#L108

How to reproduce

https://github.com/brentybh/symfony-37488
Sorry, it's not finished yet.
Because I have to bring in a retry middleware.
The Httplug simple promise CAN cover most simple use cases. But the retry middleware implementation is different...
Possible Solution

emm.. Maybe it's not strictly an issue of Symfony but a problem of Httplug....
Additional context