The Problem

Promises have 2 important methods: then and wait.
To implement Httplug's promise interface, we built HttplugPromise on top of Guzzle promise.
However, when an error occurred (Httplug NetworkException thrown) while init the request/before actually sending the request,
HttplugClient::sendAsyncRequest will return a Http\Promise\RejectedPromise, which is a dummy implementation.

If the then callable returns a promise-like object, Http\Promise\RejectedPromise will treat it as plain value.
Guzzle promise will try to resolve the promise-like value, which is an object that has then method on it.

https://github.com/guzzle/promises/blob/bbf3b200bc83c1e9298580a9f99b9be248543467/src/Promise.php#L116

To fix this, I edited src/Symfony/Component/HttpClient/HttplugClient.php.

Next, let me explain why to edit src/Symfony/Component/HttpClient/Response/HttplugPromise.php.
After the previous fix, when a Guzzle promise returned by the then callable, things will work.
However, If I return a HttplugPromiseInterface, it doesn't work, because Guzzle promise wait the return value (result)
only if it's a Guzzle promise.

https://github.com/guzzle/promises/blob/bbf3b200bc83c1e9298580a9f99b9be248543467/src/Promise.php#L63

To fix this, I referenced the wait code of Guzzle promise and edited our HttplugPromise.

How this fix make sense

So, why to return a promise from the then callable?
This let us change the promise chain according to current promise's result (fulfilled/rejected).

For example, we can retry an HTTP request if it failed.
Please take a look at my test code.