type error when specifiying a connectionTimeout for HttpClient

[miDeb]

Minimal code example:

import 'dart:io';

void main() async {
  final client = HttpClient();
  client.connectionTimeout = Duration();
  await client.openUrl(
    'get',
    Uri.parse(
      'https://example.com/',
    ),
  );
}

results in:

Unhandled exception:
type '() => Null' is not a subtype of type '(() => FutureOr<SecureSocket>)?' of 'onTimeout'
#0      Future.timeout (dart:async/future_impl.dart)
#1      _ConnectionTarget.connect.<anonymous closure> (dart:_http/http_impl.dart:2222:37)
<asynchronous suspension>
#2      _ConnectionTarget.connect.<anonymous closure> (dart:_http/http_impl.dart)
<asynchronous suspension>

OS: linux
dart: Dart SDK version: 2.13.0-0.0.dev (dev) (Thu Feb 4 08:08:19 2021 -0800) on "linux_x64"

[mit-mit]

cc @lrhn

[miDeb]

I believe the problem is the following:

Future socketFuture = task.socket; // type of task.socket: Future<Socket>
// ...
// but socketFuture is used as a Future<Socket?>, if there is a timeout it returns null:
socketFuture = socketFuture.timeout(connectionTimeout, onTimeout: () {
    _socketTasks.remove(task);
   ask.cancel();
   return null;
});

What's the proper way to convert a Future<T> to a Future<T?> ?

[lrhn]

Currently there is no easy way to just introduce nullability in the type of a future.
I'd write this as:

socketFuture = socketFuture.then<Socket?>((x) => x).timeout(....);

We could perhaps introduce a cast extension method on futures, so you could just write:

socketFuture = socketFuture.cast<Socket?>().timeout(...);

but we can't restrict it to up-casts, so it's going to be doing dynamic casts of all events, even when not necessary.
The alternative would be a Future<T?> orNull(); extension method on Future<T>.

I'll fix the dart:io library to not make that type error.

[miDeb]

It is currently unclear when/how a null safe version of dio will be released because the mantainer is inactive, but this bug will likely block a release. Is it possible to uplift this fix (if that is necessary)?
EDIT: Oh I see, that has already happended :)

[mit-mit]

This issue was cherry picked in #44989, and is available in Dart's beta channel in version 2.12.0-259.14.beta and later.