Generalize Future.traverse and Future.sequence

[markus1189]

Allow CanBuildFrom to determine the resulting collection type, instead
of forcing it to be the same as the input. This allows to do a
Future.traverse on a Map[A,B] and using collection.breakOut get
back a Future[Map[A,B]] again.

[markus1189]

/cc @axel22 any thoughts?

[axel22]

Looks like it should work.
Can't say if there are any cases where it breaks type inference, though.

[szeiger]

It looks safe to me. It should always be possible to infer the 3rd type parameter. My only concern with this change is that we're planning to move away from CanBuildFrom in the 2.13 collections, which means removing collection.breakOut as one of the few breaking changes.

[markus1189]

@szeiger Thanks for picking this up. I am also fine with just closing this if it will be obsolete soon.

[szeiger]

I suggest putting it on hold for a few months until we have a better idea of what the impact of the 2.13 collection changes will be on this PR.

[SethTisue]

if anyone wants to pick this up, please (unless you're reading this after 2.13.0-M4 is out) take it up over in https://github.com/scala/collection-strawman