Missing BuildFrom instance for IterableOnce

[MasseGuillaume]

def foo[T](rs: IterableOnce[Future[T]]): Future[T] = Future.sequence(rs)

Cannot construct a collection of type To with elements of type T based on a collection of type collection.compat.IterableOnce[scala.concurrent.Future[T]].
[error]         Future.sequence(resources)
[error]                        ^

Workaround:

def foo[T](rs: IterableOnce[Future[T]]): Future[T] = Future.sequence(rs.iterator)

[szeiger]

Attempt in scala/scala#7006, reverted in scala/scala#7088

[szeiger]

I've taken another look at these two PRs. Firstly, I think my original statement that we need a separate definition for Iterator was wrong. This should work just fine because the third type parameter is always supposed to be inferred:

  implicit def buildFromIterableOnce[A]: BuildFrom[IterableOnce[_], A, Iterator[A]] = new BuildFrom[IterableOnce[_], A, Iterator[A]] {
    def newBuilder(from: IterableOnce[_]): mutable.Builder[A, Iterator[A]] = Iterator.newBuilder
    override def fromSpecific(from: IterableOnce[_])(it: IterableOnce[A]) = it.iterator
  }

The problem with either version is that we cannot use priorities to disambiguate this against the defintions in factory objects. I'm not sure other definitions do not cause similar problems. You'd have to debug the implicit resolution process to see why the correct definitions end up being more specific in these cases.

I haven't tested this with Dotty but I suppose it would work there. This is the ambiguity you get in Scala:

Welcome to Scala 2.13.0-20190107-124924-eeea070 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_172).
Type in expressions for evaluation. Or try :help.

scala> implicitly[scala.collection.BuildFrom[scala.collection.mutable.AnyRefMap[String, String], (String, String), _]]
                 ^
       error: ambiguous implicit values:
        both method buildFromAnyRefMap in object AnyRefMap of type [K <: AnyRef, V]=> scala.collection.BuildFrom[scala.collection.mutable.AnyRefMap[_, _],(K, V),scala.collection.mutable.AnyRefMap[K,V]]
        and method buildFromIterableOnce in trait BuildFromLowPriority3 of type [A]=> scala.collection.BuildFrom[scala.collection.IterableOnce[_],A,Iterator[A]]
        match expected type scala.collection.BuildFrom[scala.collection.mutable.AnyRefMap[String,String], (String, String), _]

The only reason why buildFromAnyRefMap would not be more specific (because it clearly looks like it ought to be) is that the first two type parameters in BuildFrom are contravariant. Dotty's "contrarivariant" implicit resolution should see it as more specific.

[szeiger]

I think we can live with not having a buildFromIterableOnce and I can't think of a good solution for the problem so I'm rescheduling it to 2.14 for now.