Fix #4785: change typing rule for _* args

[allanrenucci]

In expression position, if a tree is of the form expr: _*, try to type
expr as an Array and as a Seq if there are errors

[smarter]

FWIW, the following code used to find the seq implicit but now finds the array implicit (it doesn't compile at all with 2.12):

object Test {
  implicit def array: Array[Int] = Array(0)
  implicit def seq: Seq[Int] = Seq(0)

  def foo(x: Int*) = x

  def main(args: Array[String]): Unit = {
    println(foo(implicitly: _*))
  }
}

Any reason to try the Array prototype first? Most of the time I expect users would pass a Seq, so that would be a lot of unnecessary retyping. I also wonder if you could use a union of Array or Seq as the prototype.

[allanrenucci]

Any reason to try the Array prototype first?

We could try Seq first if there were no implicit conversion from Array to Seq defined in Predef

[smarter]

Maybe try Array first only for JavaDefined methods?

[smarter]

Wait, doesn't #4785 imply that this implicit conversion won't exist with 2.13, thus solving the problem?

[allanrenucci]

No, still there.

[allanrenucci]

Anyway, relying on an implicit conversion defined in the standard library to typecheck calls to Java methods with repeated arguments seems like a very bad idea to me. What if I compile with -Yno-imports?

[odersky]

I think we agreed that we would not support this. I.e. we should allow arrays only for Java varargs.

In other words:

• If it's a Java vararg the argument has to be an array
• If it's a Scala vararg the argument has to be an immutable.Seq.'

[odersky]

On further discussion, I think it's preferable to have automatic conversions for varargs. We already have a conversion from Seq to Array when passing a Seq argument to a Java vararg. We should have a dual conversion from Array to Seq when passing an Array argument to a Scala vararg. I believe the Array-> Seq conversion should make a defensive copy of the array in order to prevent mutations leaking into the vararg.

[allanrenucci]

Superseded by #5577