Question about method '+=' on ArrayBuilder[Unit]

[FabioPinheiro]

Minimized example

scala.collection.mutable.ArrayBuilder.make[Unit] += ()

Output

Compiles on Scala 2.13.2 but not on Dotty

scala> scala.collection.mutable.ArrayBuilder.make[Unit] += ()                                                         
1 |scala.collection.mutable.ArrayBuilder.make[Unit] += ()
  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |None of the overloaded alternatives of method += in trait Growable with types
  | (elem1: Unit, elem2: Unit, elems: Unit*): 
  |  scala.collection.mutable.ArrayBuilder[Unit]
  | (elem: Unit): scala.collection.mutable.ArrayBuilder[Unit]
  |match arguments ()

Expectation

Created this issue so we can officialize if this is the intended behavior or not. In order to see what to do on scala-js/scala-js#4071

The discussion starts Gitter where @smarter and @sjrd participated
https://gitter.im/lampepfl/dotty?at=5ede17109da05a060a5b9b14

[smarter]

Scala 2 parses a += () as a.+=(()) whereas dotty parses it as a.+=(), is this intended @odersky ?

[som-snytt]

This ticket may be used to forward-port the improvement at scala/scala#7684 which is in 2.13.0.

[FabioPinheiro]

The code a += () is creating an ast Tree like InfixOp(Ident(a),Ident(+=),Tuple(List()))

After filling the Parsers.scala file with prints I added the line case Nil => unitLiteral before https://github.com/lampepfl/dotty/blob/3ea2841db764bef11871a168e8d669bdd68d5ee7/compiler/src/dotty/tools/dotc/ast/untpd.scala#L469

After this change the previous tree is replaced with InfixOp(Ident(b),Ident(+=),Parens(Literal(Constant(()))))

I probably broke a lot of stuff. But for this particular case, I got the result that I was expecting after the typer phase.
Now dotc -Xprint:type MyTest.scala prints:

package <empty> {
  final lazy module val MyTest: MyTest$ = new MyTest$()
  final module class MyTest$() extends Object(), _root_.scala.Serializable { 
    this: MyTest.type =>
    def main(): Unit = 
      {
        val a: scala.collection.mutable.ArrayBuilder[Unit] = 
          scala.collection.mutable.ArrayBuilder.make[Unit](
            scala.reflect.ClassTag.Unit
          )
        {
          a.+=(())
          ()
        }
      }
  }
}

instead

package <empty> {
  final lazy module val MyTest: MyTest$ = new MyTest$()
  final module class MyTest$() extends Object(), _root_.scala.Serializable { 
    this: MyTest.type =>
    def main(): Unit = 
      {
        val a: scala.collection.mutable.ArrayBuilder[Unit] = 
          scala.collection.mutable.ArrayBuilder.make[Unit](
            scala.reflect.ClassTag.Unit
          )
        a.+=()
      }
  }
}

[FabioPinheiro]

Without much surprise List(() => 1) does not compile now XD

[odersky]

Scala 2 parses a += () as a.+=(()) whereas dotty parses it as a.+=(), is this intended @odersky ?

I am surprised at Scala 2's behavior here. Here is another example:

  Console.println ()
  Console println ()

parses as

    Console.println();
    Console.println(())

and prints one (), not two or none. So it seems that infix applications are no longer equivalent to dotted applications. Maybe this was part of the changes to eta expansion in 2.12? EDIT: I see now that it was #7684 so a lot more recent than that.

But with the changes intended for auto-tupling in mind we should parse it the same way as Scala 2.

[som-snytt]

The Scala 2 PR refers to an earlier PR scala/scala#6974 that says "Remove adaptation of 0-arg methods under -Xsource:3.0", so that was the context. Two years ago, we weren't yet comparing Dotty behavior as carefully as now. In fact,

scala> () == ()
1 |() == ()
  |^^^^^
  |not enough arguments for method ==: (x$0: Any): Boolean

[odersky]

@som-snytt Thanks for giving the context. Things goes in the right direction.

In fact it turns out that Dotty does parse the argument as an empty tuple, but then the pretty printer fooled us in dropping the extra pair of parentheses. The Tuple gets replaced with an empty argument list in desugaring.