Why does flatten on nested Iterator not compile and why do I need type ascription?

Question

(new Iterator[List[Int]] {
  def hasNext: Boolean = ???
  def next(): List[Int] = ???
}).flatten

gives error:

value flatten is not a member of Iterator[List[Int]]
[error] possible cause: maybe a semicolon is missing before `value flatten'?
[error]     }.flatten
[error]       ^
[error] one error found

But

(new Iterator[List[Int]] {
  def hasNext: Boolean = ???
  def next(): List[Int] = ???
}: Iterator[List[Int]]).flatten

works. Also storing the iterator in a val works.

Scala version: 2.11.8

Answer 1

I believe this issue was addressed by Include the parts of a compound/refinement type in implicit scope. #5867. The problem was companion to anonymous classes used to not qualify for implicit search

I think aaf9198#diff-7c03397456d3b987549fd039d6b639c6R516 was the first to exclude refinement/anon classes from contributing to companion implicits. @odersky Can you remember the motivation?

Here is a minimal reproduction

trait A {
  def foo(): Int
}
class B {
  def bar(): String = "woohoo"
}
object A {
  implicit def aToB(a: A): B = new B
}

(new A {
  override def foo(): Int = 42
}).bar()

// Error: 
// value bar is not a member of A$A12.this.A 
// possible cause: maybe a semicolon is missing before `value bar'? }).bar();

Therefore since in 2.11 and 2.12 flatten on Iterator was provided as an extension method via flattenTraversableOnce the issue would manifest for anonymous Iterator class.

---

^{The following is my answer prior to the edit above:}

It seems to have something to do with implicit resolution in both 2.11 and 2.12. If you explicitly import flatten as an extension method via

import scala.collection.TraversableOnce.flattenTraversableOnce

then it seems to work. The issue seems to be fixed since 2.13.0-M3 where typer phase gives

collection.this.TraversableOnce.flattenTraversableOnce[Int, List]({
  final class $anon extends AnyRef with Iterator[List[Int]] {
    def <init>(): <$anon: Iterator[List[Int]]> = {
      $anon.super.<init>();
      ()
    };
    def hasNext: Boolean = scala.Predef.???;
    def next(): List[Int] = scala.Predef.???
  };
  new $anon()
})(scala.Predef.$conforms[List[Int]]).flatten

whilst in 2.13.0 release flatten no longer seem to be provided via extension method

{
  final class $anon extends AnyRef with Iterator[List[Int]] {
    def <init>(): <$anon: Iterator[List[Int]]> = {
      $anon.super.<init>();
      ()
    };
    def hasNext: Boolean = scala.Predef.???;
    def next(): List[Int] = scala.Predef.???
  };
  new $anon()
}.flatten[Int](scala.Predef.$conforms[List[Int]])

The above expansion seems to be explained by SLS 6.4 Designators

𝑒.𝑥 is typed as if it was { val 𝑦 = 𝑒; 𝑦.𝑥 }

Answer 2

I think it tries to parse this:

new Iterator[List[Int]] {
      def hasNext: Boolean = ???
      def next(): List[Int] = ???
    }.flatten

as this

new Iterator[List[Int]] ( {
      def hasNext: Boolean = ???
      def next(): List[Int] = ???
    }.flatten )

Notice where I added the parentheses ().