Add an iterator() alias to iterator

[julienrf]

We already have a scalafix rule that translates .iterator calls to .iterator(). This rule can be applied to all the code bases to make them compatible with 2.13. However, once the rule has been applied the code won’t be compatible with 2.12 (or previous Scala version) anymore…

This PR introduces an iterator() extension method for Scala 2.12 so that after the application of the scalafix rule, the code can still compile with 2.12.

A second commit makes it possible to write newBuilder() instead of newBuilder, in 2.12.

Also, related discussion: scala/collection-strawman#520

[julienrf]

I’m on the fence with the changes on parentheses. On one hand, there are some advantages of having a more disciplined used of parentheses (although, as noted by @Ichoran in the linked discussion, it is hard to find a really robust principle on when to use parentheses). But on the other hand that a source of incompatibilities between 2.13 and prior Scala versions… The trick I’m using here works for iterator() but it doesn’t always work for newBuilder(), because sometimes this newBuilder() method takes implicit parameters (typically, an Ordering or a ClassTag), and in that case the implicit conversion to the extension method is not triggered.

[szeiger]

This won't work for lazy collections. You don't want Stream.from(otherStream) to force anything.

[julienrf]

Stream uses a lazy builder, so this code doesn’t force the source. I’ve added a test for that.

[julienrf]

Now the real question is “do we really want to keep iterator() and newBuilder() in the new collections?”

[julienrf]

Closed because of scala/scala#6620