-Xsource:2.11 should disallow trailing commas under Scala 2.12?

[Sciss]

This

List(
  1,
  2,
  3,
)

is valid in Scala 2.12. If I cross-build with 2.11 as another target, I think I should be able to disallow this using

scalacOptions += "-Xsource:2.11"

but the above still compiles under Scala 2.12

[OlegYch]

fwiw i use this https://github.com/47deg/scala-commas to cross build to 2.11

[SethTisue]

I haven't seen that -source respects syntax evolution backwardly

I'm not sure if there's precedent, either. before using -Xsource:2.11 in a build of my own, I think I'd have to do a careful audit for what else changes as a result

[SethTisue]

in general, my impression is that -Xsource (and its sisters -Xfuture and -Xexperimental) are intended for testing, experimentation, and temporary use during migration, and not so much as a component of cross-building support. but then actually I don't know why would we would support backwards -Xsource, as opposed to forward -Xsource, at all

[som-snytt]

@SethTisue I think this is settled. #8361 except for the backwards part.

Frankly, I can't imagine disallowing trailing comma under any regime. That would only spur tickets to "backport trailing comma."

It's "trailing comma" singular because you only get one.

[dwijnand]

I think this should be closed. We're not going to add 2.11 source support to 2.13.