Warn about @elidable under -Xsource:3

[som-snytt]

Follow-up to #10408
Addresses scala/docs.scala-lang#2288

[som-snytt]

or maybe that is too annoying, as there is no replacement idiom.

[lrytz]

Yeah, let's keep an eye on scala/scala3#17329 and close this for now.

[SethTisue]

-Xmigration exists for this kind of thing, though I suspect it's little-used.

[SethTisue]

(Another possible venue is the Scala 3 migration tool.)

[som-snytt]

At the definition, I would just deprecate, except we need deprecate under the unfortunate -Xsource:3.

Shall I reopen this for elidable? I suggested somewhere that dotty should warn that it doesn't support it, instead of silently ignoring it.

[som-snytt]

The replacement idiom in the doc (for Scala 3) almost works in Scala 2, except that the conditional is not inlined.

Normally, if (false) is elided in refchecks. Currently, the optimizer doesn't touch if (1 == 1), for example, which is the result of inlining with constant value arg.

[lrytz]

Shall I reopen this for elidable?

You confused me there for a minute :)