Preserve singleton types for final val aliases to Java enum constants

[zielinsky]

Fixes #24750
Fixes #21881

[som-snytt]

I mentioned on the ticket that they can't be used in annotations. I tried one approach that was too clever, rewriting them as inline transparent def. Just in case there's an easy fix to the type, since enums are not constant (any more).

Edit: I guess the singleton type will be visible in the back end, at least, which is currently widened!

[zielinsky]

I mentioned on the ticket that they can't be used in annotations. I tried one approach that was too clever, rewriting them as inline transparent def. Just in case there's an easy fix to the type, since enums are not constant (any more).

Edit: I guess the singleton type will be visible in the back end, at least, which is currently widened!

@som-snytt now they can! Look at 7fee34d !:> Used exactly your insight - the singleton type is visible in backend. Should I mark this as fixing #21881?

[som-snytt]

@zielinsky that's amazing, thanks. I took a look yesterday and didn't formulate it correctly.

Yes, I think this is sufficient for the use case in #21881.

Edit: I just deleted my local branch issue/21881-post-zielinsky.

[som-snytt]

technically, in this form the val is too eager, albeit not expensive. Currently, I like inline def for local helpers. (It would be nice if it were inlined without asking.)

[som-snytt]

FSR I get the complement in the warn test.

It would fail on pattern case: NANOSECONDS, MICROSECONDS, MILLISECONDS, SECONDS, MINUTES, HOURS

I was about to inspect the back end fix.

Clean build (?) has similar warning in Duration.scala.

Edit: the difference is bootstrapped.

sbt:scala3-bootstrapped> scalac tests/pos/i24750.scala
[info] running (fork) dotty.tools.dotc.Main -d /home/amarki/projects/scala3/out/default-last-scalac-out.jar -classpath /home/amarki/projects/scala3/library/target/scala-library-bootstrapped/scala-library-3.8.2-RC1-bin-SNAPSHOT.jar tests/pos/i24750.scala
[success] Total time: 7 s, completed Jan 16, 2026, 5:34:38 PM
sbt:scala3-bootstrapped> project scala3-nonbootstrapped
[info] set current project to scala3-nonbootstrapped (in build file:/home/amarki/projects/scala3/)
sbt:scala3-nonbootstrapped> scalac tests/pos/i24750.scala
[info] running (fork) dotty.tools.dotc.Main -d /home/amarki/projects/scala3/out/default-last-scalac-out.jar -classpath /home/amarki/projects/scala3/library/target/scala-library-nonbootstrapped/scala-library-3.8.2-RC1-bin-SNAPSHOT-nonbootstrapped.jar tests/pos/i24750.scala
-- [E029] Pattern Match Exhaustivity Warning: tests/pos/i24750.scala:7:13 ------
7 |    duration.unit match
  |    ^^^^^^^^^^^^^
  |match may not be exhaustive.
  |
  |It would fail on pattern case: NANOSECONDS, MICROSECONDS, MILLISECONDS, SECONDS, MINUTES, HOURS
  |(More unmatched cases are elided)
  |
  | longer explanation available when compiling with `-explain`
1 warning found
[success] Total time: 5 s, completed Jan 16, 2026, 5:35:01 PM

[som-snytt]

I wonder if there is a reason to prefer atPhaseBeforeTransforms or conversely to avoid it.

I see to my chagrin that braceless colon syntax does not work, atPhaseBeforeTransforms:.

[som-snytt]

"Better you than me!"