scalac warns about deprecated use of identifier "then" even if it is marked as deprecated

[scabug]

In the following source code

    @deprecated("Use Then instead of then", "0.10.0-M2")
    def then[B : BaseTypeMapper](res: Column[B]) = Then(res)

scalac complains that

C:\Users\szeiger\code\scala-query\src\main\scala\scala\slick\ql\Case.scala:28: then is now a reserved word; usage as an identifier is deprecated
    def then[B : BaseTypeMapper](res: Column[B]) = Then(res)
        ^

This is not usually the case when using deprecated API. If the caller is also deprecated, no warning should be emitted.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5715?orig=1
Reporter: @szeiger
Affected Versions: 2.10.0-M3

[scabug]

@paulp said:
This is actually tricky to deal with, because we need to warn on then but not then (I don't know if jira will savage my backticks) and that information is discarded before annotations are processed.

[magnolia-k]

Warnings about reserved words are given in the lexical analysis stage(src/compiler/scala/tools/nsc/ast/parser/Scanners.scala ). Therefore, warnings are displayed in all places where they appear as identifiers.

Isn't it enough to issue warnings at the time of identifier declaration?

package example

class then

object Foo {
  val t = new then
}

[info] compiling 1 Scala source to /xxx/scala-seed-project/target/scala-2.13/classes ...
[warn] /xxx/scala-seed-project/src/main/scala/example/Hello.scala:3:7: then is a reserved word (since 2.10.0); usage as an identifier is deprecated
[warn] class then
[warn]       ^
[warn] /xxx/scala-seed-project/src/main/scala/example/Hello.scala:6:15: then is a reserved word (since 2.10.0); usage as an identifier is deprecated
[warn]   val t = new then
[warn]               ^
[warn] two warnings found

With the release of Scala3, reserved words have already been fixed, and reserved words other than then have been added.

Therefore, it would be better to remove the warning about future reserved words in the lexical analysis stage, and change it to warn against reserved words other than then, which were added in Scala3, only when declaring identifiers.

[SethTisue]

it would be better to remove the warning about future reserved words in the lexical analysis stage

agree; I would be in favor of merging a PR that did only that

and as a separate matter, if new warnings are added, I'd suggest putting those warnings behind -Xsource:3

[som-snytt]

It could be done via the much-abused "extra units of work" that run after typer.

Edit: I had a branch from 2019 that started down that path.

[som-snytt]

new then

should be

new `then`

that is, if we were still using new.

These warnings already go through runReporting, so -Wconf works.

Normally, Refchecks checks deprecated usage. It could also do the name check, as an Ident now knows if it was backticked.

For the definition, typer could clear buffered deprecations in range of the deprecated definition.

[som-snytt]

The future keywords warning was at scala/scala#9722.

My first take removed the current warning at scanner, added the usage warning in refchecks (check the name when the ref is checked) and the definition site warning piggy-backing on the future keywords warning in parser.

My second take piggybacks on -Wconf/nowarn to emit a Suppression when deprecations are seen, the equivalent of @nowarn(cat=deprecation) but does not trigger unused warning. (But the deprecation from scanner doesn't get site and origin for advanced -Wconfing.)

The hard part on first take was figuring out who was stripping the isBackquoted attachment. (It was the conversion to type name.)