Compiler plugin between parser and typer combined with macro usage causes duplicate compilation and compile error

[mrdziuban]

Compiler version

3.3.0, also the latest 3.3.1-RC5 and 3.4.0-RC1-bin-20230809-c5adafc-NIGHTLY

Minimized code

https://github.com/mrdziuban/dotty-compiler-plugin-issue/tree/duplicated-code

The compiler plugin inserts a phase between the parser and the typer, but the phase doesn't do anything. The code in the tests directory defines a simple macro in Macro.scala and a call to the macro in Test.scala.

I believe both the placement of the compiler plugin and the existence of the macro matter -- if I change either one of them, i.e. run the plugin after the typer or remove the macro, then the issue stops happening.

Output

-- [E161] Naming Error: /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Test.scala:3:0
3 |object test {
  |^
  |test is already defined as object test in /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Test.scala

When I enable -Xprint:typer, the full sbt output that shows the duplicated object test looks like this:

[info] [[syntax trees at end of                     typer]] // /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Macro.scala
[info] package example {
[info]   import scala.quoted.*
[info]   final lazy module val Macro$package: example.Macro$package =
[info]     new example.Macro$package()
[info]   final module class Macro$package() extends Object() {
[info]     this: example.Macro$package.type =>
[info]     private[example] def idMacroImpl[A >: Nothing <: Any](
[info]       implicit evidence$1: quoted.Type[A], ctx: quoted.Quotes):
[info]       quoted.Expr[A => A] =
[info]       '{
[info]         {
[info]           def $anonfun(a: A): A =
[info]             {
[info]               a
[info]             }
[info]           closure($anonfun)
[info]         }
[info]       }.apply(ctx)
[info]     inline def idMacro[A >: Nothing <: Any]: A => A =
[info]       ${
[info]         {
[info]           def $anonfun(using evidence$2: quoted.Quotes): quoted.Expr[A => A] =
[info]             example.Macro$package.inline$idMacroImpl[A](
[info]               quoted.Type.of[A](evidence$2), evidence$2)
[info]           closure($anonfun)
[info]         }
[info]       }:(A => A)
[info]     def inline$idMacroImpl[A](implicit evidence$1: quoted.Type[A],
[info]       implicit ctx: quoted.Quotes): quoted.Expr[A => A] =
[info]       example.Macro$package.idMacroImpl[A](evidence$1, ctx)
[info]   }
[info] }
[info] [[syntax trees at end of                     typer]] // /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Test.scala
[info] package example {
[info]   final lazy module val test: example.test = new example.test()
[info]   final module class test() extends Object() { this: example.test.type =>
[info]     val intId: Int => Int = example.idMacro[Int]
[info]   }
[info] }
[error] -- [E161] Naming Error: /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Test.scala:3:0
[error] 3 |object test {
[error]   |^
[error]   |test is already defined as object test in /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Test.scala
[info] [[syntax trees at end of                     typer]] // /Users/matt/dotty-compiler-plugin-issue/tests/src/main/scala/Test.scala
[info] package example {
[info]   final lazy module val test$1: example.test = new example.test()
[info]   final module class test$2() extends Object() { this: example.test.type =>
[info]     val intId: Int => Int = example.idMacro[Int]
[info]   }
[info] }

Expectation

Compilation should succeed

[smarter]

Err, you're not supposed to be able to make a non-ResearchPlugin that affects typechecking (https://docs.scala-lang.org/scala3/reference/changed-features/compiler-plugins.html#), so a plugin before typer should be rejected.

[mrdziuban]

Maybe I should have kept my mouth shut then 😬 , that would unfortunately make a compiler plugin I maintain obsolete -- https://github.com/mblink/nowarn-plugin

I would prefer to create subclasses of annotation.nowarn, but there are issues in both scala 2 and 3

• In scala 2, subclasses of annotation.nowarn suppress all warnings, not just the configured ones -- Custom annotation extending annotation.nowarn also silences exhaustivity warning bug#12367
• In scala 3, subclasses of annotation.nowarn don't seem to suppress any warnings -- https://scastie.scala-lang.org/mrdziuban/SJ5vuxfsQzC4mNKxXYQntw/6

[smarter]

Hmm I see, this is a bit annoying to support since the parameter passed to the parent class isn't reachable from the symbol of the class, but it's clear that we should either support it or deprecate extending nowarn. This isn't exactly the same, but the following works in Scala 2 and 3 and could be a good alternative I think:

import scala.annotation.nowarn

object Constants {
  final val unused = "msg=(unused|never used)"
}
import Constants._

@nowarn(unused)
object test {
  import scala.collection.immutable.SortedSet
}

[soronpo]

Could be related to #17378