Some givens ignore @compileTimeOnly annotation

[pweisenburger]

Compiler version

3.3.1-RC1-bin-20230216-2507577-NIGHTLY

Minimized code

Some givens ignore their @compileTimeOnly annotation and let code compile when it should error. See comments in the code below:

import scala.annotation.compileTimeOnly

sealed trait Test[T]

object Test:
  @compileTimeOnly("Error")
  given test0[T]: Test[T] = ???

  @compileTimeOnly("Error")
  given test1[T]: Test[T]()

  test0 // produces a compiler error, which is correct

  test1 // compiles but should not

---

EDIT: A further observation (that may be related): The following code fails to compile but should IMHO:

import scala.annotation.compileTimeOnly

sealed trait Test

object Test:
  @compileTimeOnly("Error")
  given Test()

Error message:

[error] 35 |object Test:
[error]    |       ^
[error]    |       Error
[error] one error found

[odersky]

The second failing test is due to the method ordinal that's generated for the mirror of Test.

import scala.annotation.compileTimeOnly

sealed trait Test

object Test:
  @compileTimeOnly("Error")
  given Test()

[odersky]

The first example works as expected if @compileTimeOnly is replaced with erased.

The reason it does not work with compileTimeOnly is that annotations are not copied from the class to the accessor. I believe it would be dangerous to copy all annotations, so we'd have to make a special case for @compileTimeOnly. Given that compileTimeOnly is meant to be replaced by erased, the problem will be solved eventually anyway.

[nicolasstucki]

I'm not sure that we can replace @compileTimeOnly with erased. @compileTimeOnly complements erased with the ability to give an extra message.

Probably, all @compileTimeOnly should be marked as erased. Though we should double-check in the community if there are uses where this would not work. One case that could fail (but not sure if it exists) is if we override a method with @compileTimeOnly, stating that this method should have been optimized away but is still callable through the overridden version.

[pweisenburger]

The reason it does not work with compileTimeOnly is that annotations are not copied from the class to the accessor. I believe it would be dangerous to copy all annotations, so we'd have to make a special case for @compileTimeOnly.

This sounds to me like the more principled way to control where compileTimeOnly goes are the scala.annotation.meta._ annotations. But maybe this is not possible in this case?

In any case, I would also advocate keeping the possibility to add a customized message to the compiler error.

[odersky]

We could use scala.annotation.meta.companionClass and companionMethod for that. Right now, these meta annotations are ignored in dotty. They don't work for implicit classes either.