Is it possible to set an implicitNotFound annotation inside a macro?

[soronpo]

In shapeless 2 there is this macro trick for Lazy that enables to set the annotation of a symbol. I use this concept quite a lot in my own libraries to set proper error messages for implicit constraints. Is there a way to do this inside a Scala 3 macro?

[nicolasstucki]

What operation do you need to perform?

I see that we are missing the operation to add an annotation.

[nicolasstucki]

What would the injectImplicitNotFoundError method do from the user perspective?

[soronpo]

Here is an example:

trait Foo[T]
given [T] (using Bar[T]) : Foo[T] = ???
trait Bar[T]
given [T] (using BiggerThan5[T]) : Bar[T] = ???
trait BiggerThan5[T]
inline given [T]  : BiggerThan5[T] = 
  inline compiletime.constValue[T] match 
    case i if i <= 5 => 
      compiletime.injectImplicitNotFounError("Argument is not bigger than 5")
      compiletime.error("Argument is not bigger than 5")
    case _ => new BiggerThan5[T]

val f1 = summon[Foo[3]] //expected error: Argument is not bigger than 5
val f2 = summon[Foo[6]] //OK

So injectImplicitNotFounError knows that the last implicit will be Foo and can modify its implicitNotFound accordingly. Or, we modify the error reported to consider such injected errors without modifying the implicitNotFound annotation.
Maybe we can have an argument for compiletime.error instead, like `compiletime.error("Argument is not bigger than 5", injectImplicitError = true).

[nicolasstucki]

It seems that what we need is a mechanism to tell the type to not try more implicit searches.

[nicolasstucki]

Have you tried to return a '{ compiletime.error(Argument is not bigger than 5) }? This should tell the implicit search that it found something, but will later fail (as long as the implicit argument is not dropped).

[soronpo]

Hmmm... it surprised me that it works even without the quotes. I assumed Scala 2 behavior, but something has changed, I think. I'll do some more experimentation and get back to you.