False negative unused import when there's a reference in macro-expanded code

[kubukoz]

Compiler version

3.7.4, 3.8.1-RC1-bin-20251127-fd2172d-NIGHTLY

Minimized code

1. main.scala

//> using scala 3.nightly
//> using option -Wunused:all
import ext.Reader
import ext.NonEmptyList

@main def run = locally {
  macros.demo
}

2. ext.scala

package ext

case class Reader[F[_], A, B]()

case class NonEmptyList()

3. macros.scala

import ext.Reader
import scala.quoted.*

object macros {
  inline def demo: Unit = ${ demoImpl }

  def demoImpl(
    using Quotes
  ): Expr[Unit] =
    '{
      val r: Reader[Option, String, String] = ???
      ()
    }

}

Output

[warn] ./demo.scala:4:12
[warn] unused import
[warn] import ext.NonEmptyList
[warn]            ^^^^^^^^^^^^^

Expectation

An additional warning on import ext.Reader.

[som-snytt]

Discussion at #24248 where the macro does an implicit search and "uses" the import.

But tree positions lose fidelity, so nothing indicates in which context an identifier was resolved.

I always thought that we allowed false negatives in unused warnings

I agree false negatives are less annoying than false positives, because it takes users longer to complain about them.

Test example at https://github.com/scala/scala3/tree/main/tests/warn/i24248.

Workaround:

val r: ext.Reader[Option, String, String] = ???

Tree as written:

            Inlined(Select(Ident(macros), demo), List(),
              Typed(
                Inlined(Ident(macros$), List(),
                  Block(
                    List(
                      ValDef(r,
                        AppliedTypeTree(Ident(Reader), List(Ident(Option),
                          TypeTree(),TypeTree())),
                      Ident(???))
                    ),
                  Literal(()))
                ),
              Ident(Unit))
            )

The lint that tracked import usage directly
#14524
and I think also handled implicit lookup from macros; that feature finally arrived in Scala 2.