scala3-macro-repro.scala

package repro

trait ResZero[V]:
    type Res
object ResZero:
    transparent inline given [V]: ResZero[V] = ${ macros.resZeroImpl[V] }
end ResZero

trait SomeTrait[V]:
    type Res
object SomeTrait:
    // what if I try to generalize a macro that sets any type with refinement 'type Res' to zero?
    // fails to compile (see below)
    //transparent inline given [V]: SomeTrait[V] = ${ macros.resZeroHKImpl[V, SomeTrait] }
end SomeTrait

object macros:
    import scala.quoted.*

    // compiles and works fine
    def resZeroImpl[V](using Type[V], Quotes): Expr[ResZero[V]] =
        '{ new _root_.repro.ResZero[V] { type Res = 0 } }

    // try to define a macro that sets higher kinded type:
    //def resZeroHKImpl[V, F[_] <: { type Res }](using Type[V], Type[F], Quotes): Expr[F[V]] =
    //    '{new F[V] { type Res = 0 } }

    // the above fails to compile with error:
    // [error] 20 |        '{new F[V] { type Res = 0 } }
    // [error]    |              ^^^^
    // [error]    |F[V] is not a class type

end macros