How can one match an array with quotes using Varargs?

[hmf]

I posted this question in the Discord meta-programming channel, and have had no luck. So I am trying here. Assume I have the following code:

    val s1 = ConstArray.testUnapply(Array(1, 2, 3))

I would like to match that array in the testUnapply(Array(1, 2, 3)) macro. The following match using quotes work:

      (x: Expr[Array[T]]) match
        case '{ scala.Array.apply($a:Int, $b:Int, $c:Int): scala.Array[Int]}  => 
          println(s"(a, b, c) = ($a, $b, $c)") 
          ???

However, I need to deal with a variable number of arguments. So I tried many variations that include:

        case '{ scala.Array.apply(${Varargs(Exprs(elems))}: Seq[Int])}  =>  
=>  
          Some(Array(elems*).asInstanceOf[Array[T]])
          ???

None of these work. I have also noticed that the following fails:

        case '{ scala.Array.apply[Int]($a:Int, $b:Int, $c:Int): scala.Array[Int]}  => 
          println(s"(a, b, c) = ($a, $b, $c)") 
          ???

Why does this fail? Do I have to add something related to to ClassTag? The following also fails:

      val fct = summon[FromExpr[ClassTag[Int]]]
      (x: Expr[Array[T]]) match
        case '{ scala.Array.apply( ${varArgs} : Expr[Seq[Int]])(using $fct) }  => 
          ???

Any help appreciated.

TIA

[som-snytt]

I started an answer on the forum https://users.scala-lang.org/t/scala-3-macros-quotes-how-to-match-scala-array-1-2-3/10046/2?u=som-snytt. I'll follow up here with more code. Arguably code looks better on github.

[som-snytt]

The answer was that since the signatures for Array.apply are specialized for primitives, it's necessary to match on those signatures.

def _arraylen(xs: Expr[Array[?]])(using Quotes): Expr[Int] =
  xs match
  //case '{ Array(${_}: A, (${Varargs(elems)}: Seq[A])*) } => Expr(elems.length + 1)
  case '{ Array(${_}: Int, (${Varargs(elems)}: Seq[Int])*) } => Expr(elems.length + 1)
  case '{ Array[a]((${Varargs(elems)}: Seq[a])*)($_) } => Expr(elems.length)
  case _ => Expr(-1)

or matching on the tree shape

def _arraylen(xs: Expr[Array[?]])(using Quotes): Expr[Int] =
  import quotes.reflect.*
  xs match
  case '{ Array[a]((${Varargs(elems)}: Seq[a])*)($_) } => Expr(elems.length)
  case _ =>
    xs.asTerm match
    case Inlined(_, _, Apply(_, List(elem, Typed(Repeated(elems, _), _)))) => Expr(elems.length + 1)
    case _ => Expr(-1)