Recursive macro looses elements in a constructed tuple

[letalvoj]

Related to #8779, as pointed out by further exploration by @Blaisorblade

Minimized code

import scala.compiletime._, scala.annotation.tailrec, scala.deriving.productElement

@tailrec inline def mapRec[T <: Tuple, A, B](p:Product)(i:Int)(f: A => B) <: Tuple = inline erasedValue[T] match
  case _: (A *: tt) => (f(productElement[A](p,i)) *: mapRec[tt,A,B](p)(i+1)(f)).asInstanceOf[tt]
  case _: (t *: tt) => (productElement[t](p,i) *: mapRec[tt,A,B](p)(i+1)(f)).asInstanceOf[tt]
  case _: Unit => () : Unit

mapRec[(Int, Int, String),Int,String]((1,2,"lol"))(0)(_.toString)                                                                                                                                                      

Output

val res4: (Int, String) = (1,2)

Expectation

Updated based on the apparent confusion in the comments.
Runtime fail or

val res4: (Int, Int, String) = (1,2,lol)

[odersky]

That's just a cast from Tuple2 to Tuple2 which succeeds at runtime

[Blaisorblade]

Why does dropping the casts change the return value? That was part of the intended bug report.

scala> @tailrec inline def mapRec[T <: Tuple, A, B](p:Product)(i:Int)(f: A => B) <: Tuple = inline erasedValue[T] match
     |   case _: (A *: tt) => (f(productElement[A](p,i)) *: mapRec[tt,A,B](p)(i+1)(f))
     |   case _: (t *: tt) => (productElement[t](p,i) *: mapRec[tt,A,B](p)(i+1)(f))
     |   case _: Unit => () : Unit
def mapRec[T <: Tuple, A, B](p: Product)(i: Int)(f: A => B): Tuple

scala> mapRec[(Int, Int, String),Int,String]((1,2,"lol"))(0)(_.toString)
val res0: String *: Tuple = (1,2,lol)

[odersky]

I don't know and don't have the time to look into it, but I would guess it's a consequence of #8739. If you can rule this out and find a different specific problem, then please re-open. But as it stand I do not see anyone taking this up, and in that case it's better to leave it closed.

[letalvoj]

@Blaisorblade oh I overlooked a bracket. You were casting (t *: tt) to tt. I thought that it was just the updated tail.

So the issue in this case is that the last element t *: Unit was ereased by casting it to Unit. Which is a valid behaviour, right? So this issue is closed correctly.

[Blaisorblade]

🤦 okay, that'll teach me to hurry, sorry everybody; I was trying to do something else first. The fixed code indeed works correctly.

scala> @tailrec inline def mapRec[T <: Tuple, A, B](p:Product)(i:Int)(f: A => B) <: Tuple = inline erasedValue[T] match
     |   case _: (A *: tt) => f(productElement[A](p,i)) *: mapRec[tt,A,B](p)(i+1)(f).asInstanceOf[tt]
     |   case _: (t *: tt) => productElement[t](p,i) *: mapRec[tt,A,B](p)(i+1)(f).asInstanceOf[tt]
     |   case _: Unit => () : Unit
def mapRec[T <: Tuple, A, B](p: Product)(i: Int)(f: A => B): Tuple

scala> mapRec[(Int, Int, String),Int,String]((1,2,"lol"))(0)(_.toString)
val res0: String *: Tuple = (1,2,lol)