Tailrec jammed by safer exceptions

[abgruszecki]

Compiler version

$head -1 build.sbt 
ThisBuild / scalaVersion := "3.3.1-RC1-bin-20230322-1f574e8-NIGHTLY"

Minimized code

object TestyFactorial:
  // minimal example
  import scala.annotation.tailrec
  import language.experimental.saferExceptions

  @tailrec
  final def fact(n: Int, acc: Int): Int throws Exception =
    if n < 0 then
      throw new Exception
    else if n == 0 then
      acc
    else
      fact(n - 1, acc * n)

Output

$sbt clean compile
  ...
[error]  |    fact(n - 1, acc * n)
[error]  |    ^^^^^^^^^^^^^^^^^^^^
[error]  |    Cannot rewrite recursive call: it is not in tail position

[abgruszecki]

/cc @odersky

[abgruszecki]

This seems like a reasonably simple issue. Maybe it's good for the issue spree?

[dwijnand]

Seems plausible to me too. Let's label it and if anyone disagrees they can remove it.

[scala-center-bot]

This issue was picked for the Issue Spree No. 29 of 18 April 2023 which takes place in a week from now. @bishabosha, @Linyxus, @colin4124, @nmcb will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[bishabosha]

it broke between 3.3.1-RC1-bin-20230302-8020c77-NIGHTLY and 3.3.1-RC1-bin-20230303-6ad22aa-NIGHTLY

[bishabosha]

most likely it seems that #16507 cause the issue

[bishabosha]

minimisation found by @Linyxus

object TestyFactorial:
  import scala.annotation.tailrec
  import language.experimental.erasedDefinitions

  erased class Foo1
  class Foo2

  @tailrec
  final def test1(n: Int, acc: Int): Foo1 ?=> Int =
    if n <= 0 then acc
    else test1(n - 1, acc * n)

  @tailrec
  final def test2(n: Int, acc: Int): Foo2 ?=> Int =
    if n <= 0 then acc
    else test2(n - 1, acc * n)

[bishabosha]

It seems the problem is with erasure, as I noted in #16507 (comment), before the change, the erasure of test1 is @ContextResultCount(1) @tailrec def test1(n: Int, acc: Int): Int, but now it is @tailrec final def test1(n: Int, acc: Int): Function0

This seems to involve two factors transform.ContextFunctionResults.annotateContextResults, which deliberately does not "integrate" erased context function parameters, and the second part which is whatever handles elimination of the closure for context parameters.