Backport "Preventing compilation of a @tailrec method when it does not rewrite, but an inner method does" to LTS

[WojciechMazur]

Backports #20143 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

[WojciechMazur]

This change leads to a failure in Open Community Build for sageserpent-open/kineticmerge Open CB logs, but it's an expected one. The failing code can be minimized to:

import scala.annotation.tailrec

@tailrec private def outer(i: Int): String =
  @tailrec def inner(i: Int): String =
    if i == 0 then  inner(i + 1)
    else if i == 1 then  outer(i + 1)
    else if i == 2 then outer(0)
    else "fallback"
  inner(i)

@main def Test = outer(0)

Previously it compiled but failed at runtime, now it fails at compile-time with

Compiling project (Scala 3.3.4-RC1-bin-SNAPSHOT, JVM (17))
[warn] ./test.scala:6:26
[warn] The tail recursive def outer contains a recursive call inside the non-inlined inner def inner
[warn]     else if i == 1 then  outer(i + 1)
[warn]                          ^^^^^^^^^^^^
[warn] ./test.scala:7:10
[warn] The tail recursive def outer contains a recursive call inside the non-inlined inner def inner
[warn]     else outer(0)
[warn]          ^^^^^^^^
[error] ./test.scala:3:22
[error] TailRec optimisation not applicable, method outer contains no recursive calls
[error] @tailrec private def outer(i: Int): String =
[error]      

The previous behaviour was a bug that is now going to be fixed. It gave a false guarantee of safety that could have led to unexpected StackOverflowError. However, it breaks the source compatibility guarantees thus needs to be decided by the core team on either this change should be included.

[WojciechMazur]

It was decided in the core meeting to not backport this change to the LTS