Partial tail-recursion is also optimized in scalac. It's an experiment for verifying the result of https://twitter.com/chrilves/status/1091032475801927680

console1.txt

>scalac PTailRecursion.scala

>javap -c PTailRecursion$
Compiled from "PTailRecursion.scala"
public final class PTailRecursion$ {
  public static PTailRecursion$ MODULE$;

  public static {};
    Code:
       0: new           #2                  // class PTailRecursion$
       3: invokespecial #12                 // Method "<init>":()V
       6: return

  public int test(int);
    Code:
       0: iload_1
       1: iconst_0
       2: if_icmpne     10
       5: iload_1
       6: istore_1
       7: goto          0
      10: iconst_2
      11: aload_0
      12: iload_1
      13: invokevirtual #17                 // Method test:(I)I
      16: imul
      17: ireturn
}

console2.txt

>scalac PTailRecursion2.scala
PTailRecursion2.scala:7: error: could not optimize @tailrec annotated method test: it contains a recursive call not in tail position
      2 * test(n)
              ^
one error found

PTailRecursion.scala

object PTailRecursion {
  def test(n: Int): Int = 
    if (n == 0)
      test(n)
    else
      2 * test(n)
}

PTailRecursion2.scala

object PTailRecursion2 {
  @annotation.tailrec
  def test(n: Int): Int = 
    if (n == 0)
      test(n)
    else
      2 * test(n)
}