Failed to optimize a recursive call in tail position

[scabug]

The following tail-recursive funciton foo fails to be optimized.

import scala.annotation.tailrec

sealed trait HList
case class HCons[H, T <: HList](head: H, tail: T) extends HList
case object HNil extends HList

@tailrec
def foo[L <: HList](l: L): Unit = l match {
  case HNil => ()
  case HCons(h, t) => foo(t)
}

Here is the REPL output:

Welcome to Scala 2.12.0-333187a-nightly (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_60).
Type in expressions for evaluation. Or try :help.

scala> :paste
// Entering paste mode (ctrl-D to finish)

import scala.annotation.tailrec

sealed trait HList
case class HCons[H, T <: HList](head: H, tail: T) extends HList
case object HNil extends HList

@tailrec
def foo[L <: HList](l: L): Unit = l match {
  case HNil => ()
  case HCons(h, t) => foo(t)
}

// Exiting paste mode, now interpreting.

<console>:18: error: could not optimize @tailrec annotated method foo: it contains a recursive call not in tail position
       def foo[L <: HList](l: L): Unit = l match {
                                         ^

scala> 

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9647?orig=1
Reporter: Tomas Mikula (tomas.mikula-at-gmail.com)
Affected Versions: 2.11.7, 2.12.0-M3, 2.12.0-RC1
See #8796

[scabug]

@SethTisue said:
I would have expected a "could not optimize @tailrec annotated method foo: it is called recursively with different type arguments" error.

[scabug]

@SethTisue said:
with -Xprint:all we see that after patmat runs we have:

                  val t: HList = x2.tail;
                  matchEnd5($iw.this.foo[HList](t))

so this looks like a pattern matcher issue — the transform the matcher does moves the recursive call to non-tail-position