"Error while emitting" compiler crash with edge-case macro usage

[shadaj]

As part of developing a new macro-backed feature for Slinky, shadaj/slinky#263, I found an issue where the compiler will fail to compile code that involves a macro in a very specific situation. This issue affects both Scala 2.12 and 2.13. I'm running on JDK 8, but don't believe that is related to this bug.

I've put up a minimal reproduction of this issue at https://github.com/shadaj/scala-macro-bugs.

A summary of the conditions that cause this issue (copied from the README):

• Have a macro that includes and uses a val as part of the code it emits
• Call the macro when supplying a by-name parameter to some method b
• Call method b while providing varargs to method a
• Method a is called with a parameter (default or non-default) that is not the first parameter supplied as a named parameter

Example (copied from the repo):

object Macro {
  def myMacro(): Int = macro myMacroImpl
  def myMacroImpl(c: blackbox.Context)(): c.Tree = {
    import c.universe._
    q"""{
      val ret = 0
      println(ret)
      0
    }"""
  }
}

object MacroTest {
  def a(default: Int = 0, defaultOrNonDefault: Int)(args: Int*) = ???
  def b(num: => Int): Int = ???
  a(defaultOrNonDefault = 0)(b(Macro.myMacro()))
}

The compiler fails with:

[error] Error while emitting MacroTest.scala
[error] value ret

[lrytz]

Probably some bug in the owner chain. With -Vprint:_

[[syntax trees at end of                   erasure]] // B.scala
      <artifact> val x$3: Seq = scala.collection.immutable.Seq.apply(scala.runtime.ScalaRunTime.wrapIntArray(Array[Int]{MacroTest.this.b({
  final <artifact> def $anonfun$new(): Int = ({
    val ret: Int = 0;
    scala.Predef.println(scala.Int.box(ret));
    0
  }: Int);
  (() => $anonfun$new())
})})).$asInstanceOf[Seq]();

the above looks fine, but then lambdalift treats ret like a captured value:

[[syntax trees at end of                lambdalift]] // B.scala
      <artifact> val x$3: Seq = scala.collection.immutable.Seq.apply(scala.runtime.ScalaRunTime.wrapIntArray(Array[Int]{MacroTest.this.b({
  (() => MacroTest.this.$anonfun$new$1(ret))
})})).$asInstanceOf[Seq]();
      MacroTest.this.a(x$2, 0, x$3)
    };
    final <artifact> private[this] def $anonfun$new$1(ret$1: Int): Int = ({
      val ret: Int = 0;
      scala.Predef.println(scala.Int.box(ret$1));
      0
    }: Int)

so in this.$anonfun$new$1(ret), there's no ret in scope.

I won't have time to look into this at the moment...