JS CPS transformer crash: no statements in binding of a match

[jiribenes]

Context: I was testing #841 with fun examples on my drive :)

The following program (the repro from #770!) when compiled with --no-optimize:

type Answer { Yes(); No() }

def println(ans: Answer): Unit = ans match {
  case Yes() => println("y")
  case No()  => println("n")
}

def join(left: Answer, right: Answer): Answer = (left, right) match {
  case (Yes(), Yes()) => Yes()
  case (No() ,     _) => No()
  case (_    , No() ) => No()
}

def main() = println(join(Yes(), No()))

returns an error in generator.js.TransformerCps:

Exception in thread "main" java.util.NoSuchElementException: last of empty list
        at scala.collection.immutable.Nil$.last(List.scala:665)
        at scala.collection.immutable.Nil$.last(List.scala:661)
        at effekt.generator.js.TransformerCps$.$anonfun$7(TransformerCps.scala:253)
        at scala.collection.immutable.List.map(List.scala:250)
        at effekt.generator.js.TransformerCps$.toJS(TransformerCps.scala:257)
        at effekt.generator.js.TransformerCps$.toJS$$anonfun$17(TransformerCps.scala:226)
        at effekt.generator.js.Tree$package$.stmts(Tree.scala:253)
        at effekt.generator.js.TransformerCps$.toJS$$anonfun$17(TransformerCps.scala:226)
        at effekt.generator.js.Tree$package$.stmts(Tree.scala:253)
        at effekt.generator.js.TransformerCps$.toJS$$anonfun$17(TransformerCps.scala:226)
        at effekt.generator.js.Tree$package$.stmts(Tree.scala:253)
        at effekt.generator.js.TransformerCps$.toJS$$anonfun$17(TransformerCps.scala:226)
        at effekt.generator.js.Tree$package$.stmts(Tree.scala:253)

The error comes from the stmts.last here:

effekt/effekt/shared/src/main/scala/effekt/generator/js/TransformerCps.scala

Lines 243 to 258 in 5599ca1

	// (function () { switch (sc.tag) { case 0: return f17.apply(null, sc.data) }
	case cps.Stmt.Match(sc, clauses, default) =>
	val scrutinee = toJS(sc)
	pure(js.Switch(js.Member(scrutinee, `tag`),
	clauses.map { case (tag, clause) =>
	val (e, binding) = toJS(scrutinee, tag, clause);
	val stmts = binding.stmts
	stmts.last match {
	case terminator : (js.Stmt.Return | js.Stmt.Break | js.Stmt.Continue) => (e, stmts)
	case other => (e, stmts :+ js.Break())
	}
	},
	default.map { s => toJS(s).stmts }) :: Nil)