In `<a><b/><c/><a>`, a `NodeBuffer` (which extends `ArrayBuffer`) isused to accumulate the children. The buffer is passed to`new Elem($buf: _*)`, which only works thanks to the implicit`collection.Seq[Node] => NodeSeq` declared in scala-xml.With `-Vprint:typer`:```scalascala> <a><b/></a>[[syntax trees at end of                     typer]] // <console>      private[this] val res0: scala.xml.Elem = new scala.xml.Elem(null, "a", scala.xml.Null, scala.xml.TopScope, false, (xml.this.NodeSeq.seqToNodeSeq({        val $buf: scala.xml.NodeBuffer = new scala.xml.NodeBuffer();        $buf.&+(new scala.xml.Elem(null, "b", scala.xml.Null, scala.xml.TopScope, true));        $buf      }): _*));```The implicit was not inserted in 2.12 because the varargs parameter ofElem accepted a `collection.Seq`.