Unterminated slurs in multi-voice volta alternative ending

[address@hidden]

Hello everbody

The following example produces 2 compile warnings (cannot end slur,
unterminated slur). I'm missing the slur between dis in bar 3 and d in
bar 4, both in an additional temporary voice context.

It's a snippet from a SATB arrangement with 2 alto voices put in one
staff. In order to align the text to alto 1 I chose to add temporary
voice contexts for alto 2 (which is most of the time).

(The only alternative I could imagine is to
* write alto 2 in a completely separate voice, hiding notes during
"oneVoice" portions with invisible breaks (s)
* to combine the alto 1 voice and the alto 2 voice together in one
(multi-voice alto) staff,
* add the lyrics to alto 1 (alto 2 lyrics are identical to alto 1 lyrics).

BTW: Is there a way to combine/collapse breaks within one staff in
multi-voice scenarios?

Hints and advice are highly appreciated!

Thanks in advance,
Regards
Stephan

{code}
\version "2.19.83"

global = {
  \key g \major
  \time 4/4
}

\header {
  title = "Unterminated slur"
  subtitle = "Unterminated slur in 2nd voice after 2nd repeat alternative"
  subsubtitle = "2 Alto voices with same lyrics (within a SATB
arrangement / part of ChoirStaff)"
}

altoVoices = \relative c'' {
  \global
  \repeat volta 2 {
    <<
      {
        \voiceOne
        g4 g g g
      }
      \new Voice {
        \voiceTwo
        s2 d4 d
      }
    >>
  }
  \alternative {
    {
      <<
        {
          \voiceOne
          fis2 r2
        }
        \new Voice {
          \voiceTwo
          d2 r2
        }
      >>
    }
    {
      <<
        {
          \voiceOne
          g4 r fis2(
        }
        \new Voice {
          \voiceTwo
          d4 r dis2(
        }
      >>
    }
  }
  <<
    {
      \voiceOne
      g1)
    }
    \new Voice {
      \voiceTwo
      d1)
    }
  >>


}

altoLyrics = \lyricmode {
  \repeat volta 2 {
    This is a new
  }
  \alternative {
    {
      song.
    }
    {
      song, uuh, __
    }
  }

}

\score {
  \new Staff {
      \altoVoices
  }
  \addlyrics {
    \altoLyrics
  }
}
{code}