Assertion failure when using Melody_engraver and CueVoice

[Jean Abou Samra]

> Hi there,
>
> Thank you for all your work on LilyPond!
>
> It appears to be impossible to use a Melody_engraver in a Voice that also
> includes a CueVoice. Here’s a LilyPond example that illustrates the issue:
>
> ```
> \version "2.22.0"
> \new Voice \with {
>    \consists "Melody_engraver"
>    \override Stem.neutral-direction = #'()
> } {
>    <<
>      { r }
>      \new CueVoice { c'' }
>    >>
> }
> ```
>
> LilyPond outputs:
>
> GNU LilyPond 2.22.0
> Processing `test.ly'
> Parsing...
> Interpreting music...
> Preprocessing graphical objects...
> Finding the ideal number of pages...
> Fitting music on 1 page...
> Drawing systems...Assertion failed: (d == 1 || d == -1), function at, file
> /private/tmp/lilypond-20210130-26352-lkbhbf/lilypond-2.22.0/build/../flower/include/drul-array.hh,
>   line 35.
>
> All the best,
> Nate

Hello,

The example can be reduced even further, showing
that the problem is not actually related to the
Melody_engraver or the CueVoice context:

\version "2.23.1"

\new Voice {
  \override Stem.neutral-direction = #'()
  <<
    { r }
    \new Voice { c }
  >>
}


Maybe try something like this to work around:

\version "2.23.1"

#(set-object-property! 'rest-neutral-direction 'backend-type? ly:dir?)

\layout {
  \context {
    \Voice
    \consists "Melody_engraver"
    \override Stem.neutral-direction = #'()
    \override Stem.rest-neutral-direction = #UP
    \override Stem.before-line-breaking =
       #(grob-transformer 'before-line-breaking
          (lambda (grob dummy)
            (let ((original (ly:stem::calc-default-direction grob)))
              (ly:grob-set-property!
                grob
                'default-direction
                (cond ((memv original '(-1 1))
                       original)
                      ((null? (ly:grob-object grob 'rests))
                       (ly:grob-property grob 'neutral-direction))
                     (else
                       (ly:grob-property grob 
'rest-neutral-direction)))))))
  }
}


\new Voice {
  \time 3/4
  <<
    { r }
    \new Voice \relative c'' {
      \autoBeamOff
      a8 b g f b g |
      c  b d c b c |
    }
  >>
}

Best,
Jean Abou Samra