grob-transformer and unpure-pure containers
\version "2.25.6"
\tweak Y-offset #(grob-transformer 'Y-offset (lambda (grob original) original))
\tuplet 3/2 { c'8 8 8 }
While the transformer is just a pass-through and therefore should not influence the result, this outputs:
programming error: TupletBracket.positions requested before line breaking
continuing, cross fingers
programming error: TupletBracket.Y-offset accessed before line breaking
continuing, cross fingers
As far as I understand, grob-transformer
should conceptually not be unconditionally returning an unpure-pure container. It should only do that if the original is an unpure-pure container.
But it needs to return an unpure-pure container when it's called, long before the lambdas in that unpure-pure container are actually called in the backend, which is the time when it could know if the original value actually was an unpure-pure container. So, this might be tricky.
Maybe it would work to return '()
from the generated unpure-pure container if the original was not an unpure-pure container, but I have not tested it.