`scale_fill_discrete()` no longer allows empty arguments

[jmbarbone]

scale_fill_discrete() now breaks with trailing ,

packageVersion("ggplot2")
#> [1] '4.0.0'
try(ggplot2::scale_fill_discrete(breaks = 1:2, direction = -1L,))
#> Error in list2(...) : Argument 3 can't be empty.

^{Created on 2025-10-21 with reprex v2.1.1}

packageVersion("ggplot2")
#> [1] '3.5.2'
ggplot2::scale_fill_discrete(breaks = 1:2, direction = -1L,)
#> <ggproto object: Class ScaleDiscrete, Scale, gg>
#>     aesthetics: fill
#>     axis_order: function
#>     break_info: function
#>     break_positions: function
#>     breaks: 1 2
#>     call: call
#>     clone: function
#>     dimension: function
#>     drop: TRUE
#>     expand: waiver
#>     get_breaks: function
#>     get_breaks_minor: function
#>     get_labels: function
#>     get_limits: function
#>     get_transformation: function
#>     guide: legend
#>     is_discrete: function
#>     is_empty: function
#>     labels: waiver
#>     limits: NULL
#>     make_sec_title: function
#>     make_title: function
#>     map: function
#>     map_df: function
#>     n.breaks.cache: NULL
#>     na.translate: TRUE
#>     na.value: grey50
#>     name: waiver
#>     palette: function
#>     palette.cache: NULL
#>     position: left
#>     range: environment
#>     rescale: function
#>     reset: function
#>     train: function
#>     train_df: function
#>     transform: function
#>     transform_df: function
#>     super:  <ggproto object: Class ScaleDiscrete, Scale, gg>

^{Created on 2025-10-21 with reprex v2.1.1}

[teunbrand]

Thanks for the report! Is it worth preserving empty arguments in general? This example fails in a specific case, where the scale falls back to a backward compatibility mechanism that I'm keen to get rid of altogether.

[jmbarbone]

Thanks for the report! Is it worth preserving empty arguments in general? This example fails in a specific case, where the scale falls back to a backward compatibility mechanism that I'm keen to get rid of altogether.

I think so. It seems like the use of list2() intends for that. Because more ags are added onto ..., those trailing empty values by the user get placed in the middle.