handles empty args in backwards compatible functions (#6711)

* safely handles trailing args for backwards compatibility

* adds test

* adds news item

* Revert "safely handles trailing args for backwards compatibility"

This reverts commit 6388981.

* allows empty args

---------

Co-authored-by: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>

Author: jmbarbone

NEWS.md

@@ -26,6 +26,8 @@
   legend glyphs (@teunbrand, #6594)
 * Fixed regression where `NULL`-aesthetics contributed to plot labels too 
   insistently. Now they contribute only as fallback labels (@teunbrand, #6616)
+* Fixed regression where empty arguments to colour/fill scale caused errors
+  (@jmbarbone, #6710)
 * The `theme(panel.widths, panel.heights)` setting attempts to preserve the
   plot's aspect ratio when only one of the two settings is given, and the plot 
   has a single panel (@teunbrand, #6701).

R/scale-colour.R

@@ -285,7 +285,8 @@ check_scale_type <- function(scale, name, aesthetic, scale_is_discrete = FALSE,
 scale_backward_compatibility <- function(..., scale, aesthetic, type) {
   aesthetic <- standardise_aes_names(aesthetic[1])
 
-  args <- list2(...)
+  # input ... may have trailing args, which then get stuck in the middle #6710
+  args <- dots_list(..., .ignore_empty = "all")
   args$call <- args$call %||% caller_call() %||% current_call()
 
   if (type == "binned") {

tests/testthat/test-scale-colour.R

@@ -68,3 +68,7 @@ test_that("`name` is directed correctly (#6623)", {
     expect_equal(p$name, "foobar")
   }
 })
+
+test_that("backwards compatibility allows trailing args (#6710)", {
+  expect_no_error(scale_fill_discrete(breaks = 1:2, direction = -1L, ))
+})