Fix discrete map bug

NEWS.md

@@ -1,3 +1,7 @@
+
+* Fixed bug where discrete scales could not map aesthetics only consisting of
+  `NA`s (#5623)
+
 # ggplot2 (development version)
 
 This is a minor release that turned out quite beefy. It is focused on 

R/scale-.R

@@ -926,6 +926,9 @@ ScaleDiscrete <- ggproto("ScaleDiscrete", Scale,
 
   map = function(self, x, limits = self$get_limits()) {
     n <- sum(!is.na(limits))
+    if (n < 1) {
+      return(rep(self$na.value, length(x)))
+    }
     if (!is.null(self$n.breaks.cache) && self$n.breaks.cache == n) {
       pal <- self$palette.cache
     } else {

tests/testthat/test-scales.R

@@ -716,3 +716,17 @@ test_that("Using `scale_name` prompts deprecation message", {
   expect_snapshot_warning(binned_scale("x",     "foobar", pal_identity()))
 
 })
+
+# From #5623
+test_that("Discrete scales with only NAs return `na.value`", {
+
+  x <- c(NA, NA)
+
+  sc <- scale_colour_discrete(na.value = "red")
+  sc$train(x)
+  expect_equal(sc$map(x), c("red", "red"))
+
+  sc <- scale_shape(na.value = NA_real_)
+  sc$train(x)
+  expect_equal(sc$map(x), c(NA_real_, NA_real_))
+})