TEST: Palette capability "may_return_na"

tests/testthat/test-scale-colour-continuous.R

@@ -18,3 +18,32 @@ test_that("type argument is checked for proper input", {
     scale_colour_continuous(type = "abc")
   )
 })
+
+test_that("palette with may_return_NA=FALSE works as expected", {
+  sc <- scale_fill_continuous()
+  # A palette that may return NAs, will have NAs replaced by the scale's na.value
+  # by the scale:
+  sc$palette <- structure(
+    function(x) {
+      rep(NA_character_, length(x))
+    },
+    may_return_NA = TRUE
+  )
+  sc$na.value <- "red"
+  nat <- sc$map(0.5, limits = c(0, 1))
+  expect_equal(nat, "red")
+
+  # This palette is lying, because it returns NA even though it says it can't.
+  # The scale will not replace the NA values, leading to further errors.
+  # You should not do this in production, but it helps to test:
+  sc <- scale_fill_continuous()
+  sc$palette <- structure(
+    function(x) {
+      rep(NA_character_, length(x))
+    },
+    may_return_NA = FALSE
+  )
+  sc$na.value <- "red"
+  nat <- sc$map(0.5, limits = c(0, 1))
+  expect_equal(nat, NA_character_)
+})