Using after_scale() in Geom$default_aes field

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* (internal) Using `after_scale()` in the `Geom*$default_aes()` field is now
+  evaluated in the context of data (@teunbrand, #6135)
 * Fixed bug where binned scales wouldn't simultaneously accept transformations
   and function-limits (@teunbrand, #6144).
 * Fixed bug where the `ggplot2::`-prefix did not work with `stage()` 

R/geom-.R

@@ -136,6 +136,13 @@ Geom <- ggproto("Geom",
     themed_defaults <- eval_from_theme(default_aes, theme)
     default_aes[names(themed_defaults)] <- themed_defaults
 
+    # Mark staged/scaled defaults as modifier (#6135)
+    delayed <- is_scaled_aes(default_aes) | is_staged_aes(default_aes)
+    if (any(delayed)) {
+      modifiers <- defaults(modifiers, default_aes[delayed])
+      default_aes <- default_aes[!delayed]
+    }
+
     missing_eval <- lapply(default_aes, eval_tidy)
     # Needed for geoms with defaults set to NULL (e.g. GeomSf)
     missing_eval <- compact(missing_eval)

tests/testthat/test-aes-calculated.R

@@ -146,3 +146,26 @@ test_that("stage allows aesthetics that are only mapped to start", {
   )
 
 })
+
+test_that("A geom can have scaled defaults (#6135)", {
+
+  test_geom <- ggproto(
+    NULL, GeomPoint,
+    default_aes = modify_list(
+      GeomPoint$default_aes,
+      aes(colour = after_scale(alpha(fill, 0.5)), fill = "black")
+    )
+  )
+
+  df <- data.frame(x = 1:3, fill = c("#FF0000", "#00FF00", "#0000FF"))
+
+  ld <- layer_data(
+    ggplot(df, aes(x, x, fill = I(fill))) +
+      stat_identity(geom = test_geom)
+  )
+
+  expect_equal(ld$colour, c("#FF000080", "#00FF0080", '#0000FF80'))
+
+  defaults <- get_geom_defaults(test_geom)
+  expect_equal(defaults$colour, c("#00000080"))
+})