Demote errors in legend modified expressions (#6267)

* handle after scale errors

* add test

* add news bullet

* tweak message

* conditional test

Author: teunbrand

NEWS.md

@@ -302,6 +302,8 @@
   particularly for data-points with a low radius near the center 
   (@teunbrand, #5023).
 * All scales now expose the `aesthetics` parameter (@teunbrand, #5841)
+* Staged expressions are handled more gracefully if legends cannot resolve them 
+  (@teunbrand, #6264).
 * New `theme(legend.key.justification)` to control the alignment of legend keys 
   (@teunbrand, #3669). 
 * Added `scale_{x/y}_time(date_breaks, date_minor_breaks, date_labels)` 

R/geom-.R

@@ -163,10 +163,15 @@ Geom <- ggproto("Geom",
     # If any after_scale mappings are detected they will be resolved here
     # This order means that they will have access to all default aesthetics
     if (length(modifiers) != 0) {
-      # Set up evaluation environment
-      modified_aes <- eval_aesthetics(
-        substitute_aes(modifiers), data,
-        mask = list(stage = stage_scaled)
+      modified_aes <- try_fetch(
+        eval_aesthetics(
+          substitute_aes(modifiers), data,
+          mask = list(stage = stage_scaled)
+        ),
+        error = function(cnd) {
+          cli::cli_warn("Unable to apply staged modifications.", parent = cnd)
+          data_frame0()
+        }
       )
 
       # Check that all output are valid data

tests/testthat/_snaps/guide-legend.md

@@ -0,0 +1,6 @@
+# unresolved, modified expressions throw a warning (#6264)
+
+    Unable to apply staged modifications.
+    Caused by error:
+    ! object 'prop' not found
+

tests/testthat/test-guide-legend.R

@@ -136,6 +136,15 @@ test_that("legends can be forced to display unrelated geoms", {
   )
 })
 
+test_that("unresolved, modified expressions throw a warning (#6264)", {
+  # Snapshot is unstable in lesser R versions
+  skip_if_not(getRversion() >= "4.3.0")
+  p <- ggplot(mpg, aes(drv)) +
+    geom_bar(
+      aes(fill = stage(drv, after_scale = alpha(fill, prop)))
+    )
+  expect_snapshot_warning(ggplot_build(p))
+})
 
 # Visual tests ------------------------------------------------------------