Demote errors in legend modified expressions

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Staged expressions are handled more gracefully if legends cannot resolve them 
+  (@teunbrand, #6264).
 * `guide_*()` can now accept two inside legend theme elements:
   `legend.position.inside` and `legend.justification.inside`, allowing inside
   legends to be placed at different positions. Only inside legends with the same

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 +136,13 @@ test_that("legends can be forced to display unrelated geoms", {
   )
 })
 
+test_that("unresolved, modified expressions throw a warning (#6264)", {
+  p <- ggplot(mpg, aes(drv)) +
+    geom_bar(
+      aes(fill = stage(drv, after_scale = alpha(fill, prop)))
+    )
+  expect_snapshot_warning(ggplot_build(p))
+})
 
 # Visual tests ------------------------------------------------------------