add sentinel test

tests/testthat/_snaps/StatsandGeoms.md

@@ -0,0 +1,18 @@
+# We do not need to update `silent_add()`.
+
+    Code
+      body
+    Output
+      {
+          if (is.null(scale)) {
+              return()
+          }
+          prev_aes <- self$find(scale$aesthetics)
+          if (any(prev_aes)) {
+              scalename <- self$scales[prev_aes][[1]]$aesthetics[1]
+              cli::cli_inform(c("Scale for {.field {scalename}} is already present.", 
+                  "Adding another scale for {.field {scalename}}, which will replace the existing scale."))
+          }
+          self$scales <- c(self$scales[!prev_aes], list(scale))
+      }
+

tests/testthat/test-StatsandGeoms.R

@@ -13,5 +13,13 @@ test_that("Geom and Stat ggprotos are in fact ggprotos", {
   expect_ggproto(StatEdgeDiagonal)
   expect_ggproto(StatEdgeFan)
   expect_ggproto(GeomDAGEdgePath)
-  expect_ggproto(ScalesListQuiet)
+})
+
+test_that("We do not need to update `silent_add()`.", {
+  # This is a sentinel test to see if upstream ggplot2 has made changes to
+  # the ggplot2:::Scales$add() method.
+  # If this test fails, the add method has likely changed and `silent_add()`
+  # may need to be updated in StatsandGeoms.R.
+  body <- body(environment(ggplot()$scales$add)$f)
+  expect_snapshot(body)
 })