add test

Author: teunbrand

tests/testthat/_snaps/ggproto.md

@@ -10,3 +10,7 @@
 
     `_inherit` must be a <ggproto> object, not a <data.frame> object.
 
+# circular definitions are protested
+
+    <Circular> cannot have a circular definition.
+

tests/testthat/test-ggproto.R

@@ -11,6 +11,12 @@ test_that("construction checks input", {
   expect_snapshot_error(ggproto("Test", mtcars, a = function(self, a) a))
 })
 
+test_that("circular definitions are protested", {
+  circular <- ggproto("Circular")
+  circular <- ggproto(NULL, circular)
+  expect_snapshot_error(print(circular))
+})
+
 test_that("all ggproto methods start with `{` (#6459)", {
 
   ggprotos <- Filter(