Dynamically assign pathGrob in .onLoad() to prevent packages that sub…

…class GeomPolygon from throwing an install warning. Fixes #3312. (#3331)

R/zzz.r

@@ -16,9 +16,21 @@
   })
 }
 
+# Assigning pathGrob in .onLoad ensures that packages that subclass GeomPolygon
+# do not install with error `possible error in 'pathGrob(munched$x, munched$y, ':
+# unused argument (pathId = munched$group)` despite the fact that this is correct
+# usage
+pathGrob <- NULL
+
 .onLoad <- function(...) {
   backport_unit_methods()
 
+  if (getRversion() < as.numeric_version("3.6")) {
+    pathGrob <<- function(..., pathId.lengths) {
+      grid::pathGrob(...)
+    }
+  }
+
   .zeroGrob <<- grob(cl = "zeroGrob", name = "NULL")
 
   ggplot_global$theme_current <- theme_gray()