Overwrite vars with dplyr::vars, if available (#2593)

Fixes #2585

Author: hadley

R/backports.R

@@ -6,9 +6,11 @@ if (getRversion() < 3.3) {
   absolute.units.unit.list <- getFromNamespace("absolute.units.unit.list", "grid")
   absolute.units.unit.arithmetic <- getFromNamespace("absolute.units.unit.arithmetic", "grid")
 
-  .onLoad <- function(...) {
+  backport_unit_methods <- function() {
     registerS3method("absolute.units", "unit", absolute.units.unit)
     registerS3method("absolute.units", "unit.list", absolute.units.unit.list)
     registerS3method("absolute.units", "unit.arithmetic", absolute.units.unit.arithmetic)
   }
+} else {
+  backport_unit_methods <- function() {}
 }

R/zzz.r

@@ -16,6 +16,16 @@
   })
 }
 
+.onLoad <- function(...) {
+  backport_unit_methods()
+
+  # To avoid namespace clash with dplyr.
+  # It seems surprising that this hack works
+  if (requireNamespace("dplyr", quietly = TRUE)) {
+    vars <<- dplyr::vars
+  }
+}
+
 release_questions <- function() {
   c(
     "Have you built the book?"