fix non-element theme inheritance in R 3.1 and 3.2

Author: paleolimbot

R/theme.r

@@ -612,21 +612,31 @@ merge_element.element <- function(new, old) {
   new
 }
 
-# Combine the properties of two elements
-#
-# @param e1 An element object
-# @param e2 An element object which e1 inherits from
+#' Combine the properties of two elements
+#'
+#' @param e1 An element object
+#' @param e2 An element object from which e1 inherits
+#'
+#' @noRd
+#'
 combine_elements <- function(e1, e2) {
 
   # If e2 is NULL, nothing to inherit
   if (is.null(e2) || inherits(e1, "element_blank"))  return(e1)
+
   # If e1 is NULL inherit everything from e2
   if (is.null(e1)) return(e2)
+
+  # If neither of e1 or e2 are element_* objects, return e1
+  if (!inherits(e1, "element") && !inherits(e2, "element")) return(e1)
+
   # If e2 is element_blank, and e1 inherits blank inherit everything from e2,
   # otherwise ignore e2
   if (inherits(e2, "element_blank")) {
-    if (e1$inherit.blank) return(e2)
-    else return(e1)
+    if (e1$inherit.blank)
+      return(e2)
+    else
+      return(e1)
   }
 
   # If e1 has any NULL properties, inherit them from e2

tests/testthat/test-theme.r

@@ -213,6 +213,12 @@ test_that("elements can be merged", {
   )
 })
 
+test_that("theme elements that don't inherit from element can be combined", {
+  expect_identical(combine_elements(1, NULL), 1)
+  expect_identical(combine_elements(NULL, 1), 1)
+  expect_identical(combine_elements(1, 0), 1)
+})
+
 test_that("complete plot themes shouldn't inherit from default", {
   default_theme <- theme_gray() + theme(axis.text.x = element_text(colour = "red"))
   base <- qplot(1, 1)