Preserve class when adding uneval objects

NEWS.md

@@ -17,6 +17,8 @@
 * Minor code formatting issues in examples and function parameters were
   fixed. (@hrbrmstr)
 
+* Class of aesthetic mapping is preserved when adding `aes()` objects. (#1624)
+
 # ggplot2 2.1.0
 
 ## New features

R/plot-construction.r

@@ -92,6 +92,8 @@ add_ggplot <- function(p, object, objectname) {
     p <- update_guides(p, object)
   } else if (inherits(object, "uneval")) {
       p$mapping <- defaults(object, p$mapping)
+      # defaults() doesn't copy class, so copy it.
+      class(p$mapping) <- class(object)
 
       labels <- lapply(object, deparse)
       names(labels) <- names(object)

tests/testthat/test-add.R

@@ -0,0 +1,6 @@
+context("Adding plot elements")
+
+test_that("Mapping class is preserved when adding uneval objects", {
+  p <- ggplot(mtcars) + aes(wt, mpg)
+  expect_identical(class(p$mapping), "uneval")
+})