combine fails to combine a list of characters with NA [with pull request: #2209]

[zeehio]

Hi,

Thanks to everyone working on tibble, dplyr, tidyr... In general the tidyverse packages make a lot of people very happy to use R.

I have found a rather simple case where dplyr::combine fails, and the do.call rbind equivalent works as expected. I have tested this on the latest released dplyr and the current master (b205d1f) versions, both showing the same error. I found this error trying to unnest a data frame using tidyr, but my C++ skills and dplyr knowledge are not good enough to fix it.

library(dplyr)
do.call(rbind,list("a", "b", "c", NA, "e")) # Works

combine(list("a", "b", "c", NA, "e")) # Does not work
#Error in eval(substitute(expr), envir, enclos) : 
#  Can not automatically convert from character to logical. 

combine(list("a", "b", "c", NA_character_, "e")) # Workaround

This issue affects unnest in tidyr:

library(tibble)
library(tidyr)

aa <- tibble(ID = list("a", "b", "c", NA, "e"))
bb <- unnest_(aa, "ID") # I would expect bb == tibble(ID = c("a", "b", "c", NA, "e"))

Any help will be appreciated