regroup errors on zero-row data frames

[wch]

This causes problems for zero-row data in ggvis:

dplyr::regroup(data.frame(a = numeric(0), g = character(0)), list(quote(g)))
# Error: upper value must be greater than lower value

# Slightly closer to the source of the problem:
dplyr:::grouped_df_impl(data.frame(a = numeric(0), g = character(0)), list(quote(g)), drop = TRUE)

[hadley]

@romainfrancois I think in this case we should get an index list of length 0. That will probably require flow through changes elsewhere.

[romainfrancois]

An 0-length index list is the way to go, yes. Although I'm kind of expecting that when there is a group, there is at least one element, so some things might break.

[wch]

@romainfrancois Any chance you can get to this one soonish? I've added a fair bit of workaround code for this in ggvis, and it would be nice to get rid of it. :)

[romainfrancois]

sure @wch I'll take a look.

[romainfrancois]

I think this commit took care of it. fd00ad2
(although I messed up with the issue number).

[wch]

Thanks! I think it works, but my tests still don't pass because now I'm running into #593. :-/