materialize_binding error when using rowwise on zero-row data.frame

[ambevill]

Since upgrading to 0.8.0.1, I've noticed unexpected behavior for rowwise operations if the arg has zero rows. This is an edge case, but I don't think it makes sense to require guards to check for zero-row data.frames.

---

Brief description of the problem

library(dplyr)
packageVersion('dplyr')

# any function
pass_through = function(x) { x }

# as expected, this passes an empty character array into pass_through
data.frame(a = character()) %>%
  mutate(b = pass_through(a))

# this unexpectedly crashes; output below
data.frame(a = character()) %>%
  rowwise() %>%
  mutate(b = pass_through(a))

Crash traceback:

Error: Evaluation error: subscript out of bounds. 
15.
stop(structure(list(message = "Evaluation error: subscript out of bounds.", 
    call = NULL, cppstack = NULL), class = c("Rcpp::eval_error", 
"C++Error", "error", "condition"))) 
14.
materialize_binding(index, mask_proxy_xp) 
13.
(function () 
{
    materialize_binding(index, mask_proxy_xp)
})() 
12.
pass_through(a) 
11.
mutate_impl(.data, dots, caller_env()) 
10.
mutate.tbl_df(., b = pass_through(a)) 
9.
mutate(., b = pass_through(a)) 
8.
function_list[[k]](value) 
7.
withVisible(function_list[[k]](value)) 
6.
freduce(value, `_function_list`) 
5.
`_fseq`(`_lhs`) 
4.
eval(quote(`_fseq`(`_lhs`)), env, env) 
3.
eval(quote(`_fseq`(`_lhs`)), env, env) 
2.
withVisible(eval(quote(`_fseq`(`_lhs`)), env, env)) 
1.
data.frame(a = character()) %>% rowwise() %>% mutate(b = pass_through(a))

Windows 10, installed via CRAN binary, R 3.5.1

[batpigandme]

Adding reprex with output:

library(dplyr)

# any function
pass_through = function(x) { x }

# as expected, this passes an empty character array into pass_through
data.frame(a = character()) %>%
  mutate(b = pass_through(a))
#> [1] a b
#> <0 rows> (or 0-length row.names)

# this unexpectedly crashes; output below
data.frame(a = character()) %>%
  rowwise() %>%
  mutate(b = pass_through(a))
#> Error: Evaluation error: subscript out of bounds.

^{Created on 2019-02-25 by the reprex package (v0.2.1)}

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