[Rd] tab-complete for non-syntactic names could attempt backtick-wrapping

[Michael Chirico]

Consider:

x <- list(`a b` = 1)
x$a<tab>

(i.e., press the 'tab' key after typing 'x$a')

The auto-complete mechanism will fill the buffer like so:
x$a b

This is not particularly helpful because this is now a syntax error.

It seems to me there's a simple fix -- in
utils:::specialCompletions(), we can wrap the result of
utils:::specialOpCompletionsHelper() with backticks for non-syntactic
names ([1]):

comps <- specialOpCompletionsHelper(op, suffix, prefix)
if (length(comps) == 0L) comps <- ""
+non_syntactic <- make.names(comps) != comps
+comps[non_syntactic] <- paste0("`", comps[non_syntactic], "`")
sprintf("%s%s%s", prefix, op, comps)

I'm somewhat surprised this hasn't come up before (I searched for
'completeToken', 'specialCompletions', and
'specialOpCompletionsHelper' here and on Bugzilla), so I'm checking
with the list first if I'm missing anything before filing a patch.

Mike C

[1] https://github.com/r-devel/r-svn/blob/4657f65a377cb5ef318c6548bc264e3b0f9517a0/src/library/utils/R/completion.R#L536-L538