add example of probable source of error to error message

[ahcyip]

Just a very minor proposal to add to the error message at https://github.com/r-lib/rlang/blob/1a53bd085c8e0ed41dcc56cce334dde926da7067/R/dots-ellipsis.R#L169

"These dots only exist to allow future extensions and should be empty."
"Did you misspecify an argument?"

It seems a common way to reach this error is forgetting to use c() e.g. I was using any_of("a","b","c") when it should have been any_of(c("a","b","c"))

Would it be acceptable to add "Did you misspecify an argument? e.g. forget to use c()"?" to this error message? I think it could be helpful for beginners.

[lionel-]

Thanks for the suggestion. I think this message would be too specific since dots_check_empty() is used in a wide array of cases.

We could look into more specific help in tidyselect though.

[hadley]

Maybe something like this?

if (!missing(...)) {
  abort(c(
    "`...` should be empty.",
    "i" = "Did you forget to use `c()`? You need `any_of(c('a', 'b'))`, not `any_of('a', 'b')`"
  ))
}

Looks like any_of() is the only place where this matters.

[lionel-]

Looks good. Just needs to use "must" instead of "should".

Also "You need" sounds a bit authoritative, which could confuse users when the error is due to something else. Perhaps:

! `...` must be empty.
i Did you forget to use `c()`?
  The expected syntax is `any_of(c("a", "b"))`, not `any_of("a", "b")`.