improve arg_check0 error with one permitted value

[aronatkins]

arg_check* could produce better feedback when there is only one allowed argument value.

Today: `chosen` must be one of "banana", not "apple".
Suggestion: `chosen` must be "banana", not "apple".

doit <- function() {
  chosen <- "apple"
  available <- c("banana")
  rlang::arg_match(chosen, available)
}
doit()
#> Error in `doit()`:
#> ! `chosen` must be one of "banana", not "apple".
#> Backtrace:
#>     ▆
#>  1. ├─global doit()
#>  2. │ └─rlang::arg_match(chosen, available)
#>  3. │   └─rlang::arg_match0(arg, values, error_arg, error_call = error_call)
#>  4. └─rlang:::stop_arg_match(w, values = x, error_arg = y, error_call = z)
#>  5.   └─rlang::abort(msg, call = error_call, arg = error_arg)

^{Created on 2023-06-22 with reprex v2.0.2}

[fh-mthomson]

Perhaps a duplicate of the same underlying issue?

test_fun <- function(one_allowed = c("1", "2", "3"), multiple = FALSE) {
  rlang::arg_match(arg = one_allowed, multiple = multiple)
}

# bug: should give clear error that only one value is allowed
test_fun(c("1", "2"))
#> Error in `test_fun()`:
#> ! `one_allowed` must be one of "1", "2", or "3", not "1".

# correct:
test_fun("1")
#> [1] "1"
test_fun("2")
#> [1] "2"
test_fun(c("1", "2"), multiple = TRUE)
#> [1] "1" "2"

^{Created on 2024-11-25 with reprex v2.1.1}

[fh-mthomson]

Oddly, if you pass the entire set of values, it passes? 🤔

test_fun <- function(one_allowed = c("1", "2", "3"), multiple = FALSE) {
  rlang::arg_match(arg = one_allowed, multiple = multiple)
}

test_fun(c("1", "2", "3"))
#> [1] "1"

^{Created on 2024-11-25 with reprex v2.1.1}