Limit scoping of accuracy.default()

[mitchelloharawild]

forecast/R/errors.R

Lines 244 to 254 in 465d278

	accuracy.default <- function(object, x, test = NULL, d = NULL, D = NULL, f = NULL, ...) {
	if (!is.null(f)) {
	warning("Using `f` as the argument for `accuracy()` is deprecated. Please use `object` instead.")
	object <- f
	}
	if (!any(is.element(class(object), c(
	"ARFIMA", "mforecast", "forecast", "ts", "integer", "numeric",
	"Arima", "ets", "lm", "bats", "tbats", "nnetar", "stlm", "baggedModel"
	)))) {
	stop("First argument should be a forecast object or a time series.")
	}

The error produced can be confusing when users encounter it from using other packages (for example: https://stackoverflow.com/questions/72873984/r-fabletools-accuracy-first-argument-should-be-a-forecast-object-or-a-time-ser).

The current error is specific to the forecast package, and doesn't generalise well - especially since the function also works for models and not just forecasts and time series.

The correct approach would be to register S3 methods for each of these classes and not export a default method. However at minimum I think that the error should be improved.

[robjhyndman]

Improved error message in 2906e97