[Rd] Allowing S3 methods of rounding functions to take `...`

[Davis Vaughan]

Hi all,

I would like to propose adding `...` to the signatures of the following
rounding functions:

- floor(x)
- ceiling(x)
- round(x, digits = 0)
- And possibly signif(x, digits = 6)

The purpose would be to allow S3 methods to add additional arguments as
required.

A few arguments in favor of this change:

`trunc(x, ...)` already takes dots, which sets a precedent for the others
to do so as well. It is documented in the same help file as the other
rounding functions.

Internally at the C level, a check is done to ensure that there is exactly
1 arg for floor() and ceiling(), and either 1 or 2 args for round(). The
actual names of those arguments are not checked, however, and I believe
this is what allows `round.Date(x, ...)` and `round.POSIXt(x, unit)` to
exist, solely because they have 2 arguments. It seems like this is a bit of
a hack, since you couldn't create something similar for floor, like
`floor.POSIXt(x, unit)` (not saying this should exist, it is just for
argument's sake), because the 1 argument check would error on this. I think
adding `...` to the signature of the generics would better support what is
being done here.

Additionally, I have a custom date-like S3 class of my own that I would
like to write floor(), ceiling(), and round() methods for, and they would
require passing additional arguments.

If R core would like to make this change, they could probably tweak
`do_trunc()` to be a bit more general, and use it for floor() and
ceiling(), since it already allows `...`.

A few references:

Check for 1 arg in do_math1(), used by floor() and ceiling()
https://github.com/wch/r-source/blob/fe82da3baf849fcd3cc7dbc31c6abc72b57aa083/src/main/arithmetic.c#L1270

Check for 2 args in do_Math2(), used by round()
https://github.com/wch/r-source/blob/fe82da3baf849fcd3cc7dbc31c6abc72b57aa083/src/main/arithmetic.c#L1655

do_trunc() definition that allows `...`
https://github.com/wch/r-source/blob/fe82da3baf849fcd3cc7dbc31c6abc72b57aa083/src/main/arithmetic.c#L1329-L1340

- Davis

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