[Rd] rbind has confusing result for custom sub-class (possible bug?)

[Michael Chirico]

Debugging this issue:

https://github.com/Rdatatable/data.table/issues/2008

We have custom class 'IDate' which inherits from 'Date' (it just forces
integer storage for efficiency, hence, I).

The concatenation done by rbind, however, breaks this and returns a double:

library(data.table)
DF = data.frame(date = as.IDate(Sys.Date()))
storage.mode(rbind(DF, DF)$date)
# [1] "double"

This is specific to base::rbind (data.table's rbind returns an integer as
expected); in ?rbind we see:

The method dispatching is not done via UseMethod(), but by C-internal
dispatching. Therefore there is no need for, e.g., rbind.default.
The dispatch algorithm is described in the source file
(‘.../src/main/bind.c’) as
1. For each argument we get the list of possible class memberships from the
class attribute.
2. *We inspect each class in turn to see if there is an applicable method.*
3. If we find an applicable method we make sure that it is identical to any
method determined for prior arguments. If it is identical, we proceed,
otherwise we immediately drop through to the default code.

It's not clear what #2 means -- an applicable method *for what*? Glancing
at the source code would suggest it's looking for rbind.IDate:

https://github.com/wch/r-source/blob/trunk/src/main/bind.c#L1051-L1063

const char *generic = ((PRIMVAL(op) == 1) ? "cbind" : "rbind"); // should
be rbind here
const char *s = translateChar(STRING_ELT(classlist, i)); // iterating over
the classes, should get to IDate first
sprintf(buf, "%s.%s", generic, s); // should be rbind.IDate

but adding this method (or even exporting it) is no help [ simply defining
rbind.IDate = function(...) as.IDate(NextMethod()) ]

Lastly, it appears that as.Date.IDate is called, which is causing the type
conversion:

debug(data.table:::as.Date.IDate)
rbind(DF, DF) # launches debugger
x
# [1] "2019-05-26" <-- singleton, so apparently applied to DF$date, not
c(DF$date, DF$date)
undebug(data.table:::as.Date.IDate)

I can't really wrap my head around why as.Date is being called here, and
even allowing that, why the end result is still the original class [
class(rbind(DF, DF)$date) == c('IDate', 'Date') ]

So, I'm beginning to think this might be a bug. Am I missing something?

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