[Rd] S4 fails to initialize linear hierarchies with intermediate VIRTUAL classes

[Martin Morgan]

Initialization of this simple hierarchy

   A = setClass("A", representation(x="numeric"))
   setClass("B", contains=c("VIRTUAL", "A"))
   C = setClass("C", contains="B")

fails (neat that setClass returns generators; I hadn't realized that before!)

 > C(A())
Error: evaluation nested too deeply: infinite recursion / options(expressions=)?

because in the default coerce<-,C,A-method

 > selectMethod("coerce<-", c("C", "A"))
Method Definition:

function (from, to = "A", value)
{
     .value <- as(from, "B", TRUE)
     as(.value, "A") <- value
...

as(from, "B", TRUE) correctly returns an instance of C.

Also VIRTUAL isn't mentioned on ?Classes or ?setClass and isn't documented on 
class?VIRTUAL, making me wonder whether virtual classes are actually meant to be 
supported as part of S4 (they are used regularly in Bioconductor packages)?

Martin
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