[Rd] S4 method dispatch sometimes leads to incorrect when object loaded from file?

[Martin Morgan]

Loading an S4 object from a file without first loading the library sometimes (?, 
the example below and actual example involves a virtual base class and the show 
generic) leads to incorrect dispatch (to the base class method).

The attached package reproduces the problem. It has

setClass("A")
setClass("B", contains="A")
setMethod("show", "A", function(object) cat("A\n"))
setMethod("show", "B", function(object) cat("B\n"))

with NAMESPACE

import(methods)
exportClasses(A, B)
exportMethods(show)

This creates the object and illustrated expected behavior

   ~/tmp$ R --vanilla --slave -e "library(PkgA); b = new('B'); save(b, 
file='b.Rda'); b"
   B

Loading PkgA before the object leads to correct dispatch

   ~/tmp$ R --vanilla --slave -e "library(PkgA); load(file='b.Rda'); b"
   B

but loading the object without first loading PkgA leads to dispatch to 
show,A-method.

   ~/tmp$ R --vanilla --slave -e "load(file='b.Rda'); b"
   Loading required package: PkgA
   A

Martin Morgan
-- 
Computational Biology / Fred Hutchinson Cancer Research Center
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