[Rd] S3 dispatch for S4 subclasses only works if variable "extends" is accessible from global environment

[Kirill Müller]

Scenario: An S3 method is declared for an S4 base class but called for 
an instance of a derived class.

Steps to reproduce:

 > Rscript -e "test <- function(x) UseMethod('test', x); test.Matrix <- 
function(x) 'Hi'; MatrixDispatchTest::test(Matrix::Matrix())"
Error in UseMethod("test", x) :
   no applicable method for 'test' applied to an object of class "lsyMatrix"
Calls: <Anonymous>
1: MatrixDispatchTest::test(Matrix::Matrix())

 > Rscript -e "extends <- 42; test <- function(x) UseMethod('test', x); 
test.Matrix <- function(x) 'Hi'; MatrixDispatchTest::test(Matrix::Matrix())"
[1] "Hi"

To me, it looks like a sanity check in line 655 of src/main/attrib.c is 
making wrong assumptions, but there might be other reasons. 
(https://github.com/wch/r-source/blob/780021752eb83a71e2198019acf069ba8741103b/src/main/attrib.c#L655-L656)

Same behavior in R 3.2.4, R 3.2.5 and R-devel r70420.


Best regards

Kirill