R devel - Jan 2017 - Matrix package breaks as.matrix method

Hi,
The Matrix package and the as.matrix method do not seem to be compatible inside
of a package.

Here's an example. I've created a simple package "mat" that
defines an eponymous class and as.matrix method on that class. All is well,
unless that package has the Matrix package in its Depends or Imports (and
imports, e.g. the "Diagonal" function). Then my as.matrix method stops
working, even if I'm not using any part of the Matrix package.

Here's an example on R 3.3.2:

First, create an empty package "mat" (e.g. with package.skeleton) with
one file in mat/R/mat.R with the following contents:

setClass("mat", representation(H="matrix"))
mat <- function(H) new("mat", H=H)
setMethod("as.matrix", signature("mat"), function(x, ...)
crossprod(x at H))
testmat <- function() {
    H <- matrix(1:3, 1, 3)
    M <- mat(H)
    as.matrix(M)
}

Then install the mat package :
> require(mat)
> testmat()
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    2    4    6
[3,]    3    6    9

All works fine!

Now add "Depends: Matrix" into the package's DESCRIPTION file
(alternatively add "Imports: Matrix" and
'importFrom("Matrix","Diagonal")' in the NAMESPACE).

Try again:
> require(mat)
> testmat()
Error in as.vector(data) : 
  no method for coercing this S4 class to a vector

Bug? If not, can anyone provide a work around? In my case, I'd like to mix
matrix and Matrix functions in my package, but am obviously having difficulty.

I've come across a somewhat similar report on stackoverflow
http://stackoverflow.com/questions/13812373/overloading-operator-in-r-s4-classes-and-matrix-package
regarding defining the "+" operator with the Matrix package, but I
don't think the solution or the problem quite applies.

Thanks in advance, Robert
> R.version
               _                           
platform       x86_64-pc-linux-gnu         
arch           x86_64                      
os             linux-gnu                   
system         x86_64, linux-gnu           
status                                     
major          3                           
minor          3.2                         
year           2016                        
month          10                          
day            31                          
svn rev        71607                       
language       R                           
version.string R version 3.3.2 (2016-10-31)
nickname       Sincere Pumpkin Patch