\item \code{matrix(x, n, m)} now warns in more cases where
\code{length(x)} differs from than \code{n * m}; suggested by Abby
Spurdle and Wolfgang Huber in Feb 2021 on the R-devel mailing
list.
This warning can be turned into an error by setting environment
variable \env{_R_CHECK_MATRIXDATA} to \samp{TRUE}: \command{R
CMD check --as-cran} does so unless it is already set.
in current r-devel, where the Debian-based checks are now performed with
_R_CHECK_MATRIXDATA=TRUE.