.S3methods(), typically called from methods(), again marks
methods from package base as visible.

Also, the visibility of non-base methods is again determined by
the method's presence in search().

tools::Rdiff() is now more robust against invalid strings, fixing
installation tests on Windows without Rtools installed
(PR#18530).

Fix (new) bug in hcl.colors(2, *), by Achim Zeileis (PR#18523).

head(., <illegal>) and tail(..) now produce more useful
"Error in ...." error messages, fixing PR#18362.

Package code syntax on Windows is checked in UTF-8 when UTF-8 is
the native encoding.

na.contiguous(x) now also returns the first run, when it is at
the beginning and there is a later one of the same length;
reported to R-devel, including a fix, by Georgi Boshnakov.
Further, by default, it modifies only an existing attr(*,"tsp")
but otherwise no longer sets one.

chol(<not pos.def>, pivot = <T|F>) now gives a correct error or
warning message (depending on pivot), thanks to Mikael Jagan's
(PR#18541).