• The workaround in headers R.h and Rmath.h (using namespace std;)
  for the Oracle Developer Studio compiler is no longer needed now
  C++11 is required so has been removed. A couple more usages of
  log() (which should have been std::log()) with an int argument
  are reported on Solaris.

• The undocumented limit of 4095 bytes on messages from the
  S-compatibility macros PROBLEM and MESSAGE is now documented and
  longer messages will be silently truncated rather than
  potentially causing segfaults.

• If the R_NO_SEGV_HANDLER environment variable is non-empty, the
  signal handler for SEGV/ILL/BUS signals (which offers recovery
  user interface) is not set. This allows more reliable debugging
  of crashes that involve the console.

DEPRECATED AND DEFUNCT:

• The legacy S-compatibility macros PROBLEM, MESSAGE, ERROR, WARN,
  WARNING, RECOVER, ... are deprecated and will be hidden in R
  4.2.0. R's native interface of Rf_error and Rf_warning has long
  been preferred.

BUG FIXES:

• .mapply(F, dots, .) no longer segfaults when dots is not a list
  and uses match.fun(F) as always documented; reported by Andrew
  Simmons in PR#18164.

• hist(<Date>, ...) and hist(<POSIXt>, ...) no longer pass
  arguments for rect() (such as col and density) to axis().
  (Thanks to Sebastian Meyer's PR#18171.)

• \Sexpr{ch} now preserves Encoding(ch). (Thanks to report and
  patch by Jeroen Ooms in PR#18152.)

• Setting the RNG to "Marsaglia-Multicarry" e.g., by RNGkind(), now
  warns in more places, thanks to Andr'e Gillibert's report and
  patch in PR#18168.

• gray(numeric(), alpha=1/2) no longer segfaults, fixing PR#18183,
  reported by Till Krenz.

• Fixed dnbinom(x, size=<very_small>, .., log=TRUE) regression,
  reported by Martin Morgan.

• as.Date.POSIXlt(x) now keeps names(x), thanks to Davis Vaughan's
  report and patch in PR#18188.

• model.response() now strips an "AsIs" class typically, thanks to
  Duncan Murdoch's report and other discussants in PR#18190.

• try() is considerably faster in case of an error and long call,
  as e.g., from some do.call(). Thanks to Alexander Kaever's
  suggestion posted to R-devel.

• qqline(y = <object>) such as y=I(.), now works, see also
  PR#18190.

• Non-integer mgp par() settings are now handled correctly in
  axis() and mtext(), thanks to Mikael Jagan and Duncan Murdoch's
  report and suggestion in PR#18194.

• formatC(x) returns length zero character() now, rather than ""
  when x is of length zero, as documented, thanks to Davis
  Vaughan's post to R-devel.

• removeSource(fn) now retains (other) attributes(fn).