SIGNIFICANT USER-VISIBLE CHANGES:

• The formula method of aggregate() now matches the generic in
  naming its first argument x (resolving PR#18299 by Thomas
  Soeiro).

  This means that calling aggregate() with a formula as a named
  first argument requires name formula in earlier versions of R and
  name x now, so portable code should not name the argument (code
  in many packages did).

• Calling && or || with either argument of length greater than one
  now gives a warning (which it is intended will become an error).

• Calling if() or while() with a condition of length greater than
  one gives an error rather than a warning. Consequently,
  environment variable _R_CHECK_LENGTH_1_CONDITION_ no longer has
  any effect.

• Windows users should consult the WINDOWS section below for some
  profound changes including

  • Support for 32-bit builds has been dropped.

  • UTF-8 locales are used where available.

  • The default locations for the R installation and personal
    library folder have been changed.

  Thanks to Tomas Kalibera for months of work on the Windows port
  for this release.

NEW FEATURES:

• matrix(x, n, m) now warns in more cases where length(x) differs
  from n * m, as 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 _R_CHECK_MATRIX_DATA_ to TRUE: R CMD check --as-cran
  does so unless it is already set.

• Function file_test() in package utils gains tests for symlinks,
  readability and writability.

• capabilities("libxml") is now false.

  The description of capabilities("http/ftp") now reflects that it
  refers to the default method, no longer the internal one.

• simplify2array() gains an except argument for controlling the
  exceptions used by sapply().

• Environment variables R_LIBS_USER and R_LIBS_SITE are both now
  set to the R system default if unset or empty, and can be set to
  NULL to indicate an empty list of user or site library
  directories.

• The warning for axis()(-like) calls in cases of relatively small
  ranges (typically in log-scale situations) is slightly improved
  and suppressed from explicit calls to .axisPars() as has always
  been the intention.

• The contrasts setter function contrasts<- gains an explicit
  default how.many = NULL rather than just using missing(how.many).

• grid.pretty() gains a new optional argument n = 5.

• There is a new function .pretty() with option bounds as a
  technical-utility version of pretty(). It and pretty() gain a
  new argument f.min with a better than back-compatible default.

• Function grDevices::axisTicks() and related functions such as
  graphics::axis() work better, notably for the log scale; partly
  because of the pretty() improvements, but also because care is
  taken e.g., when ylim is finite but diff(ylim) is infinite.

• nclass.FD() gains a digits option.

• The R Mathlib internal C function bd0() (called indirectly from a
  dozen probability density and distribution functions such as
  dpois(), dbinom(), dgamma(), pgamma() etc) has been
  complemented by a more sophisticated and (mostly) more accurate C
  function ebd0(), currently called only by internal dpois_raw()
  improving accuracy for R level dpois() and potentially others
  calling it such as dnbinom(), dgamma() or pgamma(). (Thanks to
  Morten Welinder's PR#15628.)

• write.ftable() gains sep = " " argument as suggested by Thomas
  Soeiro.

• The names of the locale categories supported by R's
  Sys.getlocale() and Sys.setlocale() are now provided by variable
  .LC.categories in the base namespace.

• The Date and POSIXt methods for hist() and the histogram method
  for plot() now also use the new default col = "lightgray" in
  consistency with the corresponding change to hist()'s default for
  R 4.0.0.

• hist.default() gains new fuzz argument, and the histogram plot
  method no longer uses fractional axis ticks when displaying
  counts ("Frequency").

• mapply() and hence Map() now also obey the "max-or-0-if-any"
  recycling rule, such that, e.g., Map(+, 1:3, 1[0]) is valid
  now.

• as.character(<obj>) for "hexmode" or "octmode" objects now
  fulfils the important basic rule as.character(x)[j] === as.character(x[j]).

• The set utility functions, notably intersect() have been tweaked
  to be more consistent and symmetric in their two set arguments,
  also preserving a common mode.

• substr(ch, start,end) <- new now e.g., preserves names(ch); ditto
  for substring(), thanks to a patch from Brodie Gaslam.

• plot(<lm>) gains a extend.ylim.f argument, in partial response to
  PR#15285; further PR#17784 is fixed thanks to several
  contributors and a patch by Elin Waring. The Cook's dist
  contours get customizable via cook.col and cook.lty with a
  different default color and their legend is nicer by default and
  customizable via cook.legendChanges.

• Attempting to subset an object that is not subsettable now
  signals an error of class notSubsettableError. The
  non-subsettable object is contained in the object field of the
  error condition.

• Subscript-out-of-bounds errors are now signaled as errors of
  class subscriptOutOfBoundsError.

• Stack-overflow errors are now signaled as errors inheriting from
  class stackOverflowError. See ?stackOverflowError for more
  details.

• New partly experimental Sys.setLanguage() utility, solving the
  main problem of PR#18055.

• gettext() and gettextf() get a new option trim = TRUE which when
  set to false allows translations for strings such as "Execution halted\n" typical for C code.

• An experimental implementation of hash tables is now available.
  See ?hashtab for more details.

• identical() gains a extptr.as.ref argument for requesting that
  external pointer objects be compared as reference objects.

• reorder() gets an argument decreasing which it passes to sort()
  for level creation; based on the wish and patch by Thomas Soeiro
  in PR#18243.

• as.vector() gains a data.frame method which returns a simple
  named list, also clearing a long standing 'FIXME' to enable
  as.vector(<data.frame>, mode="list"). This breaks code relying
  on as.vector(<data.frame>) to return the unchanged data frame.

• legend() is now vectorized for arguments cex, x.intersp, and
  text.width. The latter can now also be specified as a vector
  (one element for each column of the legend) or as NA for
  computing a proper column wise maximum value of strwidth(legend).
  The argument y.intersp can be specified as a vector with one
  entry for each row of the legend.

  legend() also gains new arguments title.cex and title.font.
  Thanks to Swetlana Herbrandt.

• Deparsing no longer remaps attribute names dim, dimnames, levels,
  names and tsp to historical S-compatible names (which structure()
  maps back).

• sample() and sample.int() have additional sanity checks on their
  size and n arguments.

  all.equal.numeric() gains a sanity check on its tolerance
  argument - calling all.equal(a, b, c) for three numeric vectors
  is a surprisingly common error.

  mean(na.rm =), rank(na.last =), barplot(legend.text =),
  boxplot(), contour(drawlabels =), polygon(border =) and
  methods::is(class2 =) have more robust sanity checks on their
  arguments.

  R CMD Rd2pdf (used by R CMD check) has a more robust sanity check
  on the format of \alias{} commands.

• psigamma(x, deriv) for negative x now also works for deriv = 4
  and 5; their underlying C level dpsifn() is documented in
  'Writing R Extensions'.

• The HTML help system now uses HTML5 (wish of PR#18149).

• ks.test() now provides exact p-values also with ties and MC
  p-values in the two-sample (Smirnov) case. By Torsten Hothorn.

• ks.test() gains a formula interface, with y ~ 1 for the
  one-sample (Kolmogorov) test and y ~ group for the two-sample
  (Smirnov) test. Contributed by Torsten Hothorn.

• The return value from ks.test() now has class c("ks.test", "htest") - packages using try() need to take care to use
  inherits() and not == on the class.

• New functions psmirnov(), qsmirnov() and rsmirnov() in package
  stats implementing the asymptotic and exact distributions of the
  two-sample Smirnov statistic.

• iconv() now allows sub = "c99" to use C99-style escapes for UTF-8
  inputs which cannot be converted to encoding to.

• In a forward pipe |> expression it is now possible to use a named
  argument with the placeholder _ in the rhs call to specify where
  the lhs is to be inserted. The placeholder can only appear once
  on the rhs.

• The included LAPACK sources have been updated to version 3.10.0,
  except for the four Fortran 77 routines which 3.10.0 has
  re-implemented in Fortran 90 (where the older versions have been
  retained as the R build process does not support Fortran 90).

• path.expand() and most other uses of tilde expansion now warn if
  a path would be too long if expanded. (An exception is
  file.exists(), which silently returns false.)

• trunc(<Date>, *) now supports units = "months" or "years" for
  consistency with the POSIXt method, thanks to Dirk Eddelbuettel's
  proposal in PR#18099.

• list2DF() now checks that its arguments are of the same length,
  rather than use recycling.

• The HTML help system has several new features: LaTeX-like math
  can be typeset using either KaTeX or MathJax, usage and example
  code is highlighted using Prism, and for dynamic help the output
  of examples and demos can be shown within the browser if the
  knitr package is installed. These features can be disabled by
  setting the environment variable _R_HELP_ENABLE_ENHANCED_HTML_ to
  a false value.

GRAPHICS:

• The graphics engine version, R_GE_version, has been bumped to 15
  and so packages that provide graphics devices should be
  reinstalled.

• The grid package now allows the user to specify a "vector" of
  pattern fills. The fill argument to gpar() accepts a list of
  gradients and/or patterns and the functions linearGradient(),
  radialGradient(), and pattern() have a new group argument.

  Points grobs (data symbols) can now also have a pattern fill.

  The grobCoords() function now returns a more informative and
  complex result.

• The grid package has new functions for drawing isolated groups:
  grid.group(), grid.define(), and grid.use(). These functions add
  compositing operators and affine transformations to R's graphics
  capabilities.

  The grid package also has new functions for stroking and filling
  paths: grid.stroke(), grid.fill(), and grid.fillStroke().

  A new function as.path() allows the user to specify the fill rule
  for a path that is to be used for clipping, stroking, or filling;
  available options are "winding" and "evenodd". A new function
  as.mask() allows the user to specify the type of a mask;
  available options are "alpha" and "luminance".

  These new features are only supported so far (at most) on the
  Cairo-based graphics devices and on the pdf() device.

• dev.capabilities() reports on device support for the new
  features.

• par() now warns about unnamed non-character arguments to prevent
  misuse such as {usr <- par("usr"); par(usr)}.

WINDOWS:

• R uses UTF-8 as the native encoding on recent Windows systems (at
  least Windows 10 version 1903, Windows Server 2022 or Windows
  Server 1903). As a part of this change, R uses UCRT as the C
  runtime. UCRT should be installed manually on systems older than
  Windows 10 or Windows Server 2016 before installing R.

• The default personal library on Windows, folder R\win-library\x.y
  where x.y stands for R release x.y.z, is now a subdirectory of
  Local Application Data directory (usually a hidden directory
  C:\Users\username\AppData\Local). Use shell.exec(.libPaths()[1])
  from R to open the personal library in Explorer when it is first
  in the list (PR#17842).

• R uses a new 64-bit Tcl/Tk bundle. The previous 32-bit/64-bit
  bundle had a different layout and can no longer be used.

• Make files and installer scripts for Windows have been tailored
  to Rtools42, the newly recommended 64-bit gcc 10.3 MinGW-W64 UCRT
  toolchain.

• Rtools42 by default uses the Windows security features ASLR and
  DEP; hence CRAN builds of R and packages also do.

• R now supports files Makevars.ucrt, Makefile.ucrt, configure.ucrt
  and cleanup.ucrt in packages, which are used in preference to the
  .win variants. This allows keeping the .win files around to
  support older versions of R. This feature will be removed in the
  future once support for older versions of R would no longer be
  needed.

• R.version gains a new field crt (only on Windows) to denote the C
  runtime. The value is "ucrt".

• On Windows, download.file(method = "auto") and url(method = "default") now follow Unix in using "libcurl" for all except
  file:// URIs.

• Rtools42 includes an unpatched Msys2 build of GNU tar. Paths
  including drive letters can be made to work by adding
  --force-local to environment variable TAR_OPTIONS. (Rtools40 and
  earlier included a patched version which defaulted to this
  option.)

• Installer builds of R automatically find the Rtools42 software
  collection as well as the compiler toolchain. No PATH setting is
  required from the user.

• The default installation directory of R for a user-only
  installation has been changed to the User Program Files directory
  (usually a hidden directory
  C:\Users\username\AppData\Local\Programs) to follow Windows
  conventions. Use shell.exec(R.home()) from R to open the R
  installation directory in Explorer (PR#17842).

• R now supports installation-time patching of packages. Patches
  may be installed from a supplied URL or a local directory or
  disabled. Patches are included into the installed packages for
  reference. This experimental feature may be removed in the
  future.

• libcurl is now required for building from source.

• The clipboard connection now works also with text in other than
  the current native encoding (PR#18267, with Hiroaki Yutani).
  Text is always pasted to the clipboard in UTF16-LE and the
  encoding argument is ignored.

• The internal case-changing functions are now used by default on
  Windows - this circumvents problems (for example with E acute) of
  the UCRT Windows' runtime.

• R on Windows now uses the system memory allocator. Doug Lea's
  allocator was used since R 1.2.0 to mitigate performance
  limitations seen with system allocators on earlier versions of
  Windows.

• memory.limit() and memory.size() are now stubs on Windows (as on
  Unix-alikes).

• Applications embedding R on Windows can now use additional
  callbacks, which have so far only been available only on Unix
  (PR#18286).

INSTALLATION:

• Facilities for accessing ftp:// sites are no longer tested
  (except pro tem for curlGetHeaders()) as modern browsers have
  removed support.

• R can now be built with DEFS = -DSTRICT_R_HEADERS .

PACKAGE INSTALLATION:

• R CMD INSTALL no longer tangles vignettes. This completes an R CMD build change in R 3.0.0 and affects packages built before R
  3.0.2. Such packages should be re-made with R CMD build to have
  the tangled R code of vignettes shipped with the tarball.

• USE_FC_LEN_T will become the default: this uses the correct
  prototypes for Fortran BLAS/LAPACK routines called from C/C++,
  and requires adjustment of most such calls - see 'Writing R
  Extensions' SS6.6.2. (This has been supported since R 3.6.2.)

• Package installation speed for packages installed with
  keep.source has been improved. This resolve the issue reported by
  Ofek Shilon in PR#18236.

UTILITIES:

• R CMD check can optionally report files/directories left behind
  in home, /tmp (even though TMPDIR is set) and other directories.
  See the "R Internals" manual for details.

• R CMD check now reports byte-compilation errors during
  installation. These are not usually fatal but may result in
  parts of the package not being byte-compiled.

• _R_CHECK_DEPENDS_ONLY_ can be applied selectively to examples,
  tests and/or vignettes in R CMD check: see the "R Internals"
  manual.

• _R_CHECK_SRC_MINUS_W_IMPLICIT_ now defaults to true: recent
  versions of Apple clang on macOS have made implicit function
  declarations in C into a compilation error.

• R CMD check --as-cran makes use of the environment variable
  AUTORECONF. See the "R Internals" manual SS8 for further details.

• R CMD check --use-valgrind also uses valgrind when re-building
  vignettes as some non-Sweave vignettes unhelpfully comment out
  all their code when R CMD check runs vignettes.

• Errors in re-building vignettes (unless there are LaTeX errors)
  are reported by R CMD check as ERROR rather than WARNING when
  running vignettes has been skipped (as it frequently is in CRAN
  checks and by --as-cran).

• R CMD Rd2pdf gains a --quiet option that is used by R CMD build
  when building the PDF package manual.

• R CMD Rd2pdf now always runs LaTeX in batch mode, consistent with
  Texinfo >= 6.7. The --batch option is ignored.

• R CMD build and R CMD check now include the Rd file name and line
  numbers in the error message of an \Sexpr evaluation failure.

• For packages using the \doi Rd macro (now an install-time \Sexpr)
  but no other dynamic Rd content, R CMD build now produces a
  smaller tarball and is considerably faster - skipping temporary
  package installation.

C-LEVEL FACILITIES:

• The non-API header R_ext/R-ftp-http.h is no longer provided, as
  the entry points it covered are now all defunct.

• A number of non-API declarations and macro definitions have been
  moved from the installed header Rinternals.h to the internal
  header Defn.h. Packages that only use entry points and
  definitions documented to be part of the API as specified in
  'Writing R Extensions' SS6 should not be affected.

• The macro USE_RINTERNALS no longer has any effect when compiling
  package code. Packages which also use R_NO_REMAP will need to
  ensure that the remapped names are used for calls to API
  functions that were formerly also made available as macros.

• The deprecated legacy S-compatibility macros PROBLEM, MESSAGE,
  ERROE, WARN, WARNING, RECOVER, ... are no longer defined in
  R_exts/RS.h (included by R.h). Replace these by calls to
  Rf_error and Rf_warning (defined in header R_ext/Error.h included
  by R.h).

  Header R_ext/RS.h no longer includes R_ext/Error.h.

• Header R_ext/Constants.h (included by R.h) when included from C++
  now includes the C++ header cfloat rather than the C header
  float.h (now possible as C++11 is required).

• The legacy S-compatibility macros DOUBLE_* in R_ext/Constants.h
  (included by R.h) are deprecated.

• The deprecated S-compatibility macros SINGLE_* in
  R_ext/Constants.h (included by R.h) have been removed.

• R_Calloc, R_Free and R_Realloc are preferred to their unprefixed
  forms and error messages now use the prefix. These forms were
  introduced in R 3.4.0 and are available even when
  STRICT_R_HEADERS is defined.

• rmultinom has been documented in 'Writing R Extensions' SS6 so is
  now part of the R API.

• Similarly, Rtanpi, called from R level tanpi() is now part of the
  R API.

• The long-deprecated, undocumented and non-API entry point call_R
  is no longer declared in R_ext/RS.h (included by R.h).

• The header S.h which has been unsupported since Jan 2016 has been
  removed. Use R.h instead.

DEPRECATED AND DEFUNCT:

• The (non-default and deprecated) method = "internal" for
  download.file() and url() no longer supports http:// nor ftp://
  URIs. (It is used only for file:// URIs.)

  On Windows, download.file(method = "wininet") no longer supports
  ftp:// URIs. (It is no longer the default method, which is
  "libcurl" and does.)

  On Windows, the deprecated method = "wininet" now gives a warning
  for http:// and https:// URIs for both download.file() and url().
  (It is no longer the default method.)

• On Windows, the command-line option --max-mem-size and
  environment variable R_MAX_MEM_SIZE are defunct. The memory
  allocation limit was important for 32-bit builds, but these are
  no longer supported.

• default.stringsAsFactors() is now formally deprecated, where that
  was only mentioned on its regular help page, previously. So it
  now gives a warning if called.

• unix.time() is defunct now; it had been deprecated since R 3.4.0.

BUG FIXES:

• Setting digits = 0 in format(), print.default() (and hence
  typically print()) or options() is again invalid. Its behaviour
  was platform-dependent, and it is unclear what "zero significant
  digits" should mean (PR#18098).

• Messages from C code in the cairo section of package grDevices
  are now also offered for translation, thanks to Michael Chirico's
  PR#18123.

• mean(x) with finite x now is finite also without "long.double"
  capability.

• R CMD Rd2pdf no longer leaves an empty build directory behind
  when it aborts due to an already existing output file. (Thanks
  to Sebastian Meyer's PR#18141.)

• density(x, weights = w, na.rm = TRUE) when anyNA(x) is true, now
  removes weights "in parallel" to x, fixing PR#18151, reported by
  Matthias Gondan. Additionally, it gets a subdensity option.

• Conversion of \Sexpr[]{<expR>} to LaTeX or HTML no longer
  produces long blocks of empty lines when itself contains
  several lines all producing empty output. Thanks to a report and
  patch by Ivan Krylov posted to R-devel.

• R CMD build no longer fails if a package vignette uses child
  documents and inst/doc exists. (Thanks to Sebastian Meyer's
  PR#18156.)

• When an R documentation ('help' source) file man/foo.Rd in a
  package has \donttest{..} examples with a syntax error, it is now
  signalled as ERROR and with correct line numbers relating to the
  *-Ex.R file, thanks to Duncan Murdoch and Sebastian Meyer's
  reports and patch proposals in PR#17501.

• Improved determination of the correct translation domain in
  non-base packages, addressing the combination of PR#18092 and
  PR#17998 (#c6) with reports and augmented patch #2904 by
  Suharto Anggono.

  Note that "R-base" is no longer the default domain e.g., for
  top-level calls to gettext(); rather translation needs explicit
  domain = * specification in such cases.

• identical(attrib.as.set=FALSE) now works correctly with data
  frames with default row names (Thanks to Charlie Gao's PR#18179).

• txtProgressBar() now enforces a non-zero width for argument char,
  without which no progress can be visible.

• dimnames(table(d)) is more consistent in the case where d is a
  list with a single component, thanks to Thomas Soeiro's report to
  R-devel.

  Further, table(d1, d2) now gives an error when d1 and d2 are data
  frames as suggested by Thomas in PR#18224.

• Fix for drawing semi-transparent lines and fills on the native
  Windows graphics device (PR#18219 and PR#16694). Thanks to Nick
  Ray for helpful diagnosis on Bugzilla.

• The deparser now wraps sub-expressions such as if(A) .. with
  parentheses when needed; thanks to Duncan Murdoch's PR#18232 and
  Lionel Henry's patches there.

• remove.packages() no longer tries to uninstall Priority: base
  packages, thanks to a report and suggestions by Colin Fay in
  PR#18227.

• win.metafile() now has xpinch and ypinch arguments so that the
  user can override Windows' (potentially wrong) guess at device
  dimensions.

• x[i] and x[[i]] for non-integer i should now behave in all cases
  as always documented: the index used is equivalent to
  as.integer(i) unless that would overflow where trunc(i) is used
  instead; thanks to Suharto Anggono's report and patch proposals
  in PR#17977.

• asOneSidedFormula() now associates the resulting formula with the
  global environment rather than the evaluation environment created
  for the call.

• <bibentry>$name now matches the field name case-insensitively,
  consistent with bibentry() creation and the replacement method.

• cbind() failed to detect some length mismatches with a mixture of
  time-series and non-time-series inputs.

• The default LaTeX style file Sweave.sty used by the RweaveLatex
  driver no longer loads the obsolete ae package; thanks to a
  report by Thomas Soeiro in PR#18271. Furthermore, it now skips
  \usepackage[T1]{fontenc} for engines other than pdfTeX (if
  detected) or if the new [nofontenc] option is used.

• smooth.spline() now stores its logical cv argument more safely,
  fixing a rare bug when printing, and also stores n.

• smooth.spline(x,y,*) now computes the cv.crit statistic
  correctly, also when is.unsorted(x), fixing PR#18294.

• The data.frame method of rbind() now warns when binding
  not-wholly-recycling vectors, by analogy to the default method
  (for matrices).

• setAs() finds the correct class for name to when multiple
  packages define a class with that name. Thanks to Gabor Csardi
  for the report.

• Fix for detaching a package when two classes of the same name are
  present in method signatures for the same generic. Thanks to
  Gabor Csardi for the report.

• match.arg("", c("", "a", "B")) gives a better error message, in
  part from PR#17959, thanks to Elin Waring.

• R CMD Sweave --clean no longer removes pre-existing files or
  subdirectories (PR#18242).

• The quartz() device no longer splits polylines into subpaths.
  That has caused narrowly-spaced lines with many points to always
  look solid even when dashed line type was used due to dash phase
  restarts.

• Deparsing constructs such as quote(1 + !(2) + 3) works again as
  before R 3.5.0, thanks to the report and patch in PR#18284 by
  Suharto Anggono.

• as.list(f) for a factor f now keeps names(f), fixing PR#18309.

• qbeta(.001, .9, .009) and analogous qf() calls now return a
  correct value instead of NaN or wrongly 1, all with a warning;
  thanks to the report by Ludger Goeminne in PR#18302.

• plot.lm() failed to produce the plot of residuals vs. factor
  levels (i.e., which=5 when leverages are constant) for models
  with character predictors (PR#17840).

• interaction.plot(..., xtick = TRUE) misplaced the x-axis line
  (PR#18305).

• Not strictly fixing a bug, format()ing and print()ing of
  non-finite Date and POSIXt values NaN and +/-Inf no longer show
  as NA but the respective string, e.g., Inf, for consistency with
  numeric vector's behaviour, fulfilling the wish of PR#18308.

• R CMD check no longer runs test scripts generated from
  corresponding .Rin files twice and now signals an ERROR if
  processing an .Rin script fails.

• tools::Rd2txt() used for plain-text help pages now renders \hrefs
  (if tools::Rd2txt_options(showURLs = TRUE)) and \urls with
  percent-encoding and standards-compliant delimiting style (angle
  brackets and no URL: prefix). \email is now rendered with a
  mailto: prefix.