doc updates towards the removal of C++11/14 support

git-svn-id: https://svn.r-project.org/R/trunk@89083 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

config.site

@@ -307,42 +307,6 @@
 ## Allow user to override.
 ## R_LD_LIBRARY_PATH=
 
-## C++11 compiler, optional for use in packages.
-## Introduced in R 3.1.0
-## The command which runs the C++11 compiler: defaults to $(CXX), but
-## empty if that (plus standard) cannot compile the tested C++11 code.
-## CXX11=
-
-## The 'standard' for the C++11 compiler, which is invoked as
-## $(CXX11) $(CXX11STD) for both compiling and linking DSOs/DLLs.  
-## '-std=gnu++11' and '-std=gnu++0x' are tried in turn by configure.
-## followed by '-std=c++11' and '-std=c++0x'
-## CXX11STD=
-
-## Analogues for the C++11 compiler: default to the settings for C++
-## CXX11FLAGS=
-## CXX11PICFLAGS=
-## SHLIB_CXX11LD=
-## SHLIB_CXX11LDFLAGS=
-
-## C++14 compiler, optional for use in packages.
-## Introduced in R 3.4.0
-## The command which runs the C++14 compiler: defaults to $(CXX11), but
-## empty if that (plus standard) cannot compile the tested C++14 code.
-## CXX14=
-
-## The 'standard' for the C++14 compiler, which is invoked as
-## $(CXX14) $(CXX14STD) for both compiling and linking DSOs/DLLs.  
-## '-std=gnu++14' and '-std=gnu++1y" are tried by configure, also
-## '-std=c++14' and '-std=c++1y"
-## CXX14STD=
-
-## Analogues for the C++14 compiler: default to the settings for C++11
-## CXX14FLAGS=
-## CXX14PICFLAGS=
-## SHLIB_CXX14LD=
-## SHLIB_CXX14LDFLAGS=
-
 ## C++17 compiler, optional for use in packages.
 ## Introduced in R 3.4.0
 ## The command which runs the C++17 compiler: defaults to $(CXX14), but
@@ -355,7 +319,7 @@
 ## '-std=c++17' and '-std=c++1z"
 ## CXX17STD=
 
-## Analogues for the C++17 compiler: default to the settings for C++14
+## Analogues for the C++17 compiler.
 ## CXX17FLAGS=
 ## CXX17PICFLAGS=
 ## SHLIB_CXX17LD=

doc/manual/R-exts.texi

@@ -2950,39 +2950,35 @@ define a module of the same name in multiple source files.
 @node Using C++ code
 @subsection Using C++ code
 
-@R{} can be built without a C++ compiler although one is available (but
-not necessarily installed) on all known @R{} platforms. 
-standard (`C++11').  A minor update@footnote{Some changes are linked
-from
-@uref{https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations}:
-there were also additional deprecations.} (`C++14') was published in
-December 2014 and was used by default as from @R{} 4.1.0 if supported.
-Further revisions `C++17' (in December 2017), `C++20' (with many new
-features in December 2020) and `C++23' (in October 2024) have been
-published since.  The next revision, `C++26', is expected in 2026/7 and
-several compilers already have considerable support for the current
-draft.
-
-The support in @R{} for these standards has varied over the years: this
-version of the manual only describes @R{} 4.3.0 and later.  For details
-of earlier versions, see the corresponding section in their manuals.
+A new C++ standard has been published every three years since 2011.
+C++26 was feature-frozen in October 2025 and is expected to be published
+in late 2026.
 
 The default standard for compiling @R{} packages was changed to C++17 in
-@R{} 4.3.0 if supported, and from @R{} 4.4.0 only a C++17 compiler will
-be selected as the default C++ compiler.
+@R{} 4.4.0: only a C++17 compiler will be selected as the default C++
+compiler.  Such a compiler is available (but not necessarily installed)
+on all known @R{} platforms.  It is intended that C++20 will become the
+default (`where available') in @R{} 4.6.0: it has been supported on all
+known platforms since at least 2021.
+@c The last was Intel compilers.
+
+@R{} has long allowed packages to request a C++ standard: recent
+releases support C++17, C++20, C++23 and C++26 (if a suitable compiler
+is available when @R{} was built): C++11 and C++14 were supported prior
+to @R{} 4.6.0.
 
 What standard a C++ compiler aims to support can be hard to determine:
 the value@footnote{Values @code{201103L}, @code{201402L}, @code{201703L}
 and @code{202002L} are most commonly used for C++11, C++14, C++17 and
 C++20 respectively, but some compilers set @code{1L}. For C++23 all that
 can currently be assumed is a value greater than that for C++20: for
-example @command{g++} 12 uses @code{202100L} and @command{clang++} (@I{LLVM}
-15, Apple 14) uses @code{202101L}.}  of @code{__cplusplus} may help but
-some compilers use it to denote a standard which is partially supported
-and some the latest standard which is (almost) fully supported.  On a
-Unix-alike @command{configure} will try to identify a compiler and flags
-for each of the standards: this relies heavily on the reported values of
-@code{__cplusplus}.
+example @command{g++} 12 uses @code{202100L} and @command{clang++}
+(@I{LLVM} 15, Apple 14) uses @code{202101L}.}  of @code{__cplusplus} may
+help but some compilers use it to denote a standard which is partially
+supported and some the latest standard which is (almost) fully
+supported.  On a Unix-alike @R{}'s @command{configure} will try to
+identify a compiler and flags for each of the supported standards: this
+relies heavily on the reported values of @code{__cplusplus}.
 
 @c Apple clang 14 defaults to C++98 (even 16)
 @c @I{LLVM} clang 14, 15 default to C++14, 16-20 to C++17
@@ -2998,10 +2994,10 @@ C++ standards have deprecated and later removed features (such as
 headers, types and functions).  Be aware that some current compilers
 still accept removed features in C++17 mode, such as
 @code{std::unary_function} (deprecated in C++11, removed in C++17) or
-removed in C++20 (such as @code{std::uncaught_exception}). And some
-compilers in use are far from current, notably those used for
-@acronym{CRAN} macOS builds.  The Wikipedia pages for the various
-standards list deprecated and removed (and undeprecated) features.
+removed in C++20 (such as @code{std::uncaught_exception}), and most do
+not warn on the use of deprecated features. (And some compilers in use
+are far from current.)  The Wikipedia pages for the various standards
+list deprecated and removed (and also undeprecated) features.
 
 For maximal portability a package should specify the standard it
 requires for code in its @file{src} directory by including something
@@ -3018,6 +3014,9 @@ If it has a @file{Makevars} file (or @file{Makevars.win} or
 CXX_STD = CXX23
 @end example
 
+C++20, C++23 and C++26 (from @R{} 4.5.0) can be specified in an
+analogous way.
+
 Code needing C++20 or later features can check for their presence @emph{via}
 `SD-6 feature tests'@footnote{See
 @uref{https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations}
@@ -3026,26 +3025,22 @@ or
 
 @c Ubuntu LTS releases now have 10 years of support even if announced
 @c with 8 years.
-@c Ubuntu 14.04LTS (4.8, ESM support extended until 2024-04)
 @c Ubuntu 16.04LTS (5.3, ESM support extended until 2026-04)
 @c Ubuntu 18.04LTS (7.5, on ESM support until 2028-04)
 @c Ubuntu 20.04LTS (9.4, 10.5 available)
 @c Ubuntu 22.04LTS (11.x)
-@c Centos 7 (4.8.5, ESM support until 2024-06)
 @c https://access.redhat.com/solutions/19458
 @c RHEL 7 (4.8, goes to 'extended life cycle' 2024-07 to 2028-06-30)
 @c RHEL 8 (8.x, 9.x available. full support 2024-05 + 8 more years)
 @c RHEL 9 (11.x available, full support 2027-05 + 8 more years)
 
-C++20, C++23 and C++26 (from @R{} 4.5.0) can be specified in an
-analogous way.
-
-Note that C++17 or later `support' does not mean complete support: use
+Note that `support' for a standard does not mean complete support: use
 feature tests as well as resources such as
 @uref{https://en.cppreference.com/w/cpp/compiler_support.html},
 @uref{https://gcc.gnu.org/projects/cxx-status.html} and
 @uref{https://clang.llvm.org/cxx_status.html} to see if the features you
-want to use are widely implemented.
+want to use are widely implemented. (In particular, C++20 modules were
+not.)
 
 Attempts to specify an unknown C++ standard are silently ignored: recent
 versions of @R{} throw an error for C++98 and for known standards for
@@ -3076,7 +3071,8 @@ CXXFLAGS=`"$@{R_HOME@}/bin/R" CMD config CXXFLAGS`
 AC_LANG(C++)
 @end example
 @noindent
-if no standard was specified.
+if no standard was specified. The script needs to check that @code{CXX}
+is not empty.
 
 If you want to compile C++ code in a subdirectory, make sure you pass
 down the macros to specify the appropriate compiler, e.g.@: in