more on C++ support

git-svn-id: https://svn.r-project.org/R/trunk@89081 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

doc/manual/R-admin.texi

@@ -1030,7 +1030,7 @@ C++17/20/23/26 code.
 It may be that there is no suitable flag for C++17/20/23/26 support
 with the default compiler, in which case a different compiler could be
 selected for
-@code{CXX17}/@code{CXX20}/@code{CXX23}/@code{CXX236} with
+@code{CXX17}/@code{CXX20}/@code{CXX23}/@code{CXX26} with
 its corresponding flags.
 
 If no suitable compiler/flag is found for the default C++ compiler, one
@@ -1041,32 +1041,26 @@ C++17 tests, so do this at your own risk as some packages may not compile.
 The @option{-std} flag is supported by the GCC, @command{clang++} and
 Intel compilers.  Currently accepted values are (plus some synonyms)
 @example
-g++:     c++17 gnu++17 (from 8) c++20 gnu++20 (from 10)
+g++:     c++17 gnu++17 c++2a gnu++2a(from 8) c++20 gnu++20 (from 10)
          c++23 gnu++23 c++2b gnu++2b (from 11)
          c++2c gnu++2c c++26 gnu++26 (from 14)
 Intel:   c++17 gnu++17 c++20 gnu++20 (from 2021.1)
          c++2b gnu++2b (from 2022.2) c++23 gnu++23 (at least from 2024.0)
+         c++26 gnu++26 (at least from 2025.1)
 @end example
 @noindent
 (Those for @I{LLVM} @command{clang++} are documented at
-@uref{https://clang.llvm.org/cxx_status.html}, and follow @code{g++}:
+@uref{https://clang.llvm.org/cxx_status.html} and follow @code{g++}:
 @code{-std=c++20} is supported from Clang 10, @code{-std=c++2b} from
-Clang 13 and @code{-std=c++23} and @code{-std=c++2c} from Clang 17.
+Clang 13 and @code{-std=c++23} and @code{-std=c++2c} from Clang 17 and
+currently @code{-std=c++26} (but not in that table). 
 Apple Clang supports @code{-std=c++23} and @code{-std=c++26} from 16.0.0.
 
-@c c++1y does not pass the configure test in gcc 4.9.3
-@c , with @option{-std=c++1y} (introduced@footnote{See
-@c @uref{https://gcc.gnu.org/projects/cxx-status.html#cxx14} for which
-@c C++14 features it supported.} in version 4.9.x) remaining as a deprecated
-@c synonym.
-
 `Standards' for @command{g++} starting with @samp{gnu} enable `GNU
 extensions': what those are is hard to track down.
 
 For the use of C++ in @R{} packages,
 @pxref{, , , R-exts, Writing R Extensions}.
-@c Prior to @R{} 3.6.0 the default C++ standard was
-@c that of the compiler used: currently it is C++17.
 
 @uref{https://en.cppreference.com/w/cpp/compiler_support.html} indicates
 which versions of common compilers support (parts of) which C++