updares of Fortran 2023

git-svn-id: https://svn.r-project.org/R/trunk@86530 00db46b3-68df-0310-9c12-caf00c1e9a41

doc/manual/R-exts.texi

@@ -2890,18 +2890,33 @@ selected as @samp{FC}, so platforms might be encountered which only
 support Fortran 90.  However, Fortran 95 is supported on all known
 platforms.
 
-Most compilers specified by @samp{FC} will accept Fortran 2003, 2008 or
+Most compilers specified by @samp{FC} will accept most Fortran 2003, 2008 or
 2018 code: such code should still use file extension @file{.f90}.  Most
 current platforms use @command{gfortran} where you might need to include
 @option{-std=f2003}, @option{-std=f2008} or (from version 8)
 @option{-std=f2018} in @code{PKG_FFLAGS} or @code{PKG_FCFLAGS}: the
 default is `GNU Fortran', currently Fortran 2018 (but Fortran 95 prior
 to @command{gfortran}@tie{}8) with non-standard extensions.  The other
 compilers in current use (@I{LLVM}'s @command{flang-new} and Intel's
-@command{ifx}) default to Fortran 2018.
+@command{ifx}) default to Fortran 2018@footnote{but was said to have
+complete support only from version 2023.0.0.}.
 
 It is good practice to describe a Fortran version requirement in
-@file{DESCRIPTION}'s @samp{SystemRequirements} field.
+@file{DESCRIPTION}'s @samp{SystemRequirements} field.  Note that this is
+purely for information: the package also needs a @command{configure}
+script to determine the compiler and set approrpriate option(s) and test
+that the features needed from the standard are actually supported.
+
+The Fortran 2023 released in Nov 2023: as usual compiler vedors are
+introducing support incrementally.
+For Intel's @command{ifx} see
+@uref{https://www.intel.com/content/www/us/en/developer/articles/technical/fortran-language-and-openmp-features-in-ifx.html#Fortran%20Standards}.
+For LLVM's @command{flang-new} see
+@uref{https://flang.llvm.org/docs/F202X.html}.
+@code{gfortran} does not have complete support even for the 2008 and
+2018 standards, but recent versions support the option @option{-std=f2023}.
+@c https://gcc.gnu.org/wiki/Fortran2008Status
+@c https://gcc.gnu.org/wiki/Fortran2018Status
 
 @c The Oracle @command{f95} compiler `accepts some Fortran 2003/8 features'
 @c (search for `Oracle Developer Studio 12.6: Fortran User's Guide' and