update comments on C23 support

git-svn-id: https://svn.r-project.org/R/trunk@87592 00db46b3-68df-0310-9c12-caf00c1e9a41

config.site

@@ -69,7 +69,9 @@
 
 ## Ditto for a compiler that supports C23 features.
 ## For example 'CC23=clang-15 -std=gnu2x'
-## Eventually -std=gnu23 will be supported and preferred.
+## -std=gnu2x is supported by at least GCC >= 12, LLVM clang >= 15 and Apple clang >= 16 
+## Eventually -std=gnu23 will be supported and preferred
+## (and is by GCC >= 14 and LLVM >= 18).
 ## Default to CC and CFLAGS with any -std options removed, then
 ## -std=gmu2x appended to CC.
 ## CC23=

doc/manual/R-exts.texi

@@ -3196,12 +3196,15 @@ clarifying the formerly deprecated meaning of function declarations with
 an empty parameter list to mean zero parameters. (There are many other
 additions: see for example @uref{https://en.cppreference.com/w/c/23}.)
 
+@c GCC 14, LLVM clang 19 and Apple clang 16 default to C17
+@c In 2025-01 GCC pre=15 defaulted to C23 (but did not document so).
 The @command{configure} script in recent versions of @R{} aims to choose
 a C compiler which supports C11: as the default in recent versions of
 @command{gcc}, @I{LLVM} @command{clang} and Apple @command{clang} is C17,
 that is what is likely to be chosen.  On the other hand, until @R{} 4.3.0
 the makefiles for the Windows build specified C99.  They now use the
-compiler default which for the recommended compiler is C17.
+compiler default which for the recommended compiler is C17.  Future
+versions (including @command{gcc}@tie{}15) are likely to default to C23.
 
 Packages may want to either avoid or embrace the changes in C23, and can
 do so @emph{via} specifying @samp{USE_Cnn} for 17, 23, 90 or 99 in the

src/include/R_ext/Error.h

@@ -39,11 +39,15 @@
 extern "C" {
 #endif
 
-/* C23 has a [[noreturn]] attribute supported in GCC 13 and LLVM clang
- * 15 with -std=c2x but not Apple clang 14.  All have version 202000L.
- * clang 19 has version 202301L
- * gcc pre-15 supports -std=gnu23 with version 202301L but does not
- * support (and barfs on) this attribute, so restrict to clang for now.
+/* C23 has a [[noreturn]] attribute supported in LLVM clang 15 with
+ * -std=c2x but not Apple clang 14.  That and GCC verisions 12-14 have
+ * version 202000L.
+ * clang 19 has version 202301L (and supports -std=gnu23).
+ * gcc pre-15 supports -std=gnu23 with version 202301L but barfs on
+ * this attribute, so restrict to clang for now.
+ *
+ * https://en.cppreference.com/w/c/compiler_support/23
+ * claims GCC 13 and LLVM clang 15 support [[noreturn]] ....
  *
  * In C11 there is _Noreturn * (or noreturn in header <stdnoreturn.h>).
  */