[PATCH] c++: Implement C++23 P2334R1 - #elifdef/#elifndef

[Jakub Jelinek <jakub@redhat.com>]

Hi!

This patch implements C++23 P2334R1, which is easy because Joseph has done
all the hard work for C2X already.
Unlike the C N2645 paper, the C++ P2334R1 contains one important addition
(but not in the normative text):
"While this is a new preprocessor feature and cannot be treated as a defect
report, implementations that support older versions of the standard are
encouraged to implement this feature in the older language modes as well
as C++23."
so there are different variants how to implement it.
One is in the patch below, ignores that sentence and only implements it
for -std=c++23/-std=gnu++23 like it is only implemented for -std=c23.
Another option would be to implement it also in the older GNU modes but
not in the C/CXX modes (but it would be strange if we did that just for
C++ and not for C).
Yet another option is to enable it unconditionally.
And yet another option would be to enable it unconditionally but emit
a warning (or pedwarn) when it is seen.
Note, when it is enabled for the older language modes, as Joseph wrote
in the c11-elifdef-1.c testcase, it can result e.g. in rejecting previously
valid code:
#define A
#undef B
#if 0
#elifdef A
#error "#elifdef A applied"
#endif
#if 0
#elifndef B
#error "#elifndef B applied"
#endif
Note, seems clang went the enable it unconditionally in all standard
versions of both C and C++, no warnings or anything whatsoever, so
essentially treated it as a DR that changed behavior of e.g. the above code.

2021-10-05  Jakub Jelinek  <jakub@redhat.com>

libcpp/
	* init.c (lang_defaults): Implement P2334R1, enable elifdef for
	-std=c++23 and -std=gnu++23.
gcc/testsuite/
	* g++.dg/cpp/elifdef-1.C: New test.
	* g++.dg/cpp/elifdef-2.C: New test.
	* g++.dg/cpp/elifdef-3.C: New test.

--- libcpp/init.c.jj	2021-09-02 10:01:15.954715595 +0200
+++ libcpp/init.c	2021-10-05 09:55:15.010620700 +0200
@@ -122,8 +122,8 @@ static const struct lang_flags lang_defa
   /* CXX17    */  { 1,  1,  1,  1,  1,  0,    1,  1,   1,   1,   1,    1,     1,     0,   1,      0,   1,     0,   0,   0 },
   /* GNUCXX20 */  { 1,  1,  1,  1,  1,  0,    0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   0,   0 },
   /* CXX20    */  { 1,  1,  1,  1,  1,  0,    1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   0,   0 },
-  /* GNUCXX23 */  { 1,  1,  1,  1,  1,  1,    0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   0 },
-  /* CXX23    */  { 1,  1,  1,  1,  1,  1,    1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   0 },
+  /* GNUCXX23 */  { 1,  1,  1,  1,  1,  1,    0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1 },
+  /* CXX23    */  { 1,  1,  1,  1,  1,  1,    1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1 },
   /* ASM      */  { 0,  0,  1,  0,  0,  0,    0,  0,   0,   0,   0,    0,     0,     0,   0,      0,   0,     0,   0,   0 }
 };
 
--- gcc/testsuite/g++.dg/cpp/elifdef-1.C.jj	2021-10-05 10:00:41.410057024 +0200
+++ gcc/testsuite/g++.dg/cpp/elifdef-1.C	2021-10-05 10:00:33.110173069 +0200
@@ -0,0 +1,3 @@
+// { dg-do preprocess { target { ! c++23 } } }
+
+#include "../../gcc.dg/cpp/c11-elifdef-1.c"
--- gcc/testsuite/g++.dg/cpp/elifdef-2.C.jj	2021-10-05 10:01:30.345372808 +0200
+++ gcc/testsuite/g++.dg/cpp/elifdef-2.C	2021-10-05 10:03:36.560608083 +0200
@@ -0,0 +1,4 @@
+// P2334R1
+// { dg-do preprocess { target c++23 } }
+
+#include "../../gcc.dg/cpp/c2x-elifdef-1.c"
--- gcc/testsuite/g++.dg/cpp/elifdef-3.C.jj	2021-10-05 10:01:36.029293338 +0200
+++ gcc/testsuite/g++.dg/cpp/elifdef-3.C	2021-10-05 10:03:48.896435601 +0200
@@ -0,0 +1,4 @@
+// P2334R1
+// { dg-do preprocess { target c++23 } }
+
+#include "../../gcc.dg/cpp/c2x-elifdef-1.c"

	Jakub