libcpp: Implement C++23 P2334R1 - #elifdef/#elifndef

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 ignoring that sentence and only implementing it
for -std=c++23/-std=gnu++23 like it is only implemented for -std=c2x.
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.
After feedback, this option enables #elifdef/#elifndef for -std=c2x
and -std=c++2{b,3} and enables it also for -std=gnu*, but for GNU modes
older than C2X or C++23 if -pedantic it emits a pedwarn on the directives
that either would be rejected in the corresponding -std=c* modes, e.g.
  #if 1
  #elifdef A // pedwarn if -pedantic
  #endif
or when the directives would be silently accepted, but when they are
recognized it changes behavior, so e.g.
  #define A
  #if 0
  #elifdef A // pedwarn if -pedantic
  #define M 1
  #endif
It won't pedwarn if the directives would be silently ignored and wouldn't
change anything, like:
  #define A
  #if 0
  #elifndef A
  #define M 1
  #endif
or
  #undef B
  #if 0
  #elifdef B
  #define M 1
  #endif

2021-10-06  Jakub Jelinek  <jakub@redhat.com>

libcpp/
	* init.c (lang_defaults): Implement P2334R1, enable elifdef for
	-std=c++23 and -std=gnu++23.
	* directives.c (_cpp_handle_directive): Support elifdef/elifndef if
	either CPP_OPTION (pfile, elifdef) or !CPP_OPTION (pfile, std).
	(do_elif): For older non-std modes if pedantic pedwarn about
	#elifdef/#elifndef directives that change behavior.
gcc/testsuite/
	* gcc.dg/cpp/gnu11-elifdef-1.c: New test.
	* gcc.dg/cpp/gnu11-elifdef-2.c: New test.
	* gcc.dg/cpp/gnu11-elifdef-3.c: New test.
	* gcc.dg/cpp/gnu11-elifdef-4.c: New test.
	* g++.dg/cpp/elifdef-1.C: New test.
	* g++.dg/cpp/elifdef-2.C: New test.
	* g++.dg/cpp/elifdef-3.C: New test.
	* g++.dg/cpp/elifdef-4.C: New test.
	* g++.dg/cpp/elifdef-5.C: New test.
	* g++.dg/cpp/elifdef-6.C: New test.
	* g++.dg/cpp/elifdef-7.C: New test.

2 files changed, 43 insertions, 4 deletions

diff --git a/libcpp/directives.c b/libcpp/directives.c
index 261a584..b4bc8b4 100644
--- a/libcpp/directives.c
+++ b/libcpp/directives.c
@@ -447,7 +447,11 @@ _cpp_handle_directive (cpp_reader *pfile, bool indented)
       if (dname->val.node.node->is_directive)
 	{
 	  dir = &dtable[dname->val.node.node->directive_index];
-	  if ((dir->flags & ELIFDEF) && !CPP_OPTION (pfile, elifdef))
+	  if ((dir->flags & ELIFDEF)
+	      && !CPP_OPTION (pfile, elifdef)
+	      /* For -std=gnu* modes elifdef is supported with
+		 a pedwarn if pedantic.  */
+	      && CPP_OPTION (pfile, std))
 	    dir = 0;
 	}
     }
@@ -2117,7 +2121,26 @@ do_elif (cpp_reader *pfile)
 	 are skipped and their controlling directives are processed as
 	 if they were in a group that is skipped."  */
       if (ifs->skip_elses)
-	pfile->state.skipping = 1;
+	{
+	  /* In older GNU standards, #elifdef/#elifndef is supported
+	     as an extension, but pedwarn if -pedantic if the presence
+	     of the directive would be rejected.  */
+	  if (pfile->directive != &dtable[T_ELIF]
+	      && ! CPP_OPTION (pfile, elifdef)
+	      && CPP_PEDANTIC (pfile)
+	      && !pfile->state.skipping)
+	    {
+	      if (CPP_OPTION (pfile, cplusplus))
+		cpp_error (pfile, CPP_DL_PEDWARN,
+			   "#%s before C++23 is a GCC extension",
+			   pfile->directive->name);
+	      else
+		cpp_error (pfile, CPP_DL_PEDWARN,
+			   "#%s before C2X is a GCC extension",
+			   pfile->directive->name);
+	    }
+	  pfile->state.skipping = 1;
+	}
       else
 	{
 	  if (pfile->directive == &dtable[T_ELIF])
@@ -2139,6 +2162,22 @@ do_elif (cpp_reader *pfile)
 		  if (pfile->cb.used)
 		    pfile->cb.used (pfile, pfile->directive_line, node);
 		  check_eol (pfile, false);
+		  /* In older GNU standards, #elifdef/#elifndef is supported
+		     as an extension, but pedwarn if -pedantic if the presence
+		     of the directive would change behavior.  */
+		  if (! CPP_OPTION (pfile, elifdef)
+		      && CPP_PEDANTIC (pfile)
+		      && pfile->state.skipping != skip)
+		    {
+		      if (CPP_OPTION (pfile, cplusplus))
+			cpp_error (pfile, CPP_DL_PEDWARN,
+				   "#%s before C++23 is a GCC extension",
+				   pfile->directive->name);
+		      else
+			cpp_error (pfile, CPP_DL_PEDWARN,
+				   "#%s before C2X is a GCC extension",
+				   pfile->directive->name);
+		    }
 		  pfile->state.skipping = skip;
 		}
 	    }
diff --git a/libcpp/init.c b/libcpp/init.c
index eda17a6a..5a424e2 100644
--- a/libcpp/init.c
+++ b/libcpp/init.c
@@ -122,8 +122,8 @@ static const struct lang_flags lang_defaults[] =
   /* 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 }
 };