c++: Warn on #undef/#define of remaining cpp.predefined macros [PR120778]

We already warn on #undef or pedwarn on #define (but not on #define
after #undef) of some builtin macros mentioned in cpp.predefined.

The C++26 P2843R3 paper changes it from (compile time) undefined behavior
to ill-formed.  The following patch arranges for warning (for #undef)
and pedwarn (on #define) for the remaining cpp.predefined macros.
__cpp_* feature test macros only for C++20 which added some of them
to cpp.predefined, in earlier C++ versions it was just an extension and
for pedantic diagnostic I think we don't need to diagnose anything,
__STDCPP_* and __cplusplus macros for all C++ versions where they appeared.

Like the earlier posted -Wkeyword-macro diagnostics (which is done
regardless whether the identifier is defined as a macro or not, obviously
most likely none of the keywords are defined as macros initially), this
one also warns on #undef when a macro isn't defined or later #define
after #undef.

2025-08-15  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/120778
	PR target/121520
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Implement C++26 DR 2581.  Add
	cpp_define_warn lambda and use it as well as cpp_warn where needed.
	In the if (c_dialect_cxx ()) block with __cpp_* predefinitions add
	cpp_define lambda.  Formatting fixes.
gcc/c/
	* c-decl.cc (c_init_decl_processing): Use cpp_warn instead of
	cpp_lookup and NODE_WARN bit setting.
gcc/cp/
	* lex.cc (cxx_init): Remove warn_on lambda.  Use cpp_warn instead of
	cpp_lookup and NODE_WARN bit setting or warn_on.
gcc/testsuite/
	* g++.dg/DRs/dr2581-1.C: New test.
	* g++.dg/DRs/dr2581-2.C: New test.
	* c-c++-common/cpp/pr92296-2.c: Expect warnings also on defining
	special macros after undefining them.
libcpp/
	* include/cpplib.h (struct cpp_options): Add
	suppress_builtin_macro_warnings member.
	(cpp_warn): New inline functions.
	* init.cc (cpp_create_reader): Clear suppress_builtin_macro_warnings.
	(cpp_init_builtins): Call cpp_warn on __cplusplus, __STDC__,
	__STDC_VERSION__, __STDC_MB_MIGHT_NEQ_WC__ and
	__STDCPP_STRICT_POINTER_SAFETY__ when appropriate.
	* directives.cc (do_undef): Warn on undefining NODE_WARN macros if
	not cpp_keyword_p.  Don't emit any NODE_WARN related diagnostics
	if CPP_OPTION (pfile, suppress_builtin_macro_warnings).
	(cpp_define, _cpp_define_builtin, cpp_undef): Temporarily set
	CPP_OPTION (pfile, suppress_builtin_macro_warnings) around
	run_directive calls.
	* macro.cc (_cpp_create_definition): Warn on defining NODE_WARN
	macros if they weren't previously defined and not cpp_keyword_p.
	Ignore NODE_WARN for diagnostics if
	CPP_OPTION (pfile, suppress_builtin_macro_warnings).

libcpp/directives.cc

@@ -736,7 +736,9 @@ do_undef (cpp_reader *pfile)
 
       /* Handle -Wkeyword-macro registered identifiers.  */
       bool diagnosed = false;
-      if (CPP_OPTION (pfile, cpp_warn_keyword_macro) && cpp_keyword_p (node))
+      if (CPP_OPTION (pfile, cpp_warn_keyword_macro)
+	  && !CPP_OPTION (pfile, suppress_builtin_macro_warnings)
+	  && cpp_keyword_p (node))
 	{
 	  if (CPP_OPTION (pfile, cpp_pedantic)
 	      && CPP_OPTION (pfile, cplusplus)
@@ -752,7 +754,8 @@ do_undef (cpp_reader *pfile)
 	 identifier is not currently defined as a macro name.  */
       if (cpp_macro_p (node))
 	{
-	  if (node->flags & NODE_WARN)
+	  if ((node->flags & NODE_WARN)
+	      && !CPP_OPTION (pfile, suppress_builtin_macro_warnings))
 	    {
 	      if (!diagnosed)
 		cpp_error (pfile, CPP_DL_WARNING,
@@ -769,6 +772,11 @@ do_undef (cpp_reader *pfile)
 
 	  _cpp_free_definition (node);
 	}
+      else if ((node->flags & NODE_WARN)
+	       && !CPP_OPTION (pfile, suppress_builtin_macro_warnings)
+	       && !diagnosed
+	       && !cpp_keyword_p (node))
+	cpp_error (pfile, CPP_DL_WARNING, "undefining %qs", NODE_NAME (node));
     }
 
   check_eol (pfile, false);
@@ -3087,7 +3095,9 @@ cpp_define (cpp_reader *pfile, const char *str)
     }
   buf[count] = '\n';
 
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 1;
   run_directive (pfile, T_DEFINE, buf, count);
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 0;
 }
 
 /* Like cpp_define, but does not warn about unused macro.  */
@@ -3141,7 +3151,9 @@ _cpp_define_builtin (cpp_reader *pfile, const char *str)
   char *buf = (char *) alloca (len + 1);
   memcpy (buf, str, len);
   buf[len] = '\n';
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 1;
   run_directive (pfile, T_DEFINE, buf, len);
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 0;
 }
 
 /* Process MACRO as if it appeared as the body of an #undef.  */
@@ -3152,7 +3164,9 @@ cpp_undef (cpp_reader *pfile, const char *macro)
   char *buf = (char *) alloca (len + 1);
   memcpy (buf, macro, len);
   buf[len] = '\n';
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 1;
   run_directive (pfile, T_UNDEF, buf, len);
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 0;
 }
 
 /* Replace a previous definition DEF of the macro STR.  If DEF is NULL,

libcpp/include/cpplib.h

@@ -435,6 +435,10 @@ struct cpp_options
      Presumably the usage is protected by the appropriate #ifdef.  */
   unsigned char warn_variadic_macros;
 
+  /* Non-zero means suppress diagnostics for NODE_WARN #define or #undef.
+     Used for cpp_define/cpp_undef.  */
+  unsigned char suppress_builtin_macro_warnings;
+
   /* Nonzero means warn about builtin macros that are redefined or
      explicitly undefined.  */
   unsigned char warn_builtin_macro_redefined;
@@ -1526,6 +1530,21 @@ extern cpp_comment_table *cpp_get_comments (cpp_reader *);
 extern cpp_hashnode *cpp_lookup (cpp_reader *, const unsigned char *,
 				 unsigned int);
 
+/* Set NODE_WARN flag for NAME, such that there will be diagnostics
+   for #define or #undef of NAME.  */
+
+inline void
+cpp_warn (cpp_reader *pfile, const char *name, unsigned int len)
+{
+  cpp_lookup (pfile, (const unsigned char *) name, len)->flags |= NODE_WARN;
+}
+
+inline void
+cpp_warn (cpp_reader *pfile, const char *name)
+{
+  cpp_warn (pfile, name, strlen (name));
+}
+
 typedef int (*cpp_cb) (cpp_reader *, cpp_hashnode *, void *);
 extern void cpp_forall_identifiers (cpp_reader *, cpp_cb, void *);
 

libcpp/init.cc

@@ -246,6 +246,7 @@ cpp_create_reader (enum c_lang lang, cpp_hash_table *table,
   CPP_OPTION (pfile, dollars_in_ident) = 1;
   CPP_OPTION (pfile, warn_dollars) = 1;
   CPP_OPTION (pfile, warn_variadic_macros) = 1;
+  CPP_OPTION (pfile, suppress_builtin_macro_warnings) = 0;
   CPP_OPTION (pfile, warn_builtin_macro_redefined) = 1;
   CPP_OPTION (pfile, cpp_warn_implicit_fallthrough) = 0;
   CPP_OPTION (pfile, warn_header_guard) = 0;
@@ -593,6 +594,8 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
       && (! CPP_OPTION (pfile, stdc_0_in_system_headers)
 	  || CPP_OPTION (pfile, std)))
     _cpp_define_builtin (pfile, "__STDC__ 1");
+  else if (CPP_OPTION (pfile, cplusplus))
+    cpp_warn (pfile, "__STDC__");
 
   if (CPP_OPTION (pfile, cplusplus))
     {
@@ -618,6 +621,14 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
 	_cpp_define_builtin (pfile, "__cplusplus 201103L");
       else
 	_cpp_define_builtin (pfile, "__cplusplus 199711L");
+      cpp_warn (pfile, "__cplusplus");
+      if (CPP_OPTION (pfile, lang) >= CLK_GNUCXX11)
+	{
+	  cpp_warn (pfile, "__STDC_VERSION__");
+	  cpp_warn (pfile, "__STDC_MB_MIGHT_NEQ_WC__");
+	  if (CPP_OPTION (pfile, lang) < CLK_GNUCXX23)
+	    cpp_warn (pfile, "__STDCPP_STRICT_POINTER_SAFETY__");
+	}
     }
   else if (CPP_OPTION (pfile, lang) == CLK_ASM)
     _cpp_define_builtin (pfile, "__ASSEMBLER__ 1");

libcpp/macro.cc

@@ -3412,8 +3412,11 @@ warn_of_redefinition (cpp_reader *pfile, cpp_hashnode *node,
   /* Some redefinitions need to be warned about regardless.  */
   if (node->flags & NODE_WARN)
     {
-      /* Ignore NODE_WARN on -Wkeyword-macro registered identifiers though.  */
-      if (!CPP_OPTION (pfile, cpp_warn_keyword_macro) || !cpp_keyword_p (node))
+      /* Ignore NODE_WARN on -Wkeyword-macro registered identifiers though
+	 or during cpp_define.  */
+      if (!CPP_OPTION (pfile, suppress_builtin_macro_warnings)
+	  && (!CPP_OPTION (pfile, cpp_warn_keyword_macro)
+	      || !cpp_keyword_p (node)))
 	return true;
     }
 
@@ -3954,7 +3957,9 @@ _cpp_create_definition (cpp_reader *pfile, cpp_hashnode *node,
     macro->line = name_loc;
 
   /* Handle -Wkeyword-macro registered identifiers.  */
-  if (CPP_OPTION (pfile, cpp_warn_keyword_macro) && cpp_keyword_p (node))
+  if (CPP_OPTION (pfile, cpp_warn_keyword_macro)
+      && !CPP_OPTION (pfile, suppress_builtin_macro_warnings)
+      && cpp_keyword_p (node))
     {
       if (macro->fun_like
 	  && CPP_OPTION (pfile, cplusplus)
@@ -3994,6 +3999,11 @@ _cpp_create_definition (cpp_reader *pfile, cpp_hashnode *node,
 	}
       _cpp_free_definition (node);
     }
+  else if ((node->flags & NODE_WARN)
+	   && !CPP_OPTION (pfile, suppress_builtin_macro_warnings)
+	   && !cpp_keyword_p (node))
+    cpp_error_with_line (pfile, CPP_DL_WARNING, macro->line, 0,
+			 "%qs defined", NODE_NAME (node));
 
   /* Enter definition in hash table.  */
   node->type = NT_USER_MACRO;