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).

1 files changed, 11 insertions, 0 deletions

diff --git a/libcpp/init.cc b/libcpp/init.cc
index e9732afeb2c..eb495e26eff 100644
--- a/libcpp/init.cc
+++ b/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");