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).

10 files changed, 357 insertions, 47 deletions

diff --git a/gcc/c-family/c-cppbuiltin.cc b/gcc/c-family/c-cppbuiltin.cc
index 0ba6151..fe80e1f 100644
--- a/gcc/c-family/c-cppbuiltin.cc
+++ b/gcc/c-family/c-cppbuiltin.cc
@@ -913,23 +913,42 @@ c_cpp_builtins (cpp_reader *pfile)
 
   /* encoding definitions used by users and libraries  */
   builtin_define_with_value ("__GNUC_EXECUTION_CHARSET_NAME",
-    cpp_get_narrow_charset_name (pfile), 1);
+			     cpp_get_narrow_charset_name (pfile), 1);
   builtin_define_with_value ("__GNUC_WIDE_EXECUTION_CHARSET_NAME",
-    cpp_get_wide_charset_name (pfile), 1);
-
+			     cpp_get_wide_charset_name (pfile), 1);
 
   if (c_dialect_cxx ())
-  {
-    int major;
-    parse_basever (&major, NULL, NULL);
-    cpp_define_formatted (pfile, "__GNUG__=%d", major);
-  }
+    {
+      int major;
+      parse_basever (&major, NULL, NULL);
+      cpp_define_formatted (pfile, "__GNUG__=%d", major);
+    }
 
   /* For stddef.h.  They require macros defined in c-common.cc.  */
   c_stddef_cpp_builtins ();
 
+  /* Variant of cpp_define which arranges for diagnostics on user #define
+     or #undef of the macros.  */
+  auto cpp_define_warn = [] (cpp_reader *pfile, const char *def)
+    {
+      const char *end = strchr (def, '=');
+      cpp_define (pfile, def);
+      cpp_warn (pfile, def, end ? end - def : strlen (def));
+    };
+
   if (c_dialect_cxx ())
     {
+      /* Treat all cpp_define calls in this block for macros starting
+	 with __cpp_ (for C++20 and later) or __STDCPP_ as cpp_define_warn.  */
+      auto cpp_define = [=] (cpp_reader *pfile, const char *def)
+	{
+	  if ((cxx_dialect >= cxx20 && startswith (def, "__cpp_"))
+	      || startswith (def, "__STDCPP_"))
+	    cpp_define_warn (pfile, def);
+	  else
+	    ::cpp_define (pfile, def);
+	};
+
       if (flag_weak && SUPPORTS_ONE_ONLY)
 	cpp_define (pfile, "__GXX_WEAK__=1");
       else
@@ -1101,42 +1120,65 @@ c_cpp_builtins (cpp_reader *pfile)
 	}
       if (flag_concepts && cxx_dialect > cxx14)
 	cpp_define (pfile, "__cpp_concepts=202002L");
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_concepts");
       if (flag_contracts)
 	{
 	  cpp_define (pfile, "__cpp_contracts=201906L");
 	  cpp_define (pfile, "__cpp_contracts_literal_semantics=201906L");
 	  cpp_define (pfile, "__cpp_contracts_roles=201906L");
 	}
+      else if (cxx_dialect >= cxx26)
+	cpp_warn (pfile, "__cpp_contracts");
       if (flag_modules)
 	/* The std-defined value is 201907L, but I don't think we can
 	   claim victory yet.  201810 is the p1103 date. */
 	cpp_define (pfile, "__cpp_modules=201810L");
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_modules");
       if (flag_coroutines)
 	cpp_define (pfile, "__cpp_impl_coroutine=201902L"); /* n4861, DIS */
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_impl_coroutine");
       if (flag_tm)
 	/* Use a value smaller than the 201505 specified in
 	   the TS, since we don't yet support atomic_cancel.  */
 	cpp_define (pfile, "__cpp_transactional_memory=201500L");
       if (flag_sized_deallocation)
 	cpp_define (pfile, "__cpp_sized_deallocation=201309L");
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_sized_deallocation");
       if (aligned_new_threshold)
 	{
 	  cpp_define (pfile, "__cpp_aligned_new=201606L");
 	  cpp_define_formatted (pfile, "__STDCPP_DEFAULT_NEW_ALIGNMENT__=%d",
 				aligned_new_threshold);
 	}
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_aligned_new");
+      if (cxx_dialect >= cxx17)
+	cpp_warn (pfile, "__STDCPP_DEFAULT_NEW_ALIGNMENT__");
       if (flag_new_ttp)
 	cpp_define (pfile, "__cpp_template_template_args=201611L");
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_template_template_args");
       if (flag_threadsafe_statics)
 	cpp_define (pfile, "__cpp_threadsafe_static_init=200806L");
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_threadsafe_static_init");
       if (flag_char8_t)
 	cpp_define (pfile, "__cpp_char8_t=202207L");
+      else if (cxx_dialect >= cxx20)
+	cpp_warn (pfile, "__cpp_char8_t");
 #ifndef THREAD_MODEL_SPEC
       /* Targets that define THREAD_MODEL_SPEC need to define
 	 __STDCPP_THREADS__ in their config/XXX/XXX-c.c themselves.  */
       if (cxx_dialect >= cxx11 && strcmp (thread_model, "single") != 0)
 	cpp_define (pfile, "__STDCPP_THREADS__=1");
+      else
 #endif
+	if (cxx_dialect >= cxx11)
+	  cpp_warn (pfile, "__STDCPP_THREADS__");
       if (flag_implicit_constexpr)
 	cpp_define (pfile, "__cpp_implicit_constexpr=20211111L");
     }
@@ -1293,7 +1335,13 @@ c_cpp_builtins (cpp_reader *pfile)
 	{
 	  char name[sizeof ("__STDCPP_FLOAT128_T__=1")];
 	  sprintf (name, "__STDCPP_FLOAT%d_T__=1", floatn_nx_types[i].n);
-	  cpp_define (pfile, name);
+	  cpp_define_warn (pfile, name);
+	}
+      else if (cxx_dialect >= cxx23)
+	{
+	  char name[sizeof ("__STDCPP_FLOAT128_T__")];
+	  sprintf (name, "__STDCPP_FLOAT%d_T__", floatn_nx_types[i].n);
+	  cpp_warn (pfile, name);
 	}
       char prefix[20], csuffix[20];
       sprintf (prefix, "FLT%d%s", floatn_nx_types[i].n,
@@ -1306,10 +1354,12 @@ c_cpp_builtins (cpp_reader *pfile)
   if (bfloat16_type_node)
     {
       if (c_dialect_cxx () && cxx_dialect > cxx20)
-	cpp_define (pfile, "__STDCPP_BFLOAT16_T__=1");
+	cpp_define_warn (pfile, "__STDCPP_BFLOAT16_T__=1");
       builtin_define_float_constants ("BFLT16", "BF16", "%s",
 				      "BF16", bfloat16_type_node);
     }
+  else if (cxx_dialect >= cxx23)
+    cpp_warn (pfile, "__STDCPP_BFLOAT16_T__");
 
   /* For float.h.  */
   if (targetm.decimal_float_supported_p ())
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 97b8021..77006ca 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -4843,9 +4843,8 @@ c_init_decl_processing (void)
 	    tree id = get_identifier (c_common_reswords[i].word);
 	    if (C_IS_RESERVED_WORD (id)
 		&& C_RID_CODE (id) != RID_CXX_COMPAT_WARN)
-	      cpp_lookup (parse_in,
-			  (const unsigned char *) IDENTIFIER_POINTER (id),
-			  IDENTIFIER_LENGTH (id))->flags |= NODE_WARN;
+	      cpp_warn (parse_in, IDENTIFIER_POINTER (id),
+			IDENTIFIER_LENGTH (id));
 	  }
     }
 }
diff --git a/gcc/cp/lex.cc b/gcc/cp/lex.cc
index fa377e7..da86989 100644
--- a/gcc/cp/lex.cc
+++ b/gcc/cp/lex.cc
@@ -384,47 +384,42 @@ cxx_init (void)
 	    if (IDENTIFIER_KEYWORD_P (id)
 		/* Don't register keywords with spaces.  */
 		&& IDENTIFIER_POINTER (id)[IDENTIFIER_LENGTH (id) - 1] != ' ')
-	      cpp_lookup (parse_in,
-			  (const unsigned char *) IDENTIFIER_POINTER (id),
-			  IDENTIFIER_LENGTH (id))->flags |= NODE_WARN;
+	      cpp_warn (parse_in, IDENTIFIER_POINTER (id),
+			IDENTIFIER_LENGTH (id));
 	  }
-      auto warn_on = [] (const char *name) {
-	cpp_lookup (parse_in, (const unsigned char *) name,
-		    strlen (name))->flags |= NODE_WARN;
-      };
       if (cxx_dialect >= cxx11)
 	{
-	  warn_on ("final");
-	  warn_on ("override");
-	  warn_on ("noreturn");
+	  cpp_warn (parse_in, "final");
+	  cpp_warn (parse_in, "override");
+	  cpp_warn (parse_in, "noreturn");
 	  if (cxx_dialect < cxx26)
-	    warn_on ("carries_dependency");
+	    cpp_warn (parse_in, "carries_dependency");
 	}
       if (cxx_dialect >= cxx14)
-	warn_on ("deprecated");
+	cpp_warn (parse_in, "deprecated");
       if (cxx_dialect >= cxx17)
 	{
-	  warn_on ("fallthrough");
-	  warn_on ("maybe_unused");
-	  warn_on ("nodiscard");
+	  cpp_warn (parse_in, "fallthrough");
+	  cpp_warn (parse_in, "maybe_unused");
+	  cpp_warn (parse_in, "nodiscard");
 	}
       if (cxx_dialect >= cxx20)
 	{
-	  warn_on ("likely");
-	  warn_on ("unlikely");
-	  warn_on ("no_unique_address");
+	  cpp_warn (parse_in, "likely");
+	  cpp_warn (parse_in, "unlikely");
+	  cpp_warn (parse_in, "no_unique_address");
 	}
       if (flag_modules)
 	{
-	  warn_on ("import");
-	  warn_on ("module");
+	  cpp_warn (parse_in, "import");
+	  cpp_warn (parse_in, "module");
 	}
       if (cxx_dialect >= cxx23)
-	warn_on ("assume");
+	cpp_warn (parse_in, "assume");
       if (cxx_dialect >= cxx26)
 	{
-	  warn_on ("replaceable_if_eligible");
-	  warn_on ("trivially_relocatable_if_eligible");
+	  cpp_warn (parse_in, "replaceable_if_eligible");
+	  cpp_warn (parse_in, "trivially_relocatable_if_eligible");
 	}
     }
 
diff --git a/gcc/testsuite/c-c++-common/cpp/pr92296-2.c b/gcc/testsuite/c-c++-common/cpp/pr92296-2.c
index e3f74cd..2594e47 100644
--- a/gcc/testsuite/c-c++-common/cpp/pr92296-2.c
+++ b/gcc/testsuite/c-c++-common/cpp/pr92296-2.c
@@ -39,42 +39,42 @@ filebase2 = __BASE_FILE__	/* { dg-final { scan-file-not pr92296-2.i "filebase2 =
 
 #pragma push_macro("__LINE__")
 #undef __LINE__		/* { dg-warning "undefining" } */
-#define __LINE__ 142
+#define __LINE__ 142	/* { dg-warning "defined" } */
 line1 = __LINE__	/* { dg-final { scan-file pr92296-2.i "line1 = 142" } } */
 #pragma pop_macro("__LINE__")
 line2 = __LINE__	/* { dg-final { scan-file pr92296-2.i "line2 = 45" } } */
 
 #pragma push_macro("__INCLUDE_LEVEL__")
 #undef __INCLUDE_LEVEL__	/* { dg-warning "undefining" } */
-#define __INCLUDE_LEVEL__ 42
+#define __INCLUDE_LEVEL__ 42	/* { dg-warning "defined" } */
 includelevel1 = __INCLUDE_LEVEL__	/* { dg-final { scan-file pr92296-2.i "includelevel1 = 42" } } */
 #pragma pop_macro("__INCLUDE_LEVEL__")
 includelevel2 = __INCLUDE_LEVEL__	/* { dg-final { scan-file pr92296-2.i "includelevel2 = 0" } } */
 
 #pragma push_macro("__COUNTER__")
 #undef __COUNTER__	/* { dg-warning "undefining" } */
-#define __COUNTER__ 172
+#define __COUNTER__ 172	/* { dg-warning "defined" } */
 counter1 = __COUNTER__	/* { dg-final { scan-file pr92296-2.i "counter1 = 172" } } */
 #pragma pop_macro("__COUNTER__")
 counter2 = __COUNTER__	/* { dg-final { scan-file-not pr92296-2.i "counter2 = 172" } } */
 
 #pragma push_macro("__has_attribute")
 #undef __has_attribute	/* { dg-warning "undefining" } */
-#define __has_attribute(x) 0
+#define __has_attribute(x) 0	/* { dg-warning "defined" } */
 hasattr1 = __has_attribute(noreturn)	/* { dg-final { scan-file pr92296-2.i "hasattr1 = 0" } } */
 #pragma pop_macro("__has_attribute")
 hasattr2 = __has_attribute(noreturn)	/* { dg-final { scan-file-not pr92296-2.i "hasattr2 = 0" } } */
 
 #pragma push_macro("__has_cpp_attribute")
 #undef __has_cpp_attribute	/* { dg-warning "undefining" } */
-#define __has_cpp_attribute(x) 0
+#define __has_cpp_attribute(x) 0	/* { dg-warning "defined" } */
 hasattrcpp1 = __has_cpp_attribute(noreturn)	/* { dg-final { scan-file pr92296-2.i "hasattrcpp1 = 0" } } */
 #pragma pop_macro("__has_cpp_attribute")
 hasattrcpp2 = __has_cpp_attribute(noreturn)	/* { dg-final { scan-file-not pr92296-2.i "hasattrcpp2 = 0" } } */
 
 #pragma push_macro("__has_builtin")
 #undef __has_builtin	/* { dg-warning "undefining" } */
-#define __has_builtin(x) 0
+#define __has_builtin(x) 0	/* { dg-warning "defined" } */
 hasbuiltin1 = __has_builtin(__builtin_expect)	/* { dg-final { scan-file pr92296-2.i "hasbuiltin1 = 0" } } */
 #pragma pop_macro("__has_builtin")
 hasbuiltin2 = __has_builtin(__builtin_expect)	/* { dg-final { scan-file pr92296-2.i "hasbuiltin2 = 1" } } */
diff --git a/gcc/testsuite/g++.dg/DRs/dr2581-1.C b/gcc/testsuite/g++.dg/DRs/dr2581-1.C
new file mode 100644
index 0000000..56ccb60
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2581-1.C
@@ -0,0 +1,106 @@
+// DR 2581 - Undefined behavior for predefined macros
+// { dg-do preprocess }
+// { dg-additional-options "-fcontracts" { target c++26 } }
+// { dg-additional-options "-fmodules -fcoroutines" { target c++20 } }
+
+#undef defined				// { dg-error "'defined' cannot be used as a macro name" }
+#undef __cplusplus			// { dg-warning "undefining '__cplusplus'" }
+#undef __DATE__				// { dg-warning "undefining '__DATE__'" }
+#undef __FILE__				// { dg-warning "undefining '__FILE__'" }
+#undef __LINE__				// { dg-warning "undefining '__LINE__'" }
+#undef __STDC_EMBED_NOT_FOUND__		// { dg-warning "undefining '__STDC_EMBED_NOT_FOUND__'" }
+#undef __STDC_EMBED_FOUND__		// { dg-warning "undefining '__STDC_EMBED_FOUND__'" }
+#undef __STDC_EMBED_EMPTY__		// { dg-warning "undefining '__STDC_EMBED_EMPTY__'" }
+#undef __STDC_HOSTED__			// { dg-warning "undefining '__STDC_HOSTED__'" }
+#undef __STDCPP_DEFAULT_NEW_ALIGNMENT__	// { dg-warning "undefining '__STDCPP_DEFAULT_NEW_ALIGNMENT__'" "" { target c++17 } }
+#undef __STDCPP_FLOAT16_T__		// { dg-warning "undefining '__STDCPP_FLOAT16_T__'" "" { target c++23 } }
+#undef __STDCPP_FLOAT32_T__		// { dg-warning "undefining '__STDCPP_FLOAT32_T__'" "" { target c++23 } }
+#undef __STDCPP_FLOAT64_T__		// { dg-warning "undefining '__STDCPP_FLOAT64_T__'" "" { target c++23 } }
+#undef __STDCPP_FLOAT128_T__		// { dg-warning "undefining '__STDCPP_FLOAT128_T__'" "" { target c++23 } }
+#undef __STDCPP_BFLOAT16_T__		// { dg-warning "undefining '__STDCPP_BFLOAT16_T__'" "" { target c++23 } }
+#undef __TIME__				// { dg-warning "undefining '__TIME__'" }
+#undef __STDC__				// { dg-warning "undefining '__STDC__'" }
+#undef __STDC_MB_MIGHT_NEQ_WC__		// { dg-warning "undefining '__STDC_MB_MIGHT_NEQ_WC__'" "" { target c++11 } }
+#undef __STDC_VERSION__			// { dg-warning "undefining '__STDC_VERSION__'" "" { target c++11 } }
+#undef __STDC_ISO_10646__		// { dg-warning "undefining '__STDC_ISO_10646__'" }
+#undef __STDCPP_THREADS__		// { dg-warning "undefining '__STDCPP_THREADS__'" "" { target c++11 } }
+#undef __STDCPP_STRICT_POINTER_SAFETY__	// { dg-warning "undefining '__STDCPP_STRICT_POINTER_SAFETY__'" "" { target { c++11 && c++20_down } } }
+#undef __cpp_aggregate_bases		// { dg-warning "undefining '__cpp_aggregate_bases'" "" { target c++20 } }
+#undef __cpp_aggregate_nsdmi		// { dg-warning "undefining '__cpp_aggregate_nsdmi'" "" { target c++20 } }
+#undef __cpp_aggregate_paren_init	// { dg-warning "undefining '__cpp_aggregate_paren_init'" "" { target c++20 } }
+#undef __cpp_alias_templates		// { dg-warning "undefining '__cpp_alias_templates'" "" { target c++20 } }
+#undef __cpp_aligned_new		// { dg-warning "undefining '__cpp_aligned_new'" "" { target c++20 } }
+#undef __cpp_attributes			// { dg-warning "undefining '__cpp_attributes'" "" { target c++20 } }
+#undef __cpp_auto_cast			// { dg-warning "undefining '__cpp_auto_cast'" "" { target c++23 } }
+#undef __cpp_binary_literals		// { dg-warning "undefining '__cpp_binary_literals'" "" { target c++20 } }
+#undef __cpp_capture_star_this		// { dg-warning "undefining '__cpp_capture_star_this'" "" { target c++20 } }
+#undef __cpp_char8_t			// { dg-warning "undefining '__cpp_char8_t'" "" { target c++20 } }
+#undef __cpp_concepts			// { dg-warning "undefining '__cpp_concepts'" "" { target c++20 } }
+#undef __cpp_conditional_explicit	// { dg-warning "undefining '__cpp_conditional_explicit'" "" { target c++20 } }
+#undef __cpp_constexpr			// { dg-warning "undefining '__cpp_constexpr'" "" { target c++20 } }
+#undef __cpp_constexpr_dynamic_alloc	// { dg-warning "undefining '__cpp_constexpr_dynamic_alloc'" "" { target c++20 } }
+#undef __cpp_constexpr_exceptions	// { dg-warning "undefining '__cpp_constexpr_exceptions'" "" { target c++26 } }
+#undef __cpp_constexpr_in_decltype	// { dg-warning "undefining '__cpp_constexpr_in_decltype'" "" { target c++20 } }
+#undef __cpp_constexpr_virtual_inheritance // { dg-warning "undefining '__cpp_constexpr_virtual_inheritance'" "" { target c++26 } }
+#undef __cpp_consteval			// { dg-warning "undefining '__cpp_consteval'" "" { target c++20 } }
+#undef __cpp_constinit			// { dg-warning "undefining '__cpp_constinit'" "" { target c++20 } }
+#undef __cpp_contracts			// { dg-warning "undefining '__cpp_contracts'" "" { target c++26 } }
+#undef __cpp_decltype			// { dg-warning "undefining '__cpp_decltype'" "" { target c++20 } }
+#undef __cpp_decltype_auto		// { dg-warning "undefining '__cpp_decltype_auto'" "" { target c++20 } }
+#undef __cpp_deduction_guides		// { dg-warning "undefining '__cpp_deduction_guides'" "" { target c++20 } }
+#undef __cpp_delegating_constructors	// { dg-warning "undefining '__cpp_delegating_constructors'" "" { target c++20 } }
+#undef __cpp_deleted_function		// { dg-warning "undefining '__cpp_deleted_function'" "" { target c++26 } }
+#undef __cpp_designated_initializers	// { dg-warning "undefining '__cpp_designated_initializers'" "" { target c++20 } }
+#undef __cpp_enumerator_attributes	// { dg-warning "undefining '__cpp_enumerator_attributes'" "" { target c++20 } }
+#undef __cpp_expansion_statements	// { dg-warning "undefining '__cpp_expansion_statements'" "" { target c++26 } }
+#undef __cpp_explicit_this_parameter	// { dg-warning "undefining '__cpp_explicit_this_parameter'" "" { target c++23 } }
+#undef __cpp_fold_expressions		// { dg-warning "undefining '__cpp_fold_expressions'" "" { target c++20 } }
+#undef __cpp_generic_lambdas		// { dg-warning "undefining '__cpp_generic_lambdas'" "" { target c++20 } }
+#undef __cpp_guaranteed_copy_elision	// { dg-warning "undefining '__cpp_guaranteed_copy_elision'" "" { target c++20 } }
+#undef __cpp_hex_float			// { dg-warning "undefining '__cpp_hex_float'" "" { target c++20 } }
+#undef __cpp_if_consteval		// { dg-warning "undefining '__cpp_if_consteval'" "" { target c++23 } }
+#undef __cpp_if_constexpr		// { dg-warning "undefining '__cpp_if_constexpr'" "" { target c++20 } }
+#undef __cpp_impl_coroutine		// { dg-warning "undefining '__cpp_impl_coroutine'" "" { target c++20 } }
+#undef __cpp_impl_destroying_delete	// { dg-warning "undefining '__cpp_impl_destroying_delete'" "" { target c++20 } }
+#undef __cpp_impl_three_way_comparison	// { dg-warning "undefining '__cpp_impl_three_way_comparison'" "" { target c++20 } }
+#undef __cpp_impl_reflection
+#undef __cpp_implicit_move		// { dg-warning "undefining '__cpp_implicit_move'" "" { target c++23 } }
+#undef __cpp_inheriting_constructors	// { dg-warning "undefining '__cpp_inheriting_constructors'" "" { target c++20 } }
+#undef __cpp_init_captures		// { dg-warning "undefining '__cpp_init_captures'" "" { target c++20 } }
+#undef __cpp_initializer_lists		// { dg-warning "undefining '__cpp_initializer_lists'" "" { target c++20 } }
+#undef __cpp_inline_variables		// { dg-warning "undefining '__cpp_inline_variables'" "" { target c++20 } }
+#undef __cpp_lambdas			// { dg-warning "undefining '__cpp_lambdas'" "" { target c++20 } }
+#undef __cpp_modules			// { dg-warning "undefining '__cpp_modules'" "" { target c++20 } }
+#undef __cpp_multidimensional_subscript	// { dg-warning "undefining '__cpp_multidimensional_subscript'" "" { target c++23 } }
+#undef __cpp_named_character_escapes	// { dg-warning "undefining '__cpp_named_character_escapes'" "" { target c++23 } }
+#undef __cpp_namespace_attributes	// { dg-warning "undefining '__cpp_namespace_attributes'" "" { target c++20 } }
+#undef __cpp_noexcept_function_type	// { dg-warning "undefining '__cpp_noexcept_function_type'" "" { target c++20 } }
+#undef __cpp_nontype_template_args	// { dg-warning "undefining '__cpp_nontype_template_args'" "" { target c++20 } }
+#undef __cpp_nontype_template_parameter_auto // { dg-warning "undefining '__cpp_nontype_template_parameter_auto'" "" { target c++20 } }
+#undef __cpp_nsdmi			// { dg-warning "undefining '__cpp_nsdmi'" "" { target c++20 } }
+#undef __cpp_pack_indexing		// { dg-warning "undefining '__cpp_pack_indexing'" "" { target c++26 } }
+#undef __cpp_placeholder_variables	// { dg-warning "undefining '__cpp_placeholder_variables'" "" { target c++26 } }
+#undef __cpp_pp_embed			// { dg-warning "undefining '__cpp_pp_embed'" "" { target c++26 } }
+#undef __cpp_range_based_for		// { dg-warning "undefining '__cpp_range_based_for'" "" { target c++20 } }
+#undef __cpp_raw_strings		// { dg-warning "undefining '__cpp_raw_strings'" "" { target c++20 } }
+#undef __cpp_ref_qualifiers		// { dg-warning "undefining '__cpp_ref_qualifiers'" "" { target c++20 } }
+#undef __cpp_return_type_deduction	// { dg-warning "undefining '__cpp_return_type_deduction'" "" { target c++20 } }
+#undef __cpp_rvalue_references		// { dg-warning "undefining '__cpp_rvalue_references'" "" { target c++20 } }
+#undef __cpp_size_t_suffix		// { dg-warning "undefining '__cpp_size_t_suffix'" "" { target c++23 } }
+#undef __cpp_sized_deallocation		// { dg-warning "undefining '__cpp_sized_deallocation'" "" { target c++20 } }
+#undef __cpp_static_assert		// { dg-warning "undefining '__cpp_static_assert'" "" { target c++20 } }
+#undef __cpp_static_call_operator	// { dg-warning "undefining '__cpp_static_call_operator'" "" { target c++23 } }
+#undef __cpp_structured_bindings	// { dg-warning "undefining '__cpp_structured_bindings'" "" { target c++20 } }
+#undef __cpp_template_parameters
+#undef __cpp_template_template_args	// { dg-warning "undefining '__cpp_template_template_args'" "" { target c++20 } }
+#undef __cpp_threadsafe_static_init	// { dg-warning "undefining '__cpp_threadsafe_static_init'" "" { target c++20 } }
+#undef __cpp_trivial_relocatability	// { dg-warning "undefining '__cpp_trivial_relocatability'" "" { target c++26 } }
+#undef __cpp_trivial_union
+#undef __cpp_unicode_characters		// { dg-warning "undefining '__cpp_unicode_characters'" "" { target c++20 } }
+#undef __cpp_unicode_literals		// { dg-warning "undefining '__cpp_unicode_literals'" "" { target c++20 } }
+#undef __cpp_user_defined_literals	// { dg-warning "undefining '__cpp_user_defined_literals'" "" { target c++20 } }
+#undef __cpp_using_enum			// { dg-warning "undefining '__cpp_using_enum'" "" { target c++20 } }
+#undef __cpp_variable_templates		// { dg-warning "undefining '__cpp_variable_templates'" "" { target c++20 } }
+#undef __cpp_variadic_friend		// { dg-warning "undefining '__cpp_variadic_friend'" "" { target c++26 } }
+#undef __cpp_variadic_templates		// { dg-warning "undefining '__cpp_variadic_templates'" "" { target c++20 } }
+#undef __cpp_variadic_using		// { dg-warning "undefining '__cpp_variadic_using'" "" { target c++20 } }
diff --git a/gcc/testsuite/g++.dg/DRs/dr2581-2.C b/gcc/testsuite/g++.dg/DRs/dr2581-2.C
new file mode 100644
index 0000000..1ecbe1c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2581-2.C
@@ -0,0 +1,106 @@
+// DR 2581 - Undefined behavior for predefined macros
+// { dg-do preprocess }
+// { dg-additional-options "-fcontracts" { target c++26 } }
+// { dg-additional-options "-fmodules -fcoroutines" { target c++20 } }
+
+#define defined defined				// { dg-error "'defined' cannot be used as a macro name" }
+#define __cplusplus 202400L			// { dg-error "'__cplusplus' redefined" }
+#define __DATE__				// { dg-error "'__DATE__' redefined" }
+#define __FILE__				// { dg-error "'__FILE__' redefined" }
+#define __LINE__				// { dg-error "'__LINE__' redefined" }
+#define __STDC_EMBED_NOT_FOUND__ 0		// { dg-error "'__STDC_EMBED_NOT_FOUND__' redefined" }
+#define __STDC_EMBED_FOUND__ 1			// { dg-error "'__STDC_EMBED_FOUND__' redefined" }
+#define __STDC_EMBED_EMPTY__ 2			// { dg-error "'__STDC_EMBED_EMPTY__' redefined" }
+#define __STDC_HOSTED__	1			// { dg-error "'__STDC_HOSTED__' redefined" }
+#define __STDCPP_DEFAULT_NEW_ALIGNMENT__ 1	// { dg-error "'__STDCPP_DEFAULT_NEW_ALIGNMENT__' redefined" "" { target c++17 } }
+#define __STDCPP_FLOAT16_T__ 1			// { dg-message "'__STDCPP_FLOAT16_T__' (?:re)?defined" "" { target c++23 } } 
+#define __STDCPP_FLOAT32_T__ 1			// { dg-message "'__STDCPP_FLOAT32_T__' (?:re)?defined" "" { target c++23 } } 
+#define __STDCPP_FLOAT64_T__ 1			// { dg-message "'__STDCPP_FLOAT64_T__' (?:re)?defined" "" { target c++23 } } 
+#define __STDCPP_FLOAT128_T__ 1			// { dg-message "'__STDCPP_FLOAT128_T__' (?:re)?defined" "" { target c++23 } } 
+#define __STDCPP_BFLOAT16_T__ 1			// { dg-message "'__STDCPP_BFLOAT16_T__' (?:re)?defined" "" { target c++23 } } 
+#define __TIME__				// { dg-error "'__TIME__' redefined" }
+#define __STDC__				// { dg-error "'__STDC__' redefined" }
+#define __STDC_MB_MIGHT_NEQ_WC__		// { dg-warning "'__STDC_MB_MIGHT_NEQ_WC__' defined" "" { target c++11 } }
+#define __STDC_VERSION__			// { dg-warning "'__STDC_VERSION__' defined" "" { target c++11 } }
+#define __STDC_ISO_10646__			// { dg-error "'__STDC_ISO_10646__' redefined" }
+#define __STDCPP_THREADS__			// { dg-message "'__STDCPP_THREADS__' (?:re)?defined" "" { target c++11 } }
+#define __STDCPP_STRICT_POINTER_SAFETY__	// { dg-warning "'__STDCPP_STRICT_POINTER_SAFETY__' defined" "" { target { c++11 && c++20_down } } }
+#define __cpp_aggregate_bases 201603L		// { dg-error "'__cpp_aggregate_bases' redefined" "" { target c++20 } }
+#define __cpp_aggregate_nsdmi 201304L		// { dg-error "'__cpp_aggregate_nsdmi' redefined" "" { target c++20 } }
+#define __cpp_aggregate_paren_init 201902L	// { dg-error "'__cpp_aggregate_paren_init' redefined" "" { target c++20 } }
+#define __cpp_alias_templates 200704L		// { dg-error "'__cpp_alias_templates' redefined" "" { target c++20 } }
+#define __cpp_aligned_new 201606L		// { dg-error "'__cpp_aligned_new' redefined" "" { target c++20 } }
+#define __cpp_attributes 200809L		// { dg-error "'__cpp_attributes' redefined" "" { target c++20 } }
+#define __cpp_auto_cast 202110L			// { dg-error "'__cpp_auto_cast' redefined" "" { target c++23 } }
+#define __cpp_binary_literals 201304L		// { dg-error "'__cpp_binary_literals' redefined" "" { target c++20 } }
+#define __cpp_capture_star_this 201603L		// { dg-error "'__cpp_capture_star_this' redefined" "" { target c++20 } }
+#define __cpp_char8_t 202207L			// { dg-error "'__cpp_char8_t' redefined" "" { target c++20 } }
+#define __cpp_concepts 202002L			// { dg-error "'__cpp_concepts' redefined" "" { target c++20 } }
+#define __cpp_conditional_explicit 201806L	// { dg-error "'__cpp_conditional_explicit' redefined" "" { target c++20 } }
+#define __cpp_constexpr 202406L			// { dg-error "'__cpp_constexpr' redefined" "" { target c++11 } }
+#define __cpp_constexpr_dynamic_alloc 201907L	// { dg-error "'__cpp_constexpr_dynamic_alloc' redefined" "" { target c++20 } }
+#define __cpp_constexpr_exceptions 202411L	// { dg-error "'__cpp_constexpr_exceptions' redefined" "" { target c++26 } }
+#define __cpp_constexpr_in_decltype 201711L	// { dg-error "'__cpp_constexpr_in_decltype' redefined" "" { target c++20 } }
+#define __cpp_constexpr_virtual_inheritance 202506L // { dg-error "'__cpp_constexpr_virtual_inheritance' redefined" "" { target c++26 } }
+#define __cpp_consteval 202211L			// { dg-error "'__cpp_consteval' redefined" "" { target c++20 } }
+#define __cpp_constinit 201907L			// { dg-error "'__cpp_constinit' redefined" "" { target c++20 } }
+#define __cpp_contracts 202502L			// { dg-error "'__cpp_contracts' redefined" "" { target c++26 } }
+#define __cpp_decltype 200707L			// { dg-error "'__cpp_decltype' redefined" "" { target c++20 } }
+#define __cpp_decltype_auto 201304L		// { dg-error "'__cpp_decltype_auto' redefined" "" { target c++20 } }
+#define __cpp_deduction_guides 201907L		// { dg-error "'__cpp_deduction_guides' redefined" "" { target c++17 } }
+#define __cpp_delegating_constructors 200604L	// { dg-error "'__cpp_delegating_constructors' redefined" "" { target c++20 } }
+#define __cpp_deleted_function 202403L		// { dg-error "'__cpp_deleted_function' redefined" "" { target c++26 } }
+#define __cpp_designated_initializers 201707L	// { dg-error "'__cpp_designated_initializers' redefined" "" { target c++20 } }
+#define __cpp_enumerator_attributes 201411L	// { dg-error "'__cpp_enumerator_attributes' redefined" "" { target c++20 } }
+#define __cpp_expansion_statements 202506L	// { dg-error "'__cpp_expansion_statements' redefined" "" { target c++26 } }
+#define __cpp_explicit_this_parameter 202110L	// { dg-error "'__cpp_explicit_this_parameter' redefined" "" { target c++23 } }
+#define __cpp_fold_expressions 201603L		// { dg-error "'__cpp_fold_expressions' redefined" "" { target c++20 } }
+#define __cpp_generic_lambdas 201707L		// { dg-error "'__cpp_generic_lambdas' redefined" "" { target c++14 } }
+#define __cpp_guaranteed_copy_elision 201606L	// { dg-error "'__cpp_guaranteed_copy_elision' redefined" "" { target c++20 } }
+#define __cpp_hex_float 201603L			// { dg-error "'__cpp_hex_float' redefined" "" { target c++20 } }
+#define __cpp_if_consteval 202106L		// { dg-error "'__cpp_if_consteval' redefined" "" { target c++23 } }
+#define __cpp_if_constexpr 201606L		// { dg-error "'__cpp_if_constexpr' redefined" "" { target c++20 } }
+#define __cpp_impl_coroutine 201902L		// { dg-error "'__cpp_impl_coroutine' redefined" "" { target c++20 } }
+#define __cpp_impl_destroying_delete 201806L	// { dg-error "'__cpp_impl_destroying_delete' redefined" "" { target c++20 } }
+#define __cpp_impl_three_way_comparison 201907L	// { dg-error "'__cpp_impl_three_way_comparison' redefined" "" { target c++20 } }
+#define __cpp_impl_reflection 202506L
+#define __cpp_implicit_move 202207L		// { dg-error "'__cpp_implicit_move' redefined" "" { target c++23 } }
+#define __cpp_inheriting_constructors 201511L	// { dg-error "'__cpp_inheriting_constructors' redefined" "" { target c++20 } }
+#define __cpp_init_captures 201803L		// { dg-error "'__cpp_init_captures' redefined" "" { target c++14 } }
+#define __cpp_initializer_lists 200806L		// { dg-error "'__cpp_initializer_lists' redefined" "" { target c++20 } }
+#define __cpp_inline_variables 201606L		// { dg-error "'__cpp_inline_variables' redefined" "" { target c++20 } }
+#define __cpp_lambdas 200907L			// { dg-error "'__cpp_lambdas' redefined" "" { target c++20 } }
+#define __cpp_modules 201907L			// { dg-error "'__cpp_modules' redefined" "" { target c++20 } }
+#define __cpp_multidimensional_subscript 202211L // { dg-error "'__cpp_multidimensional_subscript' redefined" "" { target c++23 } }
+#define __cpp_named_character_escapes 202207L	// { dg-error "'__cpp_named_character_escapes' redefined" "" { target c++23 } }
+#define __cpp_namespace_attributes 201411L	// { dg-error "'__cpp_namespace_attributes' redefined" "" { target c++20 } }
+#define __cpp_noexcept_function_type 201510L	// { dg-error "'__cpp_noexcept_function_type' redefined" "" { target c++20 } }
+#define __cpp_nontype_template_args 201911L	// { dg-error "'__cpp_nontype_template_args' redefined" "" { target c++17 } }
+#define __cpp_nontype_template_parameter_auto 201606L // { dg-error "'__cpp_nontype_template_parameter_auto' redefined" "" { target c++20 } }
+#define __cpp_nsdmi 200809L			// { dg-error "'__cpp_nsdmi' redefined" "" { target c++20 } }
+#define __cpp_pack_indexing 202311L		// { dg-error "'__cpp_pack_indexing' redefined" "" { target c++26 } }
+#define __cpp_placeholder_variables 202306L	// { dg-error "'__cpp_placeholder_variables' redefined" "" { target c++26 } }
+#define __cpp_pp_embed 202502L			// { dg-error "'__cpp_pp_embed' redefined" "" { target c++26 } }
+#define __cpp_range_based_for 202211L		// { dg-error "'__cpp_range_based_for' redefined" "" { target c++11 } }
+#define __cpp_raw_strings 200710L		// { dg-error "'__cpp_raw_strings' redefined" "" { target c++20 } }
+#define __cpp_ref_qualifiers 200710L		// { dg-error "'__cpp_ref_qualifiers' redefined" "" { target c++20 } }
+#define __cpp_return_type_deduction 201304L	// { dg-error "'__cpp_return_type_deduction' redefined" "" { target c++20 } }
+#define __cpp_rvalue_references 200610L		// { dg-error "'__cpp_rvalue_references' redefined" "" { target c++20 } }
+#define __cpp_size_t_suffix 202011L		// { dg-error "'__cpp_size_t_suffix' redefined" "" { target c++23 } }
+#define __cpp_sized_deallocation 201309L	// { dg-error "'__cpp_sized_deallocation' redefined" "" { target c++20 } }
+#define __cpp_static_assert 202306L		// { dg-error "'__cpp_static_assert' redefined" "" { target c++11 } }
+#define __cpp_static_call_operator 202207L	// { dg-error "'__cpp_static_call_operator' redefined" "" { target c++23 } }
+#define __cpp_structured_bindings 202411L	// { dg-error "'__cpp_structured_bindings' redefined" "" { target c++17 } }
+#define __cpp_template_parameters 202502L
+#define __cpp_template_template_args 201611L	// { dg-error "'__cpp_template_template_args' redefined" "" { target c++20 } }
+#define __cpp_threadsafe_static_init 200806L	// { dg-error "'__cpp_threadsafe_static_init' redefined" "" { target c++20 } }
+#define __cpp_trivial_relocatability 202502L	// { dg-error "'__cpp_trivial_relocatability' redefined" "" { target c++26 } }
+#define __cpp_trivial_union 202502L
+#define __cpp_unicode_characters 200704L	// { dg-error "'__cpp_unicode_characters' redefined" "" { target c++17 } }
+#define __cpp_unicode_literals 200710L		// { dg-error "'__cpp_unicode_literals' redefined" "" { target c++20 } }
+#define __cpp_user_defined_literals 200809L	// { dg-error "'__cpp_user_defined_literals' redefined" "" { target c++20 } }
+#define __cpp_using_enum 201907L		// { dg-error "'__cpp_using_enum' redefined" "" { target c++20 } }
+#define __cpp_variable_templates 201304L	// { dg-error "'__cpp_variable_templates' redefined" "" { target c++20 } }
+#define __cpp_variadic_friend 202403L		// { dg-error "'__cpp_variadic_friend' redefined" "" { target c++26 } }
+#define __cpp_variadic_templates 200704L	// { dg-error "'__cpp_variadic_templates' redefined" "" { target c++20 } }
+#define __cpp_variadic_using 201611L		// { dg-error "'__cpp_variadic_using' redefined" "" { target c++20 } }
diff --git a/libcpp/directives.cc b/libcpp/directives.cc
index 47fb8fb..b6dd36a 100644
--- a/libcpp/directives.cc
+++ b/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,
diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index bbd88e5..5190ff7 100644
--- a/libcpp/include/cpplib.h
+++ b/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 *);
 
diff --git a/libcpp/init.cc b/libcpp/init.cc
index e9732af..eb495e2 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");
diff --git a/libcpp/macro.cc b/libcpp/macro.cc
index a47e1fe..9867098 100644
--- a/libcpp/macro.cc
+++ b/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;