On Wed, Aug 13, 2025 at 10:37:35AM -0400, Jason Merrill wrote:
It looks to me like we're supposed to complain whether or not they're
defined.
Ok, here is a new version.
For __cpp_* macros which aren't defined solely based on cxx_dialect >= cxxXX
but instead being guarded on various switches as well as for __STDC_* and
__STDCPP_* macros mentioned in the standards it will mark them as NODE_WARN
even if they aren't defined and tweaks libcpp so that it warns also when
trying to #undef NODE_WARN non-keyword macro if not defined as macro or
#define NODE_WARN non-keyword macro when previously not defined.
The only exception is __STDC_ISO_10646__ because that is normally defined
in glibc <stdc-predef.h> (and is marked NODE_WARN at that point), but no
diagnostics if -nostdinc or on targets without stdc-predef.h.
So far lightly tested, ok for trunk if it passes full bootstrap/regtest?
2025-08-13 Jakub Jelinek <ja...@redhat.com>
PR preprocessor/120778
gcc/c-family/
* c-cppbuiltin.cc (c_cpp_builtins): Implement C++26 DR 2581. Add
cpp_define_warn and cpp_warn lambdas and use them. In the
if (c_dialect_cxx ()) block with __cpp_* predefinitions add
cpp_define lambda. Formatting fixes.
gcc/
* config/i386/i386-c.cc (ix86_target_macros): Temporarily remove
NODE_WARN flag from __STDCPP_FLOAT16_T__ and __STDCPP_BFLOAT16_T__
nodes around calling cpp_undefine on those.
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/
* init.cc (cpp_init_builtins): Mark __cplusplus, __STDC__,
__STDC_VERSION__, __STDC_MB_MIGHT_NEQ_WC__ and
__STDCPP_STRICT_POINTER_SAFETY__ nodes as NODE_WARN when appropriate.
* directives.cc (do_undef): Warn on undefining NODE_WARN macros if
not cpp_keyword_p.
* macro.cc (_cpp_create_definition): Warn on defining NODE_WARN
macros if they weren't previously defined and not cpp_keyword_p.
--- gcc/c-family/c-cppbuiltin.cc.jj 2025-08-13 17:32:37.963278509 +0200
+++ gcc/c-family/c-cppbuiltin.cc 2025-08-13 18:22:43.032506685 +0200
@@ -913,23 +913,49 @@ 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_lookup (pfile, (const unsigned char *) def,
+ end ? end - def : strlen (def))->flags |= NODE_WARN;
+ };
+ /* Just mark DEF for warning even when not defined. */
+ auto cpp_warn = [] (cpp_reader *pfile, const char *def)
+ {
+ cpp_lookup (pfile, (const unsigned char *) def,
+ strlen (def))->flags |= NODE_WARN;
+ };
+
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
@@ -1100,42 +1126,65 @@ 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 >= cxx14)
+ cpp_warn (pfile, "__STDCPP_THREADS__");
if (flag_implicit_constexpr)
cpp_define (pfile, "__cpp_implicit_constexpr=20211111L");
}
@@ -1292,7 +1341,13 @@ 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,
@@ -1305,10 +1360,12 @@ 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 ())
--- gcc/config/i386/i386-c.cc.jj 2025-05-20 08:14:04.979425818 +0200
+++ gcc/config/i386/i386-c.cc 2025-08-13 18:52:57.340677017 +0200
@@ -901,8 +901,24 @@ ix86_target_macros (void)
{
if (c_dialect_cxx () && cxx_dialect > cxx20)
{
+ cpp_lookup (parse_in,
+ (const unsigned char *) "__STDCPP_FLOAT16_T__",
+ sizeof ("__STDCPP_FLOAT16_T__")
+ - 1)->flags &= ~NODE_WARN;
cpp_undef (parse_in, "__STDCPP_FLOAT16_T__");
+ cpp_lookup (parse_in,
+ (const unsigned char *) "__STDCPP_FLOAT16_T__",
+ sizeof ("__STDCPP_FLOAT16_T__")
+ - 1)->flags |= NODE_WARN;
+ cpp_lookup (parse_in,
+ (const unsigned char *) "__STDCPP_BFLOAT16_T__",
+ sizeof ("__STDCPP_BFLOAT16_T__")
+ - 1)->flags &= ~NODE_WARN;
cpp_undef (parse_in, "__STDCPP_BFLOAT16_T__");
+ cpp_lookup (parse_in,
+ (const unsigned char *) "__STDCPP_BFLOAT16_T__",
+ sizeof ("__STDCPP_BFLOAT16_T__")
+ - 1)->flags |= NODE_WARN;
}
}
--- gcc/testsuite/g++.dg/DRs/dr2581-1.C.jj 2025-08-13 17:34:05.259190318 +0200
+++ gcc/testsuite/g++.dg/DRs/dr2581-1.C 2025-08-13 18:33:52.942064983 +0200
@@ -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 } }
--- gcc/testsuite/g++.dg/DRs/dr2581-2.C.jj 2025-08-13 17:34:05.259190318
+0200
+++ gcc/testsuite/g++.dg/DRs/dr2581-2.C 2025-08-13 18:35:33.141805226 +0200
@@ -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-error "'__STDCPP_FLOAT16_T__' redefined"
"" { target { c++23 && float16 } } }
+#define __STDCPP_FLOAT32_T__ 1 // { dg-error "'__STDCPP_FLOAT32_T__' redefined"
"" { target { c++23 && float32 } } }
+#define __STDCPP_FLOAT64_T__ 1 // { dg-error "'__STDCPP_FLOAT64_T__' redefined"
"" { target { c++23 && float64 } } }
+#define __STDCPP_FLOAT128_T__ 1 // { dg-error "'__STDCPP_FLOAT128_T__'
redefined" "" { target { c++23 && float128 } } }
+#define __STDCPP_BFLOAT16_T__ 1 // { dg-error "'__STDCPP_BFLOAT16_T__'
redefined" "" { target { c++23 && bfloat16 } } }
+#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-error "'__STDCPP_THREADS__'
redefined" "" { 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 } }
--- gcc/testsuite/c-c++-common/cpp/pr92296-2.c.jj 2020-01-12
11:54:37.002404522 +0100
+++ gcc/testsuite/c-c++-common/cpp/pr92296-2.c 2025-08-13 18:32:38.596000455
+0200
@@ -39,42 +39,42 @@ filebase2 = __BASE_FILE__ /* { dg-final
#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" } } */
--- libcpp/init.cc.jj 2025-08-01 15:55:51.128693211 +0200
+++ libcpp/init.cc 2025-08-13 18:36:36.634006963 +0200
@@ -593,6 +593,9 @@ cpp_init_builtins (cpp_reader *pfile, in
&& (! 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_lookup (pfile, (const unsigned char *) "__STDC__",
+ sizeof ("__STDC__") - 1)->flags |= NODE_WARN;
