https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121114
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <[email protected]>: https://gcc.gnu.org/g:62c126db6b6017011dcbe6945aab371ab48f8ded commit r16-6038-g62c126db6b6017011dcbe6945aab371ab48f8ded Author: Jakub Jelinek <[email protected]> Date: Thu Dec 11 19:37:22 2025 +0100 libstdc++: Implement C++26 P3378R2 - constexpr exception types The following patch attempts to implement the C++26 P3378R2 - constexpr exception types paper. This is quite complicated, because most of these classes which should be constexpr-ized use solely or mostly out of line definitions in libstdc++, both for historical, code size and dual ABI reasons, so that one can throw these as exceptions between TUs with old vs. new (or vice versa) ABIs. For this reason, logic_error/runtime_error and classes derived from it have the old ABI std::string object inside of them and the exported APIs from libstdc++.so.6 ensure the right thing. Now, because new invoked during constant evaluation needs to be deleted during the same constant evaluation and can't leak into the constant expressions, I think we don't have to use COW strings under the hood (which aren't constexpr I guess because of reference counting/COW) and we can use something else, the patch uses heap allocated std::string object (where __cow_constexpr_string class has just a pointer to that). As I think we still want to hide the ugly details if !consteval in the library, the patch exports 8 __cow_string class symbols (6 existing which were previously just not exported and 2 new ones) and if !consteval calls those through extern "C" _Zmangled_name symbols. The functions are always_inline. And then logic_error etc. have for C++26 (precisely for __cpp_lib_constexpr_exceptions >= 202502L) constexpr definitions of cdtors/methods. This results in slightly larger code (a few insns at most) at runtime for C++26, e.g. instead of calling say some logic error cdtor/method with 2 arguments it calls some __cow_string one with 2 arguments but + 8 bytes pointer additions on both. The patch also removes the __throw_format_error forward declaration which apparently wasn't needed for anything as all __throw_format_error users were either in <format> or included <format> before the uses, reverts the https://gcc.gnu.org/pipermail/libstdc++/2025-July/062598.html patch and makes sure __throw_* functions (only those for exception types which the P3378R2 or P3068R5 papers made constexpr usable and there are actually constexpr/consteval uses of those) are constexpr for C++26 constexpr exceptions. The patch does that by splitting the bits/functexcept.h header: 1) bits/functexcept.h stays for the __throw_* functions which are (at least for now) never constexpr (the <ios>, <system_error>, <future> and <functional> std::exception derived classes) or are never used or never used in constexpr/consteval contexts (<exception>, <typeinfo> std::exception derived classes and std::range_error). 2) bits/new_{throw,except}.h for __throw_bad_alloc/__throw_bad_array_new_length and std::bad_alloc/std::bad_array_new_length (where <new> includes <bits/new_except.h> and <bits/new_throw.h> as well for the C++26 constexpr exceptions case) 3) for the most complicated <stdexcept> stuff, one header addition to bits/stdexcept.h one header for the __throw_logic_error etc. forward declarations, one header for the __throw_logic_error etc. definitions and one header without header guards which will depending on __glibcxx_exc_in_string include one or the other because <string> vs. <string_view> vs. <stdexcept> have heavy interdependencies 2025-12-11 Jakub Jelinek <[email protected]> PR libstdc++/121114 libstdc++-v3/ * include/bits/version.def: Implement C++26 P3378R2 - constexpr exception types. (constexpr_exceptions): Change value from 1 to 202502, remove no_stdname and TODO comments. * include/bits/version.h: Regenerate. * src/c++11/cow-stdexcept.cc (__cow_string(const char*)): New ctor. (__cow_string::c_str()): New method. * config/abi/pre/gnu.ver (GLIBCXX_3.4.35): Export 8 __cow_string symbols. * include/bits/new_except.h: New file. * include/bits/new_throw.h: New file. * include/bits/stdexcept_throw.h: New file. * include/bits/stdexcept_throwdef.h: New file. * include/bits/stdexcept_throwfwd.h: New file. * include/std/stdexcept: Include bits/stdexcept_except.h and move everything after <string> include except for std::range_error into include/bits/stdexcept_except.h. (std::range_error): If __cpp_lib_constexpr_exceptions >= 202502L make all cdtors and methods constexpr. * include/bits/stdexcept_except.h: New file. * include/std/optional (__glibcxx_want_constexpr_exceptions): Define before including bits/version.h. (bad_optional_access::what): Make constexpr for __cpp_lib_constexpr_exceptions >= 202502L. (__throw_bad_optional_access): Likewise. * include/std/expected (__glibcxx_want_constexpr_exceptions): Define before including bits/version.h. (bad_expected_access): Make cdtors and all methods constexpr for __cpp_lib_constexpr_exceptions >= 202502L. * include/std/format (__glibcxx_want_constexpr_exceptions): Define before including bits/version.h. (_GLIBCXX_CONSTEXPR_FORMAT_ERROR): Define and undef later. (format_error): Use _GLIBCXX_CONSTEXPR_FORMAT_ERROR on ctors. * include/std/variant (__glibcxx_want_constexpr_exceptions): Define before including bits/version.h. (_GLIBCXX_CONSTEXPR_BAD_VARIANT_ACCESS): Define and undef later. (bad_variant_access): Use it on ctors and what() method. (__throw_bad_variant_access): Use it here too. * testsuite/18_support/exception/version.cc: Adjust expected __cpp_lib_constexpr_exceptions value. * testsuite/19_diagnostics/runtime_error/constexpr.cc: New test. * testsuite/19_diagnostics/headers/stdexcept/version.cc: New test. * testsuite/19_diagnostics/logic_error/constexpr.cc: New test. * testsuite/20_util/expected/observers.cc (test_value_throw): Change return type to bool from void, return true at the end, add test to dereference what() first character. Make it constexpr for __cpp_lib_constexpr_exceptions >= 202502L and add static_assert. * testsuite/20_util/expected/version.cc: Add tests for __cpp_lib_constexpr_exceptions value. * testsuite/20_util/variant/constexpr.cc: For __cpp_lib_constexpr_exceptions >= 202502L include <string>. (test_get): New function if __cpp_lib_constexpr_exceptions >= 202502L, assert calling it is true. * testsuite/20_util/variant/version.cc: Add tests for __cpp_lib_constexpr_exceptions value. * testsuite/20_util/optional/constexpr/observers/3.cc: Include testsuite_hooks.h. (eat, test01): New functions. Assert test01() is true. * testsuite/20_util/optional/version.cc: Add tests for __cpp_lib_constexpr_exceptions value. * include/std/future: Add #include <bits/functexcept.h>. * include/std/shared_mutex: Include <bits/new_throw.h>. * include/std/flat_map: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/std/syncstream: Remove <bits/functexcept.h> include. * include/std/flat_set: Likewise. * include/std/bitset: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/std/string_view: Don't include <bits/functexcept.h>, include <bits/stdexcept_throw.h> early if __glibcxx_exc_in_string is not defined and include <bits/stdexcept_throw.h> at the end of the header again if __glibcxx_exc_in_string is 2 and C++26 constexpr exceptions are enabled. (__glibcxx_exc_in_string): Define if __glibcxx_exc_in_string wasn't defined before including <bits/stdexcept_throw.h>. * include/std/array: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/std/inplace_vector: Likewise. * include/std/string: Include <bits/stdexcept_except.h> and <bits/stdexcept_throw.h> after bits/basic_string.tcc include if C++26 constexpr exceptions are enabled and include <bits/stdexcept_throw.h> instead of <bits/functexcept.h> early. (__glibcxx_exc_in_string): Define early to 1, undefine at the end. * include/std/deque: Include <bits/stdexcept_throw.h>. * include/bits/new_allocator.h: Include <bits/new_throw.h> instead of <bits/functexcept.h>. * include/bits/stl_algobase.h: Remove <bits/functexcept.h> include. * include/bits/stl_vector.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/bits/memory_resource.h: Include <bits/new_throw.h> instead of <bits/functexcept.h>. * include/bits/functexcept.h: Guard everything after includes with #if _GLIBCXX_HOSTED. (__throw_bad_alloc, __throw_bad_array_new_length, __throw_logic_error, __throw_domain_error, __throw_invalid_argument, __throw_length_error, __throw_out_of_range, __throw_out_of_range_fmt, __throw_runtime_error, __throw_overflow_error, __throw_underflow_error): Move declarations to other headers - <bits/new_throw.h> and <bits/stdexcept_throwfwd.h>. * include/bits/stl_map.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/bits/hashtable_policy.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/bits/formatfwd.h (std::__throw_format_error): Remove declaration. * include/bits/specfun.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/bits/basic_ios.h: Include <bits/functexcept.h>. * include/bits/locale_classes.h: Likewise. * include/tr1/cmath: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/tr1/memory: Remove <bits/functexcept.h> include. * include/tr1/array: Include <bits/stdexcept_throw.h>. * include/ext/vstring_util.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/ext/bitmap_allocator.h: Include <bits/new_throw.h> instead of <bits/functexcept.h>. * include/ext/mt_allocator.h: Likewise. * include/ext/malloc_allocator.h: Likewise. * include/ext/debug_allocator.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/ext/concurrence.h: Include <bits/exception_defines.h> instead of <bits/functexcept.h>. * include/ext/throw_allocator.h: Include <bits/new_throw.h> and <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/ext/string_conversions.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/ext/pool_allocator.h: Include <bits/new_throw.h> instead of <bits/functexcept.h>. * include/ext/ropeimpl.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * include/tr2/dynamic_bitset: Likewise. * include/experimental/optional: Include <bits/exception_defines.h> instead of <bits/functexcept.h>. * include/Makefile.am (bits_freestanding): Add ${bits_srcdir}/{new,stdexcept}_{except,throw}.h and ${bits_srcdir}/stdexcept_throw{fwd,def}.h. * include/Makefile.in: Regenerate. * src/c++17/floating_from_chars.cc: Remove <bits/functexcept.h> include. * src/c++11/regex.cc: Likewise. * src/c++11/functexcept.cc: Likewise. * src/c++11/snprintf_lite.cc: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * src/c++11/thread.cc: Include <bits/functexcept.h>. * testsuite/util/testsuite_hooks.h: Include <bits/stdexcept_throw.h> instead of <bits/functexcept.h>. * testsuite/util/io/verified_cmd_line_input.cc: Include <bits/exception_defines.h> instead of <bits/functexcept.h>. * testsuite/20_util/allocator/105975.cc: Expect different diagnostics for C++26. * testsuite/23_containers/inplace_vector/access/capacity.cc: Remove #error, guard if consteval { return; } with #ifndef __cpp_lib_constexpr_exceptions. * testsuite/23_containers/inplace_vector/access/elem.cc: Likewise. * testsuite/23_containers/inplace_vector/cons/1.cc: Likewise. * testsuite/23_containers/inplace_vector/cons/from_range.cc: Likewise. * testsuite/23_containers/inplace_vector/modifiers/single_insert.cc: Likewise. * testsuite/23_containers/inplace_vector/modifiers/assign.cc: Likewise. * testsuite/23_containers/inplace_vector/modifiers/multi_insert.cc: Likewise. * libsupc++/new: Include <bits/new_except.h>. (std::bad_alloc, std::bad_array_new_length): Move defintion to <bits/new_except.h>. libgomp/ * omp.h.in: Include <bits/new_throw.h> instead of <bits/functexcept.h>. gcc/testsuite/ * g++.dg/tree-ssa/pr110819.C: Guard scan-tree-dump-not delete on c++23_down and add comment explaining why C++26 fails that. * g++.dg/tree-ssa/pr96945.C: Likewise. * g++.dg/tree-ssa/pr109442.C: Likewise. * g++.dg/tree-ssa/pr116868.C: Likewise. * g++.dg/tree-ssa/pr58483.C: Likewise.
