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 <jakub@redhat.com>
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.
The structure of these functions likely dates from the time before G++
fully supported C++14 extended constexpr, so that the throw expression
had to be the operand of a conditional expression. That is not true now,
so we can use a more straightforward version of the code.
We can also simplify the declaration of __throw_bad_optional_access by
using the C++11-style [[noreturn]] attribute so that a separate
declaration isn't needed.
libstdc++-v3/ChangeLog:
* include/experimental/optional (__throw_bad_optional_access):
Replace GNU attribute with C++11 attribute.
(optional::value, optional::value_or): Use if statements
instead of conditional expressions.
* include/std/optional (__throw_bad_optional_access)
(optional::value, optional::value_or): Likewise.
The <algorithm> header includes <utility>, with a comment referring to
UK-300, a National Body comment on the C++11 draft. That comment
proposed to move std::swap to <utility> and then require <algorithm> to
include <utility>. The comment was rejected, so we do not need to
implement the suggestion. For backwards compatibility with C++03 we do
want <algorithm> to define std::swap, but it does so anyway via
<bits/move.h>. We don't need the whole of <utility> to do that.
A few other headers that need std::swap can include <bits/move.h> to
get it, instead of <utility>.
There are several headers that include <utility> to get std::pair, but
they can use <bits/stl_pair.h> to get it without also including the
rel_ops namespace and other contents of <utility>.
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
* include/std/algorithm: Do not include <utility>.
* include/std/functional: Likewise.
* include/std/regex: Include <bits/stl_pair.h> instead of
<utility>.
* include/debug/map.h: Likewise.
* include/debug/multimap.h: Likewise.
* include/debug/multiset.h: Likewise.
* include/debug/set.h: Likewise.
* include/debug/vector: Likewise.
* include/bits/fs_path.h: Likewise.
* include/bits/unique_ptr.h: Do not include <utility>.
* include/experimental/any: Likewise.
* include/experimental/executor: Likewise.
* include/experimental/memory: Likewise.
* include/experimental/optional: Likewise.
* include/experimental/socket: Use __exchange instead
of std::exchange.
* src/filesystem/ops-common.h: Likewise.
* testsuite/20_util/default_delete/48631_neg.cc: Adjust expected
errors to not use a hardcoded line number.
* testsuite/20_util/default_delete/void_neg.cc: Likewise.
* testsuite/20_util/specialized_algorithms/uninitialized_copy/constrained.cc:
Include <utility> for std::as_const.
* testsuite/20_util/specialized_algorithms/uninitialized_default_construct/constrained.cc:
Likewise.
* testsuite/20_util/specialized_algorithms/uninitialized_move/constrained.cc:
Likewise.
* testsuite/20_util/specialized_algorithms/uninitialized_value_construct/constrained.cc:
Likewise.
* testsuite/23_containers/vector/cons/destructible_debug_neg.cc:
Adjust dg-error line number.
The std::__addressof function is always constexpr, even in C++14, so we
can just use that.
* include/experimental/bits/lfts_config.h: Improve doc markup.
* include/experimental/optional: Improve docs.
(_Has_addressof_mem, _Has_addressof_free, _Has_addressof)
(__constexpr_addressof): Remove.
(optional::operator->()): Use std::__addressof().
* include/std/optional (optional::operator->()): Adjust whitespace.
* testsuite/experimental/optional/constexpr/observers/2.cc: Check
that operator-> is still constexpr with overloaded operator&. Change
to compile-only test.
* testsuite/experimental/optional/constexpr/observers/3.cc: Change to
compile-only test.
From-SVN: r270826
Constrain optional's __constexpr_addressof in its return type
and use a constexpr addressof for optional, if available.
* include/experimental/optional (__constexpr_addressof):
Constrain in the return type instead of in a template parameter.
(_Has_addressof_mem)
(_Has_addressof_free, _Has_addressof, __constexpr_addressof):
Guard with #ifndef __cpp_lib_addressof_constexpr.
(operator->()): Use std::__addressof if it's constexpr.
From-SVN: r243298
