[PATCH] c++, libstdc++, v5: Implement C++26 P3068R5 - constexpr exceptions [PR117785]
Jason Merrill
jason@redhat.com
Tue Jul 15 12:30:33 GMT 2025
On 7/15/25 7:49 AM, Jonathan Wakely wrote:
> On Thu, 10 Jul 2025 at 09:06, Jakub Jelinek wrote:
>>
>> On Wed, Jul 09, 2025 at 06:45:41PM -0400, Jason Merrill wrote:
>>>> + && reduced_constant_expression_p (val))
>>>
>>> And a value doesn't need to be constant to be printable, we should be able
>>> to print it unconditionally.
>>
>> Sure, the question is if printing non-constant value is better for users.
>> The change to do unconditionally the %qE results in
>> /usr/src/gcc/gcc/testsuite/g++.dg/cpp26/constexpr-eh12.C:71:49: error: uncaught exception '(E*)(& heap )'
>> while previously it was
>> /usr/src/gcc/gcc/testsuite/g++.dg/cpp26/constexpr-eh12.C:71:49: error: uncaught exception of type 'E*'
>> I've kept the conditional for now but if you really want that change, can remove it
>> in the 2 spots and tweak constexpr-eh12.C test's expectations.
>>
>>> Is there a reason not to add it to heap_vars here?
>>
>> In the earlier patch I had:
>>
>>>> + case CXA_ALLOCATE_EXCEPTION:
>> ...
>>>> + tree var = cxa_allocate_exception (loc, type, size_zero_node);
>>>> + ctx->global->heap_vars.safe_push (var);
>>>> + ctx->global->put_value (var, NULL_TREE);
>> vs.
>>>> + case CXA_BAD_CAST:
>>>> + case CXA_BAD_TYPEID:
>>>> + case CXA_THROW_BAD_ARRAY_NEW_LENGTH:
>> ...
>>>> + tree var = cxa_allocate_exception (loc, type, size_one_node);
>>>> + tree ctor
>>>> + = build_special_member_call (var, complete_ctor_identifier,
>>>> + NULL, type, LOOKUP_NORMAL,
>>>> + ctx->quiet ? tf_none
>>>> + : tf_warning_or_error);
>>>> + if (ctor == error_mark_node)
>>>> + {
>>>> + *non_constant_p = true;
>>>> + return call;
>>>> + }
>>>> + ctx->global->heap_vars.safe_push (var);
>>
>> and so it wasn't added to heap_vars if build_special_member_call failed.
>> But thinking about it now, we really don't care about what is in heap_vars
>> if *non_constant_p, so I've made the change.
>>
>> The rest incorporated into the following version of the patch, passes
>> GXX_TESTSUITE_STDS=98,11,14,17,20,23,26 make check-g++ RUNTESTFLAGS="--target_board=unix/-fpie dg.exp='constexpr-eh* constexpr-asm-5.C static_assert1.C feat-cxx26.C constexpr-throw.C constexpr-84192.C constexpr-dynamic*.C consteval34.C constexpr-new27.C consteval-memfn1.C constexpr-typeid5.C constexpr-ellipsis*'"
>> so far.
>>
>> 2025-07-10 Jakub Jelinek <jakub@redhat.com>
>>
>> PR c++/117785
>> gcc/c-family/
>> * c-cppbuiltin.cc (c_cpp_builtins): Predefine
>> __cpp_constexpr_exceptions=202411L for C++26.
>> gcc/cp/
>> * constexpr.cc: Implement C++26 P3068R5 - constexpr exceptions.
>> (class constexpr_global_ctx): Add caught_exceptions and
>> uncaught_exceptions members.
>> (constexpr_global_ctx::constexpr_global_ctx): Initialize
>> uncaught_exceptions.
>> (returns, breaks, continues, switches): Move earlier.
>> (throws): New function.
>> (exception_what_str, diagnose_std_terminate,
>> diagnose_uncaught_exception): New functions.
>> (enum cxa_builtin): New type.
>> (cxx_cxa_builtin_fn_p, cxx_eval_cxa_builtin_fn): New functions.
>> (cxx_eval_builtin_function_call): Add jump_target argument. Call
>> cxx_eval_cxa_builtin_fn for __builtin_eh_ptr_adjust_ref. Adjust
>> cxx_eval_constant_expression calls, if it results in jmp_target,
>> set *jump_target to it and return.
>> (cxx_bind_parameters_in_call): Add jump_target argument. Pass
>> it through to cxx_eval_constant_expression. If it sets *jump_target,
>> break.
>> (fold_operand): Adjust cxx_eval_constant_expression caller.
>> (cxx_eval_assert): Likewise. If it set jmp_target, return true.
>> (cxx_eval_internal_function): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression. Return early if
>> *jump_target after recursing on args.
>> (cxx_eval_dynamic_cast_fn): Likewise. Don't set reference_p for
>> C++26 with -fexceptions.
>> (cxx_eval_thunk_call): Add jump_target argument. Pass it through
>> to cxx_eval_constant_expression.
>> (cxx_set_object_constness): Likewise. Don't set TREE_READONLY if
>> throws (jump_target).
>> (cxx_eval_call_expression): Add jump_target argument. Pass it
>> through to cxx_eval_internal_function, cxx_eval_builtin_function_call,
>> cxx_eval_thunk_call, cxx_eval_dynamic_cast_fn and
>> cxx_set_object_constness. Pass it through also
>> cxx_eval_constant_expression on arguments, cxx_bind_parameters_in_call
>> and cxx_fold_indirect_ref and for those cases return early if
>> *jump_target. Call cxx_eval_cxa_builtin_fn for cxx_cxa_builtin_fn_p
>> functions. For cxx_eval_constant_expression on body, pass address of
>> cleared jmp_target automatic variable, if it throws propagate to
>> *jump_target and make it non-cacheable. For C++26 don't diagnose
>> calls to non-constexpr functions before cxx_bind_parameters_in_call
>> could report some argument throwing an exception.
>> (cxx_eval_unary_expression): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression and return early if
>> *jump_target after the call.
>> (cxx_fold_pointer_plus_expression): Likewise.
>> (cxx_eval_binary_expression): Likewise and similarly for
>> cxx_fold_pointer_plus_expression call.
>> (cxx_eval_conditional_expression): Pass jump_target to
>> cxx_eval_constant_expression on first operand and return early if
>> *jump_target after the call.
>> (cxx_eval_vector_conditional_expression): Add jump_target argument.
>> Pass it through to cxx_eval_constant_expression for all 3 arguments
>> and return early if *jump_target after any of those calls.
>> (get_array_or_vector_nelts): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression.
>> (eval_and_check_array_index): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression calls and return early after
>> each of them if *jump_target.
>> (cxx_eval_array_reference): Likewise.
>> (cxx_eval_component_reference): Likewise.
>> (cxx_eval_bit_field_ref): Likewise.
>> (cxx_eval_bit_cast): Likewise. Assert CHECKING_P call doesn't
>> throw or return.
>> (cxx_eval_logical_expression): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression calls and return early after
>> each of them if *jump_target.
>> (cxx_eval_bare_aggregate): Likewise.
>> (cxx_eval_vec_init_1): Add jump_target argument. Pass it through
>> to cxx_eval_bare_aggregate and recursive call. Pass it through
>> to get_array_or_vector_nelts and cxx_eval_constant_expression
>> and return early after it if *jump_target.
>> (cxx_eval_vec_init): Add jump_target argument. Pass it through
>> to cxx_eval_constant_expression and cxx_eval_vec_init_1.
>> (cxx_union_active_member): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression and return early after it
>> if *jump_target.
>> (cxx_fold_indirect_ref_1): Add jump_target argument. Pass it
>> through to cxx_union_active_member and recursive calls.
>> (cxx_eval_indirect_ref): Add jump_target argument. Pass it through
>> to cxx_fold_indirect_ref_1 calls and to recursive call, in which
>> case return early after it if *jump_target.
>> (cxx_fold_indirect_ref): Add jump_target argument. Pass it through
>> to cxx_fold_indirect_ref and cxx_eval_constant_expression calls and
>> return early after those if *jump_target.
>> (cxx_eval_trinary_expression): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression calls and return early after
>> those if *jump_target.
>> (cxx_eval_store_expression): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression and eval_and_check_array_index
>> calls and return early after those if *jump_target.
>> (cxx_eval_increment_expression): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression calls and return early after
>> those if *jump_target.
>> (label_matches): Handle VAR_DECL case.
>> (cxx_eval_statement_list): Remove local_target variable and
>> !jump_target handling. Handle throws (jump_target) like returns or
>> breaks.
>> (cxx_eval_loop_expr): Remove local_target variable and !jump_target
>> handling. Pass it through to cxx_eval_constant_expression. Handle
>> throws (jump_target) like returns.
>> (cxx_eval_switch_expr): Pass jump_target through to
>> cxx_eval_constant_expression on cond, return early after it if
>> *jump_target.
>> (build_new_constexpr_heap_type): Add jump_target argument. Pass it
>> through to cxx_eval_constant_expression calls, return early after
>> those if *jump_target.
>> (merge_jump_target): New function.
>> (cxx_eval_constant_expression): Make jump_target argument no longer
>> defaulted, don't test jump_target for NULL. Pass jump_target
>> through to recursive calls, cxx_eval_call_expression,
>> cxx_eval_store_expression, cxx_eval_indirect_ref,
>> cxx_eval_unary_expression, cxx_eval_binary_expression,
>> cxx_eval_logical_expression, cxx_eval_array_reference,
>> cxx_eval_component_reference, cxx_eval_bit_field_ref,
>> cxx_eval_vector_conditional_expression, cxx_eval_bare_aggregate,
>> cxx_eval_vec_init, cxx_eval_trinary_expression, cxx_fold_indirect_ref,
>> build_new_constexpr_heap_type, cxx_eval_increment_expression,
>> cxx_eval_bit_cast and return earlyu after some of those if
>> *jump_target as needed.
>> (cxx_eval_constant_expression) <case TARGET_EXPR>: For C++26 push
>> also CLEANUP_EH_ONLY cleanups, with NULL_TREE marker after them.
>> (cxx_eval_constant_expression) <case RETURN_EXPR>: Don't override
>> *jump_target if throws (jump_target).
>> (cxx_eval_constant_expression) <case TRY_CATCH_EXPR, case TRY_BLOCK,
>> case MUST_NOT_THROW_EXPR, case TRY_FINALLY_EXPR, case CLEANUP_STMT>:
>> Handle C++26 constant expressions.
>> (cxx_eval_constant_expression) <case CLEANUP_POINT_EXPR>: For C++26
>> with throws (jump_target) evaluate the CLEANUP_EH_ONLY cleanups as
>> well, and if not throws (jump_target) skip those. Set *jump_target
>> if some of the cleanups threw.
>> (cxx_eval_constant_expression) <case THROW_EXPR>: Recurse on operand
>> for C++26.
>> (cxx_eval_outermost_constant_expr): Diagnose uncaught exceptions both
>> from main expression and cleanups, diagnose also
>> break/continue/returns from the main expression. Handle
>> CLEANUP_EH_ONLY cleanup markers. Don't diagnose mutable poison stuff
>> if non_constant_p. Use different diagnostics for non-deleted heap
>> allocations if they were allocated by __cxa_allocate_exception.
>> (callee_might_throw): New function.
>> (struct check_for_return_continue_data): Add could_throw field.
>> (check_for_return_continue): Handle AGGR_INIT_EXPR and CALL_EXPR and
>> set d->could_throw if they could throw.
>> (potential_constant_expression_1): For CALL_EXPR allow
>> cxx_dynamic_cast_fn_p calls. For C++26 set *jump_target to void_node
>> for calls that could throw. For C++26 if call to non-constexpr call
>> is seen, try to evaluate arguments first and if they could throw,
>> don't diagnose call to non-constexpr function nor return false.
>> Adjust check_for_return_continue_data initializers and set
>> *jump_target to void_node if data.could_throw_p. For C++26 recurse on
>> THROW_EXPR argument. Add comment explaining TRY_BLOCK handling with
>> C++26 exceptions. Handle throws like returns in some cases.
>> * cp-tree.h (MUST_NOT_THROW_NOEXCEPT_P, MUST_NOT_THROW_THROW_P,
>> MUST_NOT_THROW_CATCH_P, DECL_EXCEPTION_REFCOUNT): Define.
>> (DECL_LOCAL_DECL_P): Fix comment typo, VARIABLE_DECL -> VAR_DECL.
>> (enum cp_built_in_function): Add CP_BUILT_IN_EH_PTR_ADJUST_REF,
>> (handler_match_for_exception_type): Declare.
>> * call.cc (handler_match_for_exception_type): New function.
>> * except.cc (initialize_handler_parm): Set MUST_NOT_THROW_CATCH_P
>> on newly created MUST_NOT_THROW_EXPR.
>> (begin_eh_spec_block): Set MUST_NOT_THROW_NOEXCEPT_P.
>> (wrap_cleanups_r): Set MUST_NOT_THROW_THROW_P.
>> (build_throw): Add another TARGET_EXPR whose scope spans
>> until after the __cxa_throw call and copy pointer value from ptr
>> to it and use it in __cxa_throw argument.
>> * tree.cc (builtin_valid_in_constant_expr_p): Handle
>> CP_BUILT_IN_EH_PTR_ADJUST_REF.
>> * decl.cc (cxx_init_decl_processing): Initialize
>> __builtin_eh_ptr_adjust_ref FE builtin.
>> * pt.cc (tsubst_stmt) <case MUST_NOT_THROW_EXPR>: Copy the
>> MUST_NOT_THROW_NOEXCEPT_P, MUST_NOT_THROW_THROW_P and
>> MUST_NOT_THROW_CATCH_P flags.
>> * cp-gimplify.cc (cp_gimplify_expr) <case CALL_EXPR>: Error on
>> non-folded CP_BUILT_IN_EH_PTR_ADJUST_REF calls.
>> gcc/testsuite/
>> * g++.dg/cpp0x/constexpr-ellipsis2.C: Expect different diagnostics for
>> C++26.
>> * g++.dg/cpp0x/constexpr-throw.C: Likewise.
>> * g++.dg/cpp1y/constexpr-84192.C: Expect different diagnostics.
>> * g++.dg/cpp1y/constexpr-throw.C: Expect different diagnostics for
>> C++26.
>> * g++.dg/cpp1z/constexpr-asm-5.C: Likewise.
>> * g++.dg/cpp26/constexpr-eh1.C: New test.
>> * g++.dg/cpp26/constexpr-eh2.C: New test.
>> * g++.dg/cpp26/constexpr-eh3.C: New test.
>> * g++.dg/cpp26/constexpr-eh4.C: New test.
>> * g++.dg/cpp26/constexpr-eh5.C: New test.
>> * g++.dg/cpp26/constexpr-eh6.C: New test.
>> * g++.dg/cpp26/constexpr-eh7.C: New test.
>> * g++.dg/cpp26/constexpr-eh8.C: New test.
>> * g++.dg/cpp26/constexpr-eh9.C: New test.
>> * g++.dg/cpp26/constexpr-eh10.C: New test.
>> * g++.dg/cpp26/constexpr-eh11.C: New test.
>> * g++.dg/cpp26/constexpr-eh12.C: New test.
>> * g++.dg/cpp26/constexpr-eh13.C: New test.
>> * g++.dg/cpp26/constexpr-eh14.C: New test.
>> * g++.dg/cpp26/constexpr-eh15.C: New test.
>> * g++.dg/cpp26/feat-cxx26.C: Change formatting in __cpp_pack_indexing
>> and __cpp_pp_embed test. Add __cpp_constexpr_exceptions test.
>> * g++.dg/cpp26/static_assert1.C: Expect different diagnostics for
>> C++26.
>> * g++.dg/cpp2a/consteval34.C: Likewise.
>> * g++.dg/cpp2a/consteval-memfn1.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic4.C: For C++26 add std::exception and
>> std::bad_cast definitions and expect different diagnostics.
>> * g++.dg/cpp2a/constexpr-dynamic6.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic7.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic8.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic9.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic11.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic14.C: Likewise.
>> * g++.dg/cpp2a/constexpr-dynamic18.C: Likewise.
>> * g++.dg/cpp2a/constexpr-new27.C: New test.
>> * g++.dg/cpp2a/constexpr-typeid5.C: New test.
>> libstdc++-v3/
>> * include/bits/version.def (constexpr_exceptions): New.
>> * include/bits/version.h: Regenerate.
>> * libsupc++/exception (std::bad_exception::bad_exception): Add
>> _GLIBCXX26_CONSTEXPR.
>> (std::bad_exception::~bad_exception, std::bad_exception::what): For
>> C++26 add constexpr and define inline.
>> * libsupc++/exception.h (std::exception::exception,
>> std::exception::operator=): Add _GLIBCXX26_CONSTEXPR.
>> (std::exception::~exception, std::exception::what): For C++26 add
>> constexpr and define inline.
>> * libsupc++/exception_ptr.h (std::make_exception_ptr): Add
>> _GLIBCXX26_CONSTEXPR. For if consteval use just throw with
>> current_exception() in catch.
>> (std::exception_ptr::exception_ptr(void*)): For C++26 add constexpr
>> and define inline.
>> (std::exception_ptr::exception_ptr()): Add _GLIBCXX26_CONSTEXPR.
>> (std::exception_ptr::exception_ptr(const exception_ptr&)): Likewise.
>> Use __builtin_eh_ptr_adjust_ref if consteval and compiler has it
>> instead of _M_addref.
>> (std::exception_ptr::exception_ptr(nullptr_t)): Add
>> _GLIBCXX26_CONSTEXPR.
>> (std::exception_ptr::exception_ptr(exception_ptr&&)): Likewise.
>> (std::exception_ptr::operator=): Likewise.
>> (std::exception_ptr::~exception_ptr): Likewise. Use
>> __builtin_eh_ptr_adjust_ref if consteval and compiler has it
>> instead of _M_release.
>> (std::exception_ptr::swap): Add _GLIBCXX26_CONSTEXPR.
>> (std::exception_ptr::operator bool): Likewise.
>> (std::exception_ptr::operator==): Likewise.
>> * libsupc++/nested_exception.h
>> (std::nested_exception::nested_exception): Add _GLIBCXX26_CONSTEXPR.
>> (std::nested_exception::operator=): Likewise.
>> (std::nested_exception::~nested_exception): For C++26 add constexpr
>> and define inline.
>> (std::nested_exception::rethrow_if_nested): Add _GLIBCXX26_CONSTEXPR.
>> (std::nested_exception::nested_ptr): Likewise.
>> (std::_Nested_exception::_Nested_exception): Likewise.
>> (std::throw_with_nested, std::rethrow_if_nested): Likewise.
>> * libsupc++/new (std::bad_alloc::bad_alloc): Likewise.
>> (std::bad_alloc::operator=): Likewise.
>> (std::bad_alloc::~bad_alloc): For C++26 add constexpr and define
>> inline.
>> (std::bad_alloc::what): Likewise.
>> (std::bad_array_new_length::bad_array_new_length): Add
>> _GLIBCXX26_CONSTEXPR.
>> (std::bad_array_new_length::~bad_array_new_length): For C++26 add
>> constexpr and define inline.
>> (std::bad_array_new_length::what): Likewise.
>> * libsupc++/typeinfo (std::bad_cast::bad_cast): Add
>> _GLIBCXX26_CONSTEXPR.
>> (std::bad_cast::~bad_cast): For C++26 add constexpr and define inline.
>> (std::bad_cast::what): Likewise.
>> (std::bad_typeid::bad_typeid): Add _GLIBCXX26_CONSTEXPR.
>> (std::bad_typeid::~bad_typeid): For C++26 add constexpr and define
>> inline.
>> (std::bad_typeid::what): Likewise.
>>
>
>
> I'm seeing some unexpected libstdc++ test failures since this commit
> (confirmed by bisection).
>
> $ export GLIBCXX_TESTSUITE_STDS=26
> $ make check RUNTESTFLAGS="conformance.exp=25_algo*debug/constexpr*neg.cc
> --target_board=unix/-D_GLIBCXX_DEBUG"
>
> FAIL: 25_algorithms/copy/debug/constexpr_neg.cc -std=gnu++26 (test
> for errors, line )
> FAIL: 25_algorithms/copy/debug/constexpr_neg.cc -std=gnu++26 (test
> for excess errors)
> FAIL: 25_algorithms/copy_backward/debug/constexpr_neg.cc -std=gnu++26
> (test for excess errors)
> FAIL: 25_algorithms/equal/debug/constexpr_neg.cc -std=gnu++26 (test
> for errors, line )
> FAIL: 25_algorithms/equal/debug/constexpr_neg.cc -std=gnu++26 (test
> for excess errors)
> FAIL: 25_algorithms/lower_bound/debug/constexpr_partitioned_neg.cc
> -std=gnu++26 (test for excess errors)
> FAIL: 25_algorithms/lower_bound/debug/constexpr_partitioned_pred_neg.cc
> -std=gnu++26 (test for excess errors)
> FAIL: 25_algorithms/lower_bound/debug/constexpr_valid_range_neg.cc
> -std=gnu++26 (test for excess errors)
> FAIL: 25_algorithms/upper_bound/debug/constexpr_partitioned_neg.cc
> -std=gnu++26 (test for excess errors)
> FAIL: 25_algorithms/upper_bound/debug/constexpr_partitioned_pred_neg.cc
> -std=gnu++26 (test for excess errors)
> FAIL: 25_algorithms/upper_bound/debug/constexpr_valid_range_neg.cc
> -std=gnu++26 (test for excess errors)
>
>
> The difference is in the diagnostic we get from the expansion of this macro:
>
> #define _GLIBCXX_DEBUG_VERIFY_AT_F(_Cond,_ErrMsg,_File,_Line,_Func) \
> do { \
> if (__builtin_expect(!bool(_Cond), false)) \
> __gnu_debug::_Error_formatter::_S_at(_File, _Line, _Func) \
> ._ErrMsg._M_error(); \
> } while (false)
>
>
> The dg-error lines in the tests expect a "non-constant condition"
> error and the location to match:
> // { dg-error "_Error_formatter::_M_error()" "" { target *-*-* } 0 }
>
> but now the error points to the _Error_formatter::_S_at(...) function.
>
> The diagnostic only changed for C++26, and I'm not sure why (the _S_at
> function isn't constexpr, so why wasn't it failing there before?)
In C++23, we could see that the outermost thing in the expression is a
call to _M_error, and it's non-constexpr, so we give up at that point
and never look at the call to _S_at.
In C++26, evaluating the arguments to a function (including the object
argument) might throw before we get to calling the non-constexpr
function, so we need to evaluate them before giving up.
> Is this change in diagnostics expected?
> I already have a patch to make the necessary changes to the tests, but
> I wanted to understand why it is required.
>
> The fixes are all of this form:
>
> -// { dg-error "_Error_formatter::_M_error()" "" { target *-*-* } 0 }
> +// { dg-error "_Error_formatter::(_M_error|_S_at)" "" { target *-*-* } 0 }
>
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