GCC accepts this code: template <class T> struct S { S() requires false = default; }; static_assert(__is_trivial(S<int>)); There are some related DRs that GCC doesn't implement (like DR1363), but I don't think this falls under any of them.
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>: https://gcc.gnu.org/g:9690fb3a43e5cf26a5fb853283d4200df312a640 commit r15-2146-g9690fb3a43e5cf26a5fb853283d4200df312a640 Author: Marek Polacek <polacek@redhat.com> Date: Tue Jun 18 16:49:24 2024 -0400 c++: implement DR1363 and DR1496 for __is_trivial [PR85723] is_trivial was introduced in <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2230.html> which split POD into is_trivial and is_standard_layout. Later came CWG 1363. Since struct A { A() = default; A(int = 42) {} }; cannot be default-initialized, it should not be trivial, so the definition of what is a trivial class changed. Similarly, CWG 1496 concluded that struct B { B() = delete; }: should not be trivial either. P0848 adjusted the definition further to say "eligible". That means that template<typename T> struct C { C() requires false = default; }; should not be trivial, either, since C::C() is not eligible. Bug 85723 reports that we implement none of the CWGs. I chose to fix this by using type_has_non_deleted_trivial_default_ctor which uses locate_ctor which uses build_new_method_call, which would be used by default-initialization as well. With that, all __is_trivial problems I could find in the Bugzilla are fixed, except for PR96288, which may need changes to trivially-copyable, so I'm not messing with that now. I hope this has no ABI implications. There's effort undergoing to remove "trivial class" from the core language as it's not really meaningful. So the impact of this change should be pretty low except to fix a few libstdc++ problems. PR c++/108769 PR c++/58074 PR c++/115522 PR c++/85723 gcc/cp/ChangeLog: * class.cc (type_has_non_deleted_trivial_default_ctor): Fix formatting. * tree.cc (trivial_type_p): Instead of TYPE_HAS_TRIVIAL_DFLT, use type_has_non_deleted_trivial_default_ctor. gcc/testsuite/ChangeLog: * g++.dg/warn/Wclass-memaccess.C: Add dg-warning. * g++.dg/ext/is_trivial1.C: New test. * g++.dg/ext/is_trivial2.C: New test. * g++.dg/ext/is_trivial3.C: New test. * g++.dg/ext/is_trivial4.C: New test. * g++.dg/ext/is_trivial5.C: New test. * g++.dg/ext/is_trivial6.C: New test.
Fixed for GCC 15; thanks for the bug report.