https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121068
--- Comment #12 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Jason Merrill <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:fdbc5ff61b471076cc9c758fb6c30d62f7ef1c56 commit r16-2432-gfdbc5ff61b471076cc9c758fb6c30d62f7ef1c56 Author: Jason Merrill <ja...@redhat.com> Date: Wed Jul 16 11:52:45 2025 -0400 c++: constexpr union placement new [PR121068] The note and example in [class.union] p6 think that placement new can be used to change the active member of a union, but we didn't support that for array members in constant-evaluation even after implementing P1330 and P2747. First I tried to address this by introducing a CLOBBER_BEGIN_OBJECT for the entire array, but that broke the resolution of LWG3436, which invokes 'new T[1]' for an array T, and trying to clobber a multidimensional array when the actual object is single-dimensional breaks. So I've raised that issue with the committee. Until that is resolved, this patch takes a simpler approach: allow initialization of an element of an array to make the array the active member of a union. PR c++/121068 gcc/cp/ChangeLog: * constexpr.cc (cxx_eval_store_expression): Allow ARRAY_REFs when activating an array member of a union. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/constexpr-union6.C: Expect x5 to work. * g++.dg/cpp26/constexpr-new4.C: New test.
