https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102163
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-11 branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:59c6831682dfa8bec2b5a62bdc85739924970808 commit r11-9080-g59c6831682dfa8bec2b5a62bdc85739924970808 Author: Patrick Palka <ppa...@redhat.com> Date: Tue Sep 14 11:22:12 2021 -0400 c++: empty union member activation during constexpr [PR102163] Here, the union's constructor is defined to activate its empty data member _M_rest, but during constexpr evaluation of this constructor the subobject constructor call O::O(&_M_rest, 42) doesn't produce a side effect that actually activates the member, so the union still appears uninitialized after its constructor has run. This patch fixes this by using a dummy MODIFY_EXPR in this situation, whose evaluation ensures the member gets activated. PR c++/102163 gcc/cp/ChangeLog: * constexpr.c (cxx_eval_call_expression): After evaluating a subobject constructor call for an empty union member, produce a side effect that makes sure the member gets activated. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/constexpr-empty17.C: New test. (cherry picked from commit de07cff96abd43f6f65dcf333958899c2ec42598)
