https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105491
--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-12 branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:e30b73bad9486f11b6b0022ae4a3edfc0f9da4bb commit r12-8445-ge30b73bad9486f11b6b0022ae4a3edfc0f9da4bb Author: Patrick Palka <ppa...@redhat.com> Date: Wed Jun 1 08:47:25 2022 -0400 c++: constexpr init of union sub-aggr w/ base [PR105491] Here ever since r10-7313-gb599bf9d6d1e18, reduced_constant_expression_p in C++11/14 is rejecting the marked sub-aggregate initializer (of type S) W w = {.D.2445={.s={.D.2387={.m=0}, .b=0}}}; ^ ultimately because said initializer has CONSTRUCTOR_NO_CLEARING set, hence the function must verify that all fields of S are initialized. And before C++17 it doesn't expect to see base class fields (since next_initializable_field skips over them), so the presence thereof causes r_c_e_p to return false. The reason r10-7313-gb599bf9d6d1e18 causes this is because in that commit we began using CONSTRUCTOR_NO_CLEARING to precisely track whether we're in middle of activating a union member. This ends up affecting clear_no_implicit_zero, which recurses into sub-aggregate initializers only if the outer initializer has CONSTRUCTOR_NO_CLEARING set. After that commit, the outer union initializer above no longer has the flag set at this point and so clear_no_implicit_zero no longer recurses into the marked inner initializer. But arguably r_c_e_p should be able to accept the marked initializer regardless of whether CONSTRUCTOR_NO_CLEARING is set. The primary bug therefore seems to be that r_c_e_p relies on next_initializable_field which skips over base class fields in C++11/14. To fix this, this patch introduces a new helper function next_subobject_field which is like next_initializable_field except that it never skips base class fields, and makes r_c_e_p use it. This patch then renames next_initializable_field to next_aggregate_field (and makes it skip over vptr fields again). NB: This minimal backport of r13-211-g0c7bce0ac184c0 for 12.2 just adds next_subobject_field and makes reduced_constant_expression_p use it. PR c++/105491 gcc/cp/ChangeLog: * constexpr.cc (reduced_constant_expression_p): Use next_subobject_field instead. * cp-tree.h (next_subobject_field): Declare. * decl.cc (next_subobject_field): Define. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/constexpr-union7.C: New test. * g++.dg/cpp0x/constexpr-union7a.C: New test. * g++.dg/cpp2a/constinit17.C: New test. (cherry picked from commit 0c7bce0ac184c057bacad9c8e615ce82923835fd)
