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From: "nathanieloshead at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111226] New: constexpr doesn't detect change of union to
 empty member
Date: Tue, 29 Aug 2023 13:21:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111226

            Bug ID: 111226
           Summary: constexpr doesn't detect change of union to empty
                    member
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathanieloshead at gmail dot com
  Target Milestone: ---

While working on a patch for PR101631, I found that the following code is
currently incorrectly handled by GCC: (https://godbolt.org/z/1YevacMK3)


struct Empty {};

union U {
  int x;
  Empty e;
};

constexpr int foo() {
  U u{ 10 };
  u.e =3D {};
  return u.x;  // incorrectly accepted, even pre-C++20
}
constexpr auto y =3D foo();

constexpr Empty bar() {
  U u{ 10 };
  u.e =3D {};
  return u.e;  // incorrectly errors, thinks active member is still 'x'
}
constexpr auto x =3D bar();


The cause seems to be that the zero-sized trivial assignment is removed in
call.cc (since PR43075) and after constant folding is no longer in the tree
that the constexpr handling machinery receives.=