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From: "natattak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97566] New: [[no_unique_address]] causes miscompiles when
 mixed with EBO in constexpr context
Date: Sun, 25 Oct 2020 03:37:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97566

            Bug ID: 97566
           Summary: [[no_unique_address]] causes miscompiles when mixed
                    with EBO in constexpr context
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: natattak at gmail dot com
  Target Milestone: ---

Example: (https://godbolt.org/z/Y5q5br)

  #include <memory>
  #include <cassert>

  // error disappears if E doesn't inherit from B
  struct B {};
  struct E : B {};

  struct counter {
    constexpr counter() =3D default;
    constexpr void inc() {
      size++;
    }

    // error disappears if you remove or reorder this value
    int unused =3D 0;
    int size =3D 0;
    [[no_unique_address]] E empty =3D {};
  };

  constexpr int test() {
    counter x;
    x.inc();
    return x.size;
  }

  int main() {
    assert(test() =3D=3D 1);        // works, unless optimisations enabled
    static_assert(test() =3D=3D 1); // fails, always
  }

In particular, the runtime assertion only passes on -O0; any higher
optimisation e.g. -O1 causes it to fail.

Compiled as `g++ -std=3Dc++20`.=