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From: "konstantin.vladimirov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/115233] New: constexpr RVO result is not determined as
 constant epression
Date: Sun, 26 May 2024 12:16:17 +0000
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X-Bugzilla-Version: 14.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115233

            Bug ID: 115233
           Summary: constexpr RVO result is not determined as constant
                    epression
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: konstantin.vladimirov at gmail dot com
  Target Milestone: ---

Consider I have this type:

---

struct Type {
  int a;
  const int& b;
  constexpr Type() : a(1), b(a) {}
};

constexpr Type t1;
constexpr int c11 =3D t1.a;
constexpr int c12 =3D t1.b;

---

c11 and c12 are fine for both gcc and clang

Now lets introduce constexpr function which is subject to RVO:

constexpr auto get() {
  return Type();
}

Now try this:

constexpr Type t2 =3D get();
constexpr int c2 =3D t2.a;

This code compiles fine with clang but not with gcc.

Live example on godbolt: https://godbolt.org/z/zf3YvebbG

According to [expr.const] and [class.copy.elision] result of constexpr func=
tion
is necessary RVO and is contant expression, so t2 construction is fine and =
t2.a
is constant expression as well.=