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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/103879] error: accessing value of variant::_Copy_ctor_base
 through a 'const variant<string>' glvalue in a constant expression
Date: Wed, 05 Jan 2022 16:20:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103879

--- Comment #7 from =E5=BA=B7=E6=A1=93=E7=91=8B <hewillk at gmail dot com> =
---
(In reply to Patrick Palka from comment #6)
> Reduced C++14 rejects-valid testcase:
>=20
> struct A {
>   int n =3D 42;
> };
>=20
> struct B : A { };
>=20
> struct C {
>   B b;
> };
>=20
> constexpr int f() {
>   C c;
>   A& a =3D static_cast<A&>(c.b);
>   B& b =3D static_cast<B&>(a);
>   return b.n;
> }
>=20
> static_assert(f() =3D=3D 42, "");

Is this the same issue?

#include <string>
#include <ranges>

int main() {
  using namespace std::string_literals;
  static_assert(std::ranges::distance(
      std::views::iota(0, 2) | std::ranges::views::transform([](int) { retu=
rn
""s; })
                             | std::ranges::views::join));
}

https://godbolt.org/z/8nve43zvW=