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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113060] std::variant converting
 constructor/assignment is non-conforming after P2280?
Date: Mon, 19 Feb 2024 08:07:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113060

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #4 from Jiang An <de34 at live dot cn> ---
Hi, I found that we can support std::variant<float>(IC{}) with C++17 core
language rules (as modified by P2280R4), and thus P3146 should be updated.

Proof-of-concept example (https://gcc.godbolt.org/z/174sx397f):

```
#include <type_traits>
#include <utility>

template<class Ti>
struct array_element_initialization_tester {
    Ti elems[1];
};

template<class Ti, class Tp>
auto test_array_element_initializable(Tp&& t)
    -> decltype((void)
array_element_initialization_tester<Ti>{{std::forward<Tp>(t)}});

template<class Ti, class Tp, class =3D void>
constexpr bool is_array_element_initializable_from =3D false;
template<class Ti, class Tp>
constexpr bool is_array_element_initializable_from<Ti, Tp,
    decltype(::test_array_element_initializable<Ti>(std::declval<Tp>()))> =
=3D
true;

template <typename T,
std::enable_if_t<is_array_element_initializable_from<float, T>, int> =3D 0>
void FUN(float);

int main() {
    using IC =3D std::integral_constant<int, 42>;
    FUN<IC>(IC{});
    IC ic;
    FUN<IC&>(ic);
}
```

Note that this example adds a mediate function template
(test_array_element_initializable) to "reduce" the non-constexpr-ness of
std::declval.=