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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96645] [9/10/11 Regression] std::variant default constructor
Date: Mon, 17 Aug 2020 14:40:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96645

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And libc++'s std::variant is still affected by the same issue, but instead =
of
the default constructor being deleted it just has the wrong exception
specification:

#include <variant>

void testVarStruct()
{
    struct DataWithStruct {
        struct A {
            int number =3D 5; // compiles, if remove initialization
        };

        using Member =3D std::variant<A>;
        Member data;
        static_assert(std::is_nothrow_default_constructible_v<Member>);
    };
}

The difference is that GCC defines the std::variant default constructor as
defaulted, but libc++ defines it as:

constexpr variant()
noexcept(is_nothrow_default_constructible_v<variant_alternative<0, Types...=
>)

That means the constructor isn't deleted for libc++, it just has the wrong
noexcept-specifier.

It looks like the nested class DataWithStruct::A isn't considered complete
until after DataWithStruct is complete, but I'm not sure why that is.=