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From: "dev at hrookami dot icu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/96645] New: std::variant default constructor
Date: Mon, 17 Aug 2020 07:58:08 +0000
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X-Bugzilla-Version: 9.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96645

            Bug ID: 96645
           Summary: std::variant default constructor
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dev at hrookami dot icu
  Target Milestone: ---

Appears in version 9 and above.
Flags: -std=3Dc++17

Code:

#include <variant>

void testVariant()
{
    struct DataWithVariant {
        using Member =3D std::variant<int, float>;
        Member d;
    };
    auto d =3D DataWithVariant{};
}

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

        struct B {
            bool flag =3D false;
        };

        using Member =3D std::variant<A, B>;
        Member data;
    };
    auto d =3D DataWithStruct{};
}

int main() {
    testVariant();
    testVarStruct();
}




Compiler output:
<source>: In function 'void testVarStruct()':

<source>:26:29: error: use of deleted function 'std::variant<_Types>::varia=
nt()
[with _Types =3D {testVarStruct()::DataWithStruct::A,
testVarStruct()::DataWithStruct::B}]'

   26 |     auto d =3D DataWithStruct{};

      |                             ^

In file included from <source>:1:

/opt/compiler-explorer/gcc-9.1.0/include/c++/9.1.0/variant:1287:7: note:
'std::variant<_Types>::variant() [with _Types =3D
{testVarStruct()::DataWithStruct::A, testVarStruct()::DataWithStruct::B}]' =
is
implicitly deleted because the default definition would be ill-formed:

 1287 |       variant() =3D default;

      |       ^~~~~~~

/opt/compiler-explorer/gcc-9.1.0/include/c++/9.1.0/variant:1287:7: error: u=
se
of deleted function 'constexpr std::_Enable_default_constructor<false,
_Tag>::_Enable_default_constructor() [with _Tag =3D
std::variant<testVarStruct()::DataWithStruct::A,
testVarStruct()::DataWithStruct::B>]'

In file included from
/opt/compiler-explorer/gcc-9.1.0/include/c++/9.1.0/variant:38,

                 from <source>:1:

/opt/compiler-explorer/gcc-9.1.0/include/c++/9.1.0/bits/enable_special_memb=
ers.h:110:15:
note: declared here

  110 |     constexpr _Enable_default_constructor() noexcept =3D delete;

      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~





It compiles if move nested structs in global namespace or remove initialize=
r.=