[Bug libstdc++/96645] New: std::variant default constructor

[dev at hrookami dot icu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96645

            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=c++17

Code:

#include <variant>

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

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

        struct B {
            bool flag = false;
        };

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

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




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

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

   26 |     auto d = 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 =
{testVarStruct()::DataWithStruct::A, testVarStruct()::DataWithStruct::B}]' is
implicitly deleted because the default definition would be ill-formed:

 1287 |       variant() = default;

      |       ^~~~~~~

/opt/compiler-explorer/gcc-9.1.0/include/c++/9.1.0/variant:1287:7: error: use
of deleted function 'constexpr std::_Enable_default_constructor<false,
_Tag>::_Enable_default_constructor() [with _Tag =
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_members.h:110:15:
note: declared here

  110 |     constexpr _Enable_default_constructor() noexcept = delete;

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





It compiles if move nested structs in global namespace or remove initializer.