[Bug libstdc++/93034] New: variant not constexpr in c++17 mode

[rene.r...@fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93034

            Bug ID: 93034
           Summary: variant not constexpr in c++17 mode
           Product: gcc
           Version: 7.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rene.r...@fu-berlin.de
  Target Milestone: ---

Hi there, 

I couldn't find a related issue but maybe I just missed it.
But it seems that the copy and move constructor of std::variant are not
declared constexpr for gcc-7.

```
#include <variant>

using variant_t = std::variant<int>;

inline constexpr variant_t v1{};  // OK
inline constexpr variant_t v2{v1}; // Error
inline constexpr variant_t v3{std::move(v1)}; // Error
inline constexpr variant_t v4{3}; // OK
```

Here is the corresponding error:
```
<source>:6:33: error: call to non-constexpr function
'std::variant<_Types>::variant(const std::variant<_Types>&) [with _Types =
{int}]'

 inline constexpr variant_t v2{v1}; // Error

                                 ^

In file included from <source>:1:0:

/opt/compiler-explorer/gcc-7.5.0/include/c++/7.5.0/variant:943:7: note:
'std::variant<_Types>::variant(const std::variant<_Types>&) [with _Types =
{int}]' is not usable as a constexpr function because:

       variant(const variant&) = default;

       ^~~~~~~

/opt/compiler-explorer/gcc-7.5.0/include/c++/7.5.0/variant:407:7: note:
defaulted constructor calls non-constexpr
'std::__detail::__variant::_Variant_base<_Types>::_Variant_base(const
std::__detail::__variant::_Variant_base<_Types>&) [with _Types = {int}]'

       _Variant_base(const _Variant_base& __rhs)

       ^~~~~~~~~~~~~

<source>:7:44: error: call to non-constexpr function
'std::variant<_Types>::variant(const std::variant<_Types>&) [with _Types =
{int}]'

 inline constexpr variant_t v3{std::move(v1)}; // Error

                                            ^

Compiler returned: 1
```

This compiles fine with g++8 and g++9 and trunk.


Compiled on the compiler explorer using g++-7.5 with flags: -std=c++17 -Wall
-Wextra -pedantic

Any help in finding a solution for this would be really appreciated.