Created attachment 41038 [details]
Example that a tuple of a variant can't be constructed

Hi gcc-team,

first of all, I'm not sure if this bug(?) should be filed here. If not please move it to the correct component.

I currently was trying to create a constexpr tuple of a constexpr variant as value, which apparently doesn't work, even though I can create a constexpr variant.

I get the following error:

```
In file included from bug_gcc_constexpr_tuple_of_variant.cpp:2:0:
/usr/local/bin/gcc-7/include/c++/7.0.1/tuple: In function ‘int main()’:
bug_gcc_constexpr_tuple_of_variant.cpp:8:51:   in constexpr expansion of ‘std::make_tuple(_Elements&& ...) [with _Elements = {const std
::variant<unsigned char, short unsigned int, unsigned int>&}]()’
bug_gcc_constexpr_tuple_of_variant.cpp:8:51:   in constexpr expansion of ‘std::tuple<std::variant<unsigned char, short unsigned int, un
signed int> >(std::forward<const std::variant<unsigned char, short unsigned int, unsigned int>&>(__args#0))’
/usr/local/bin/gcc-7/include/c++/7.0.1/tuple:609:33:   in constexpr expansion of ‘((std::tuple<std::variant<unsigned char, short unsign
ed int, unsigned int> >*)this)->std::tuple<std::variant<unsigned char, short unsigned int, unsigned int> >::<anonymous>.std::_Tuple_imp
l<0, std::variant<unsigned char, short unsigned int, unsigned int> >::_Tuple_impl(__elements#0)’
/usr/local/bin/gcc-7/include/c++/7.0.1/tuple:361:21: error: ‘constexpr std::_Head_base<_Idx, _Head, false>::_Head_base(const _Head&) [w
ith long unsigned int _Idx = 0; _Head = std::variant<unsigned char, short unsigned int, unsigned int>]’ called in a constant expression
       : _Base(__head) { }
                     ^
/usr/local/bin/gcc-7/include/c++/7.0.1/tuple:125:17: note: ‘constexpr std::_Head_base<_Idx, _Head, false>::_Head_base(const _Head&) [wi

th long unsigned int _Idx = 0; _Head = std::variant<unsigned char, short unsigned int, unsigned int>]’ is not usable as a constexpr fun
ction because:
       constexpr _Head_base(const _Head& __h)
                 ^~~~~~~~~~
/usr/local/bin/gcc-7/include/c++/7.0.1/tuple:126:25: error: call to non-constexpr function ‘std::variant<_Types>::variant(const std::va
riant<_Types>&) [with _Types = {unsigned char, short unsigned int, unsigned int}]’
       : _M_head_impl(__h) { }
                         ^
In file included from bug_gcc_constexpr_tuple_of_variant.cpp:3:0:
/usr/local/bin/gcc-7/include/c++/7.0.1/variant:932:7: note: ‘std::variant<_Types>::variant(const std::variant<_Types>&) [with _Types =
{unsigned char, short unsigned int, unsigned int}]’ is not usable as a constexpr function because:
       variant(const variant&) = default;
       ^~~~~~~
/usr/local/bin/gcc-7/include/c++/7.0.1/variant:399:7: note: defaulted constructor calls non-constexpr ‘std::__detail::__variant::_Varia
nt_base<_Types>::_Variant_base(const std::__detail::__variant::_Variant_base<_Types>&) [with _Types = {unsigned char, short unsigned in
t, unsigned int}]’
       _Variant_base(const _Variant_base& __rhs)
       ^~~~~~~~~~~~~
zsh: exit 1     g++-7 -std=c++1z -Wall -Wextra bug_gcc_constexpr_tuple_of_variant.cpp
```

Maybe you can tell me what I'm doing wrong.

Best regards
marehr