formatter<string_view> incompatible with #include<variant>

[jcelerier]

The following code works fine if #include <variant> is removed (tested with g++-8.2) :

#include <variant>
#include <fmt/format.h>

struct foo { };
namespace fmt
{
template <>
struct formatter<foo> final
    : formatter<string_view>
{
  template <typename ParseContext>
  constexpr auto parse(ParseContext &ctx) { return ctx.begin(); }

  template <typename FormatContext>
  auto format(const foo& e, FormatContext &ctx) {
    return formatter<string_view>::format("foo", ctx);
  }
};
}

int main() {
    fmt::format("{}", foo{});
}

The error is :

/usr/include/c++/8.2.1/variant: In instantiation of ‘constexpr decltype(auto) std::visit(_Visitor&&, _Variants&& ...) [with _Visitor = fmt::v5::arg_formatter<fmt::v5::output_range<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >; _Variants = {fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >}]’:
include/fmt/format.h:3234:17:   required from ‘decltype (ctx.out()) fmt::v5::formatter<T, Char, typename std::enable_if<fmt::v5::internal::format_type<typename fmt::v5::buffer_context<Char>::type, T>::value>::type>::format(const T&, FormatContext&) [with FormatContext = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; T = fmt::v5::basic_string_view<char>; Char = char; typename std::enable_if<fmt::v5::internal::format_type<typename fmt::v5::buffer_context<Char>::type, T>::value>::type = void; decltype (ctx.out()) = std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >]’
test/array.cpp:16:53:   required from ‘auto fmt::v5::formatter<foo>::format(const foo&, FormatContext&) [with FormatContext = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>]’
include/fmt/core.h:580:5:   required from ‘static void fmt::v5::internal::value<Context>::format_custom_arg(const void*, Context&) [with T = foo; Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>]’
include/fmt/core.h:563:19:   required from ‘fmt::v5::internal::value<Context>::value(const T&) [with T = foo; Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>]’
include/fmt/core.h:591:58:   required from ‘constexpr fmt::v5::internal::init<Context, T, TYPE>::operator fmt::v5::internal::value<Context>() const [with Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; T = const foo&; fmt::v5::internal::type TYPE = (fmt::v5::internal::type)13]’
include/fmt/core.h:995:35:   required from ‘typename std::enable_if<IS_PACKED, fmt::v5::internal::value<Context> >::type fmt::v5::internal::make_arg(const T&) [with bool IS_PACKED = true; Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; T = foo; typename std::enable_if<IS_PACKED, fmt::v5::internal::value<Context> >::type = fmt::v5::internal::value<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >]’
include/fmt/core.h:1053:51:   required from ‘fmt::v5::format_arg_store<Context, Args>::format_arg_store(const Args& ...) [with Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; Args = {foo}]’
include/fmt/core.h:1383:40:   required from ‘std::__cxx11::basic_string<typename fmt::v5::internal::format_string_traits<S>::char_type> fmt::v5::format(const String&, const Args& ...) [with String = char [3]; Args = {foo}; typename fmt::v5::internal::format_string_traits<S>::char_type = char]’
test/array.cpp:22:28:   required from here
/usr/include/c++/8.2.1/variant:1375:23: error: ‘class fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ has no member named ‘valueless_by_exception’
       if ((__variants.valueless_by_exception() || ...))
            ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1380:17: error: no matching function for call to ‘get<0>(fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >)’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:216:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(std::pair<_Tp1, _Tp2>&)’
     get(std::pair<_Tp1, _Tp2>& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:216:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:221:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type&& std::get(std::pair<_Tp1, _Tp2>&&)’
     get(std::pair<_Tp1, _Tp2>&& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:221:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:226:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr const typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(const std::pair<_Tp1, _Tp2>&)’
     get(const std::pair<_Tp1, _Tp2>& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:226:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:231:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr const typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type&& std::get(const std::pair<_Tp1, _Tp2>&&)’
     get(const std::pair<_Tp1, _Tp2>&& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:231:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:240:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp& std::get(std::pair<_T1, _T2>&)’
     get(pair<_Tp, _Up>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:240:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:245:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp& std::get(const std::pair<_T1, _T2>&)’
     get(const pair<_Tp, _Up>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:245:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:250:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp&& std::get(std::pair<_T1, _T2>&&)’
     get(pair<_Tp, _Up>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:250:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:255:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp&& std::get(const std::pair<_T1, _T2>&&)’
     get(const pair<_Tp, _Up>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:255:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:260:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp& std::get(std::pair<_Up, _Tp>&)’
     get(pair<_Up, _Tp>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:260:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:265:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp& std::get(const std::pair<_Up, _Tp>&)’
     get(const pair<_Up, _Tp>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:265:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:270:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp&& std::get(std::pair<_Up, _Tp>&&)’
     get(pair<_Up, _Tp>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:270:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:275:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp&& std::get(const std::pair<_Up, _Tp>&&)’
     get(const pair<_Up, _Tp>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:275:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1329:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >& std::get(std::variant<_Types ...>&)’
     get(variant<_Types...>& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1329:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1340:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >&& std::get(std::variant<_Types ...>&&)’
     get(variant<_Types...>&& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1340:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1351:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >& std::get(const std::variant<_Types ...>&)’
     get(const variant<_Types...>& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1351:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1362:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >&& std::get(const std::variant<_Types ...>&&)’
     get(const variant<_Types...>&& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1362:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:881:27: note: candidate: ‘template<class _Tp, class ... _Types> constexpr _Tp& std::get(std::variant<_Types ...>&)’
     constexpr inline _Tp& get(variant<_Types...>& __v)
                           ^~~
/usr/include/c++/8.2.1/variant:881:27: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:890:28: note: candidate: ‘template<class _Tp, class ... _Types> constexpr _Tp&& std::get(std::variant<_Types ...>&&)’
     constexpr inline _Tp&& get(variant<_Types...>&& __v)
                            ^~~
/usr/include/c++/8.2.1/variant:890:28: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:900:33: note: candidate: ‘template<class _Tp, class ... _Types> constexpr const _Tp& std::get(const std::variant<_Types ...>&)’
     constexpr inline const _Tp& get(const variant<_Types...>& __v)
                                 ^~~
/usr/include/c++/8.2.1/variant:900:33: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:909:34: note: candidate: ‘template<class _Tp, class ... _Types> constexpr const _Tp&& std::get(const std::variant<_Types ...>&&)’
     constexpr inline const _Tp&& get(const variant<_Types...>&& __v)
                                  ^~~
/usr/include/c++/8.2.1/variant:909:34: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: error: no matching function for call to ‘get<0>(fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >)’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:216:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(std::pair<_Tp1, _Tp2>&)’
     get(std::pair<_Tp1, _Tp2>& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:216:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:221:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type&& std::get(std::pair<_Tp1, _Tp2>&&)’
     get(std::pair<_Tp1, _Tp2>&& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:221:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:226:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr const typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(const std::pair<_Tp1, _Tp2>&)’
     get(const std::pair<_Tp1, _Tp2>& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:226:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:231:5: note: candidate: ‘template<long unsigned int _Int, class _Tp1, class _Tp2> constexpr const typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type&& std::get(const std::pair<_Tp1, _Tp2>&&)’
     get(const std::pair<_Tp1, _Tp2>&& __in) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:231:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::pair<_Tp1, _Tp2>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8.2.1/variant:37,
                 from test/array.cpp:1:
/usr/include/c++/8.2.1/utility:240:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp& std::get(std::pair<_T1, _T2>&)’
     get(pair<_Tp, _Up>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:240:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:245:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp& std::get(const std::pair<_T1, _T2>&)’
     get(const pair<_Tp, _Up>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:245:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:250:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp&& std::get(std::pair<_T1, _T2>&&)’
     get(pair<_Tp, _Up>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:250:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:255:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp&& std::get(const std::pair<_T1, _T2>&&)’
     get(const pair<_Tp, _Up>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:255:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:260:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp& std::get(std::pair<_Up, _Tp>&)’
     get(pair<_Up, _Tp>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:260:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:265:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp& std::get(const std::pair<_Up, _Tp>&)’
     get(const pair<_Up, _Tp>& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:265:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:270:5: note: candidate: ‘template<class _Tp, class _Up> constexpr _Tp&& std::get(std::pair<_Up, _Tp>&&)’
     get(pair<_Up, _Tp>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:270:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/utility:275:5: note: candidate: ‘template<class _Tp, class _Up> constexpr const _Tp&& std::get(const std::pair<_Up, _Tp>&&)’
     get(const pair<_Up, _Tp>&& __p) noexcept
     ^~~
/usr/include/c++/8.2.1/utility:275:5: note:   template argument deduction/substitution failed:
In file included from test/array.cpp:1:
/usr/include/c++/8.2.1/variant:1329:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >& std::get(std::variant<_Types ...>&)’
     get(variant<_Types...>& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1329:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1340:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >&& std::get(std::variant<_Types ...>&&)’
     get(variant<_Types...>&& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1340:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1351:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >& std::get(const std::variant<_Types ...>&)’
     get(const variant<_Types...>& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1351:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:1362:5: note: candidate: ‘template<long unsigned int _Np, class ... _Types> constexpr std::variant_alternative_t<_Np, std::variant<_Types ...> >&& std::get(const std::variant<_Types ...>&&)’
     get(const variant<_Types...>&& __v)
     ^~~
/usr/include/c++/8.2.1/variant:1362:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1380:17: note:   ‘fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ is not derived from ‘const std::variant<_Types ...>’
      std::get<0>(std::forward<_Variants>(__variants))...));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8.2.1/variant:881:27: note: candidate: ‘template<class _Tp, class ... _Types> constexpr _Tp& std::get(std::variant<_Types ...>&)’
     constexpr inline _Tp& get(variant<_Types...>& __v)
                           ^~~
/usr/include/c++/8.2.1/variant:881:27: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:890:28: note: candidate: ‘template<class _Tp, class ... _Types> constexpr _Tp&& std::get(std::variant<_Types ...>&&)’
     constexpr inline _Tp&& get(variant<_Types...>&& __v)
                            ^~~
/usr/include/c++/8.2.1/variant:890:28: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:900:33: note: candidate: ‘template<class _Tp, class ... _Types> constexpr const _Tp& std::get(const std::variant<_Types ...>&)’
     constexpr inline const _Tp& get(const variant<_Types...>& __v)
                                 ^~~
/usr/include/c++/8.2.1/variant:900:33: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:909:34: note: candidate: ‘template<class _Tp, class ... _Types> constexpr const _Tp&& std::get(const std::variant<_Types ...>&&)’
     constexpr inline const _Tp&& get(const variant<_Types...>&& __v)
                                  ^~~
/usr/include/c++/8.2.1/variant:909:34: note:   template argument deduction/substitution failed:
/usr/include/c++/8.2.1/variant:1385:55: error: ‘class fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >’ has no member named ‘index’
       auto __func_ptr = __vtable._M_access(__variants.index()...);
                                            ~~~~~~~~~~~^~~~~
In file included from test/array.cpp:2:
include/fmt/format.h: In instantiation of ‘decltype (ctx.out()) fmt::v5::formatter<T, Char, typename std::enable_if<fmt::v5::internal::format_type<typename fmt::v5::buffer_context<Char>::type, T>::value>::type>::format(const T&, FormatContext&) [with FormatContext = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; T = fmt::v5::basic_string_view<char>; Char = char; typename std::enable_if<fmt::v5::internal::format_type<typename fmt::v5::buffer_context<Char>::type, T>::value>::type = void; decltype (ctx.out()) = std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >]’:
test/array.cpp:16:53:   required from ‘auto fmt::v5::formatter<foo>::format(const foo&, FormatContext&) [with FormatContext = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>]’
include/fmt/core.h:580:5:   required from ‘static void fmt::v5::internal::value<Context>::format_custom_arg(const void*, Context&) [with T = foo; Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>]’
include/fmt/core.h:563:19:   required from ‘fmt::v5::internal::value<Context>::value(const T&) [with T = foo; Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>]’
include/fmt/core.h:591:58:   required from ‘constexpr fmt::v5::internal::init<Context, T, TYPE>::operator fmt::v5::internal::value<Context>() const [with Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; T = const foo&; fmt::v5::internal::type TYPE = (fmt::v5::internal::type)13]’
include/fmt/core.h:995:35:   required from ‘typename std::enable_if<IS_PACKED, fmt::v5::internal::value<Context> >::type fmt::v5::internal::make_arg(const T&) [with bool IS_PACKED = true; Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; T = foo; typename std::enable_if<IS_PACKED, fmt::v5::internal::value<Context> >::type = fmt::v5::internal::value<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >]’
include/fmt/core.h:1053:51:   required from ‘fmt::v5::format_arg_store<Context, Args>::format_arg_store(const Args& ...) [with Context = fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>; Args = {foo}]’
include/fmt/core.h:1383:40:   required from ‘std::__cxx11::basic_string<typename fmt::v5::internal::format_string_traits<S>::char_type> fmt::v5::format(const String&, const Args& ...) [with String = char [3]; Args = {foo}; typename fmt::v5::internal::format_string_traits<S>::char_type = char]’
test/array.cpp:22:28:   required from here
include/fmt/format.h:3234:17: error: could not convert ‘std::visit<fmt::v5::arg_formatter<fmt::v5::output_range<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >, fmt::v5::basic_format_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> > >(fmt::v5::arg_formatter<fmt::v5::output_range<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >((* & ctx), (&((fmt::v5::formatter<fmt::v5::basic_string_view<char> >*)this)->fmt::v5::formatter<fmt::v5::basic_string_view<char> >::specs_.fmt::v5::internal::dynamic_format_specs<char>::<anonymous>)), fmt::v5::internal::make_arg<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>, fmt::v5::basic_string_view<char> >((* & val)))’ from ‘void’ to ‘fmt::v5::internal::context_base<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>, char>::iterator’ {aka ‘std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >’}
     return visit(arg_formatter<range_type>(ctx, &specs_),
            ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  internal::make_arg<FormatContext>(val));
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[2]    5708 exit 1     LANG=C g++ test/array.cpp -I include -DFMT_HEADER_ONLY=1 -std=c++17

[vitaut]

Should be fixed in 36c8d66. Thanks for reporting!

[jcelerier]

ah, thanks for looking into it. I closed it because I noticed that on my machine I had two concurrent fmt, one in my repo and one in /usr/include installed by some package - removing the /usr/include one fixed my problems.