to_string_view interferes with dynamic_format_arg_store

[BillyDonahue]

The presence of a to_string_view(T) function (visible by ADL as usual) breaks dynamic_format_arg_store.push_back(T).

https://gcc.godbolt.org/z/j1xdje

The problem is probably just the use of detail::is_string<T> in the private stored_type<T> metafunction.
Anything with a to_string_view(T) will have a detail::is_string<T>::value == true.

https://github.com/fmtlib/fmt/blob/master/include/fmt/args.h#L97-L99

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

#include <iostream>
#include <string>
#include <string_view>

struct UserType {
  std::string name;
#ifdef HAS_TO_STRING_VIEW
  friend std::string_view to_string_view(const UserType& ut) { return ut.name; }
#endif
};

template <>
class fmt::formatter<UserType> : fmt::formatter<std::string_view> {
    using Base = fmt::formatter<std::string_view>;
public:
    using Base::parse;
    template <typename FC>
    auto format(const UserType& ut, FC&& fc) {
        return Base::format(ut.name,fc);
    }
};

void print(const std::string& s) { std::cout << s << std::endl; }

int main() {
  UserType ut{"Alice"};
  print(format(FMT_STRING("sanity check: {}"), ut));  // works
  {
    fmt::dynamic_format_arg_store<fmt::format_context> args;
    args.push_back(fmt::arg("userRef", ut));  // std::string(ut) fails
    print(vformat(FMT_STRING("named: {userRef}"), args));
  }
  {
    fmt::dynamic_format_arg_store<fmt::format_context> args;
    args.push_back(fmt::arg("userRef", std::cref(ut)));  // std::string(ut) fails
    print(vformat(FMT_STRING("named,cref: {userRef}"), args));
  }
}

When we build with -DHAS_TO_STRING_VIEW, we get a cascade of failure:
The salient part is here:

/opt/compiler-explorer/libs/fmt/trunk/include/fmt/args.h: In instantiation of 'constexpr fmt::v7::detail::dynamic_arg_list::typed_node<T>::typed_node(const Arg&) [with Arg = UserType; T = std::__cxx11::basic_string<char>]':
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/args.h:54:52:   required from 'const T& fmt::v7::detail::dynamic_arg_list::push(const Arg&) [with T = std::__cxx11::basic_string<char>; Arg = UserType]'
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/args.h:201:64:   required from 'void fmt::v7::dynamic_format_arg_store<Context>::push_back(const fmt::v7::detail::named_arg<typename Context::char_type, T>&) [with T = UserType; Context = fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char>; typename Context::char_type = char]'
<source>:33:43:   required from here
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/args.h:43:57: error: no matching function for call to 'std::__cxx11::basic_string<char>::basic_string(const UserType&)'
   43 |     FMT_CONSTEXPR typed_node(const Arg& arg) : value(arg) {}
      |                                                         ^

[vitaut]

Thanks for reporting. We should definitely fix this but is there any particular reason you define to_string_view for your struct?

[BillyDonahue]

IRL it's a legacy string_view-like type and we used the to_string_view overload to cheaply make it formattable without having to write a whole fmt::formatter. The formatter specialization in the repro here is just to show that the code compiles without the -DHAS_TO_STRING_VIEW.

When I upgraded from libfmt-6.1.1 to -7.1.3, I started using FMT_COMPILE and dynamic_format_arg_store and hitting this issue and #2181.

I've since written a fmt::formatter specialization for the type, to work around these issues.

[vitaut]

Fixed in 6ae402f.