to_string_view bug with suggested fix (related to issue #1282)

[adnsv]

Custom type stringers defined with to_string_view fail to compile:

The source for this problem is in <core.h>:

template <typename S, typename = void> struct char_t_impl {};
template <typename S> struct char_t_impl<S, enable_if_t<is_string<S>::value>> {
  using result = decltype(to_string_view(std::declval<S>()));
  using type = typename result::char_type;
};

In this code, result::char_type could be replaced with result::value_type, or better yet with result::traits_type::char_type.

Here is a temporary workaround that I'm using:

namespace NS {

struct MyStruct{
...
string_view sv() const {...}
...
};

inline std::string_view to_string_view(const MyStruct& v)
{
    return {v.sv()};
}
}; // namespace NS

// somewhere in the root namespace...
template <> struct fmt::internal::char_t_impl<NS::MyType> {
    using result = decltype(to_string_view(std::declval<NS::MyType>()));
    using type = typename result::traits_type::char_type;
};

[vitaut]

Good catch, thanks! Fixed in b55ea58.