"{}"_format(x) does not see to_string_view(x) anymore

[BillyDonahue]

I'd expect this to work, and it does in libfmt 5.3.0.
https://gcc.godbolt.org/z/kVkM09 (clang-8.0, libfmt-5.3.0)

It is broken in trunk libfmt:
https://gcc.godbolt.org/z/mNHi1D (clang-8.0, libfmt-trunk)

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

namespace u {
struct X {};
fmt::string_view to_string_view(const X&) { return "abc"; }
}  // namespace u

std::string func(const u::X& x) {
    return fmt::format("X:{}", x);
}

std::string lit(const u::X& x) {
    using namespace fmt::literals;
    return "X:{}"_format(x);
}

Only the _format literal is broken. The fmt::format call is ok in both cases.

In file included from <source>:2:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:2115:7: error: call to deleted constructor of 'conditional_t<has_formatter<X, buffer_context<char_type> >::value, formatter<u::X, char_type>, internal::fallback_formatter<X, char_type> >' (aka 'fmt::v5::internal::fallback_formatter<u::X, char, void>')
      f;
      ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:2126:23: note: in instantiation of function template specialization 'fmt::v5::internal::parse_format_specs<u::X, fmt::v5::basic_parse_context<char, fmt::v5::internal::error_handler> >' requested here
        parse_funcs_{&parse_format_specs<Args, parse_context_type>...} {}
                      ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:2173:54: note: in instantiation of member function 'fmt::v5::internal::format_string_checker<char, fmt::v5::internal::error_handler, u::X>::format_string_checker' requested here
  format_string_checker<Char, ErrorHandler, Args...> checker(s, eh);
                                                     ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:3539:9: note: in instantiation of function template specialization 'fmt::v5::internal::do_check_format_string<char, fmt::v5::internal::error_handler, u::X>' requested here
        do_check_format_string<Char, error_handler, Args...>(
        ^
<source>:15:25: note: in instantiation of function template specialization 'fmt::v5::internal::udl_formatter<char, 'X', ':', '{', '}'>::operator()<u::X>' requested here
    return "X:{}"_format(x);
                        ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:602:3: note: 'fallback_formatter' has been explicitly marked deleted here
  fallback_formatter() = delete;
  ^
1 error generated.
Compiler returned: 1

[vitaut]

Fixed in 4912cff. Thanks for reporting!