Support binding positional arguments to named arguments

[espakm]

I get a compiler error when trying to use named arguments in the format string of a log message.

To reproduce:

quill::Logger* logger = quill::Frontend::create_or_get_logger(
    "root", quill::Frontend::create_or_get_sink<quill::ConsoleSink>("sink_id_1"));
std::string whoever = "you";
LOG_INFO(logger, "Hello, {}!", whoever);                              // GOOD
fmt::println("Hello, {whoever}!", fmt::arg("whoever", whoever));      // GOOD
LOG_INFO(logger, "Hello, {whoever}!", fmt::arg("whoever", whoever));  // BAD

The error:

In file included from /Users/miklos.espak/.conan/data/quill/8.0.0/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/quill/Frontend.h:9:
In file included from /Users/miklos.espak/.conan/data/quill/8.0.0/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/quill/Logger.h:11:
/Users/miklos.espak/.conan/data/quill/8.0.0/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/quill/core/Codec.h:62:3: error: static assertion failed due to requirement 'always_false_v<fmt::detail::named_arg<char, std::string>>': 
+------------------------------------------------------------------------------+
|                       Missing Codec for Type 'Arg'                           |
+------------------------------------------------------------------------------+

Error: A codec for the specified type 'Arg' is not available.

Possible solutions:
1. If Arg is an STL type:
   - Ensure you have included the necessary headers for the specific STL type you are using from the quill/std folder.

2. If Arg is a user-defined type:
   - Define a custom Codec for your type.
   - Consider passing the value as a string instead.

Note: The specific type of 'Arg' can be found in the compiler error message.
      Look for the instantiation of 'codec_not_found_for_type<Arg>' in the error output.
      The compiler should indicate what type 'Arg' represents in this instance.

For more information see https://quillcpp.readthedocs.io/en/latest/cheat_sheet.html

  static_assert(
  ^
/Users/miklos.espak/.conan/data/quill/8.0.0/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/quill/core/Codec.h:141:15: note: in instantiation of function template specialization 'quill::detail::codec_not_found_for_type<fmt::detail::named_arg<char, std::string>>' requested here
      detail::codec_not_found_for_type<Arg>();
              ^
/Users/miklos.espak/.conan/data/quill/8.0.0/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/quill/core/Codec.h:300:47: note: in instantiation of member function 'quill::Codec<fmt::detail::named_arg<char, std::string>>::compute_encoded_size' requested here
  ((total_sum += Codec<remove_cvref_t<Args>>::compute_encoded_size(conditional_arg_size_cache, args)), ...);
                                              ^
/Users/miklos.espak/.conan/data/quill/8.0.0/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/quill/Logger.h:128:15: note: in instantiation of function template specialization 'quill::detail::compute_encoded_size_and_cache_string_lengths<fmt::detail::named_arg<char, std::string>>' requested here
      detail::compute_encoded_size_and_cache_string_lengths(
              ^

I want to use named args in a code generator that generates log functions. (It used to generate ones that use Boost::logging, but would like to switch them over to Quill.)

It would be nice to have this working out of the box, but some hints how to get around it by writing a custom codec (for fmt::detail::named_arg, I guess), would also be much approciated.

[odygrd]

Hey, libfmt named arguments are not supported in the same way as in libfmt. The reason is because quill is internally using them also for switching to the json structured level logging mode. for example

There is a restriction on the order. You need to provide all arguments in order, then in the format string you can specify a {name}. Then each name from the format string is mapped automatically to the corresponding argument in order.

By default they are replaced in the log message. But you can also print them by adding '%(named_args)` to the pattern formatter.

See this example https://godbolt.org/z/xYb96Wv37

[espakm]

I see, thanks for the clarification.
It is, actually, useful to print the named args by '%(named_args)', thanks for showing that.

The two features do not seem to be conflicting to me, there could also be quill::arg() to bind an actual (positional) argument to a named argument with an underlying codec.

For this, the title of this issue could be changed to "Support binding positional arguments to named arguments".

But that's fine, I can also change my code generator to rearrange the args. So this can be closed as well.