So I was trying out the custom serialization is simdjson and I could not get the example to compile in C++20 with clang or msvc.

The issue turns out to be that in the definition for serialize_tag it is declared to require custom_deserializable<T>. This is a problem since it means you cannot create a type that is serializable without also writing a custom deserializer, and also because the type in the concept does not use std::remove_cv it means the constness would have to match in the serializer.

The code, based on the example provided here illustrates the issue https://github.com/simdjson/simdjson/blob/master/doc/builder.md#example--string-builder,
as it does not compile in C++20 mode in MSVC or clang

#include <string>
#include <vector>
#include <stdint.h>
#include <simdjson.h>

struct Car {
	std::string make;
	std::string model;
	int64_t year;
	std::vector<float> tire_pressure;
};

namespace simdjson {

	template <typename builder_type>
	void tag_invoke(serialize_tag, builder_type& builder, const Car& car) {
		builder.start_object();
		builder.append_key_value("make", car.make);
		builder.append_comma();
		builder.append_key_value("model", car.model);
		builder.append_comma();
		builder.append_key_value("year", car.year);
		builder.append_comma();
		builder.append_key_value("tire_pressure", car.tire_pressure);
		builder.end_object();
	}
} // namespace simdjson

int main()
{
	simdjson::builder::string_builder output_json;

	Car c = { "Toyota", "Corolla", 2017, {30.0,30.2,30.513,30.79} };

	output_json.append_key_value("car", c);
}

msvc gives call to object of class type 'simdjson::serialize_tag': no matching call operator found

and clang gives no matching function for call to object of type 'const struct serialize_tag'

I am using the 4.2.2 release version on windows acquired through vcpkg

I think the requires clause from serialize_tag should be removed or replaced with a new custom_serializable concept