error: no template named 'adl_serializer';

[mobergmann]

I have this code to serialize the lambda_strategy_t type.

#include <variant>

#include <nlohmann/json.hpp>

namespace tef::optimizers
{

struct lambda_strategy
{
    struct direct
    {
        double val;
    };
 
    struct mlu_threshold
    {
        double val;
    };

    lambda_strategy() = delete;
};

using lambda_strategy_t = std::variant<lambda_strategy::direct, lambda_strategy::mlu_threshold>;

namespace nlohmann
{
template <>
struct adl_serializer<lambda_strategy_t>
{
    static void to_json(json &j, const lambda_strategy_t &strat)
    {
        j = nlohmann::json{};
        j["direct"] = nullptr;
        j["mlu_threshold"] = nullptr;

        const auto direct = std::get_if<lambda_strategy::direct>(&strat);
        const auto mlu_threshold = std::get_if<lambda_strategy::mlu_threshold>(&strat);
        // either direct or mlu_threshold can be set in a variant
        if (direct)
        {
            j["direct"] = direct->val;
            j["mlu_threshold"] = nullptr;
        }
        if (mlu_threshold)
        {
            j["direct"] = nullptr;
            j["mlu_threshold"] = mlu_threshold->val;
        }
    }

    static void from_json(const json &j, lambda_strategy_t &strat)
    {
        const auto &direct = j.at("direct");
        const auto &mlu_threshold = j.at("mlu_threshold");

        // ensure, that either direct or mlu_threshold is set, because we are using a variant
        if (direct == nullptr and mlu_threshold == nullptr)
        {
            throw std::logic_error("either 'direct' or 'mlu_threshold' has to be set");
        }
        else if (direct != nullptr and mlu_threshold != nullptr)
        {
            throw std::logic_error("only 'direct' or 'mlu_threshold' can be set");
        }
        else if (direct != nullptr)
        {
            strat = lambda_strategy::direct{direct};
        }
        else // if (mlu_threshold != nullptr)
        {
            strat = lambda_strategy::mlu_threshold{mlu_threshold};
        }
    }
};
} // namespace nlohmann
} // namespace tef::optimizers

But I can't compile the project, I get the error:

error: no template named 'adl_serializer'; did you mean '::nlohmann::adl_serializer'?
struct adl_serializer<lambda_strategy_t>
       ^~~~~~~~~~~~~~
       ::nlohmann::adl_serializer
dependencies/nlohmann_json/include/nlohmann/adl_serializer.hpp:22:8: note: '::nlohmann::adl_serializer' declared here
struct adl_serializer
       ^

I wan't to be able to use the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE macro for a struct, which has a lambda_strategy_t attribute.

[gregmarr]

namespace tef::optimizers
{
namespace nlohmann
{

The adl_serializer type is in ::nlohmann, you are trying to specialize it in ::tef::optimizers::nlohmann. You need to close your namespace first.