C++ enum class std::size_t implicit conversion

There is no implicit conversion here. From enum:

There are no implicit conversions from the values of a scoped enumerator to integral types, although static_cast may be used to obtain the numeric value of the enumerator.

So, you have to use static_cast.

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There are some workarounds which are based on static_cast. For instance, one might make use of std::underlying_type:

template<typename T>
constexpr auto get_idx(T value)
{
    return static_cast<std::underlying_type_t<T>>(value);
}

And then:

const auto& key = std::get<get_idx(ParameterKey::KEY)>(*parameterPointer);

The whole purpose of enum class is to not be implicitly convertible to int, so there is no implicit conversion.

You could create your own get version:

template <ParameterKey key, typename Tuple>
decltype(auto) get(Tuple &&tuple) {
    return std::get<static_cast<std::underlying_type_t<ParameterKey>>(key)>(tuple);
}

Then:

const auto& key = get<ParameterKey::KEY>(*parameterPointer);