struct validate_set
{
    std::string_view key;
    span<std::string_view> allowed_values;
};

constexpr std::array allowedValues{
    validate_set{ "foo"sv, { "bar"sv, "baz"sv } },
    validate_set{ "hello"sv, { "world"sv } }
};

constexpr std::array allowedValues{
    validate_set{ "foo"sv, "barbaz"sv },
    validate_set{ "hello"sv, "world"sv }
};

constexpr std::initializer_list list0 = { "bar"sv, "baz"sv };
constexpr std::initializer_list list1 = { "world"sv };
constexpr std::array allowedValues{
    validate_set{ "foo"sv, list0 },
    validate_set{ "hello"sv, list1 }
};