template <typename S>
concept Socket = requires(S socket, std::uint8_t* p, std::size_t n) {
   { socket.receive(p, n) } -> std::size_t;
   { socket.receive } -> std::size_t(std::uint8_t*, std::size_t);
};

struct UdpSocket {
   std::size_t receive(std::uint8_t* p, std::size_t n);
};

struct IoVec {
   // a bunch of buffers here.
};

struct TcpSocket {
   std::size_t receive(std::uint8_t* p, std::size_t n);
   std::size_t receive(IoVec& io);
};

//static_assert(Socket);
static_assert(Socket);

template <typename S> requires Socket
std::size_t receive_at_least(S& socket, std::uint8_t* p, std::size_t n, std::size_t nmax) {
   assert(n <= nmax);
   std::size_t received = 0;

   while (received < n) {
       received += socket.receive(p + received, nmax - received);
   }

   return received;
}

std::size_t foo(UdpSocket& s);
std::size_t foo(UdpSocket& s) {
   std::uint8_t buffer[4096];
   return receive_at_least(s, buffer, 2048, sizeof(buffer));
}

template <typename S>
concept Socket = requires(S socket, std::uint8_t* p, std::size_t n) {
   { socket.receive(p, n) } -> std::size_t;
};

struct TcpSocket {
   std::size_t receive(std::uint8_t* p, std::size_t n);
   std::size_t receive(std::span<std::byte> buffer);
};

static_assert(Socket);

template <typename S>
concept Socket = requires(S socket, std::uint8_t* p, std::size_t n) {
   { socket.receive(p, n) } -> std::same_as<std::size_t>;
};

struct TcpSocket {
   std::size_t receive(std::uint8_t* p, std::size_t n);
   std::size_t receive(std::span<std::byte> buffer);
};

static_assert(Socket);

template <typename S>
concept Socket = requires(S socket, std::uint8_t* p, std::size_t n) {
   { socket.receive(p, n) } -> std::same_as<std::size_t>;
   { &S::receive } -> std::same_as<std::size_t (S::*)(std::uint8_t*, std::size_t)>;
};

struct UdpSocket {
   std::size_t receive(std::uint8_t* p, std::uint8_t n);
};

struct UdpSocketCorrect {
   std::size_t receive(std::uint8_t* p, std::size_t n);
};

struct TcpSocket {
   std::size_t receive(std::uint8_t* p, std::size_t n);
   std::size_t receive(std::span<std::byte> buffer);
};

static_assert(!Socket);
static_assert( Socket);
static_assert( Socket);