I see there is fixes for correct type cast.

Previously within pack.hpp was declaraion:

void append_buffer(const char* buf, size_t len)
    { m_stream.write(buf, len); }

Now there is couple of compile time overloads to respect valid type cast when len parameter is not size_t. But, what if method ::write has overloads ? In this case &Stream::write will not be compiled (because compiler has ambiguity).

I have class Writer with 'primary' write method

virtual void write(const std::byte*, size_t size) = 0;

and second 'utility' method:

void write(const char* bytes, size_t size)
{
	write(reinterpret_cast<const std::byte*>(bytes), size);
}

Second method introduced especially to using with msgpack-c, but for first parameter compatibility. As temporary solution i remove second 'utility' method from my Writer class and 'fix' pack.hpp as

void append_buffer(const char* buf, size_t len)
{
    append_buffer(m_stream, &Stream::write, buf, len);
}

template<typename S, typename R, typename Ptr, typename Size>
static void append_buffer(S& s, R (S::*) (Ptr, Size), const char* buf, size_t len)
{
  s.write(reinterpret_cast<Ptr>(buf), static_cast<Size>(len));
}