File asio\impl\serial_port_base.ipp generates warning C4242 at line 511 due to conversion between unsigned int and BYTE. I have observed this when compiling with Visual Studio 2019 in an MFC project.

the original line:
storage.ByteSize = value_;

the addition of a static cast fixes the issue in my local copy of the source code.
storage.ByteSize = static_cast<BYTE>(value_);

Strictly speaking there is no run time bug, however this issue is troublesome for developers treating warnings as errors.