Hi,
I'm using VS2022 17.10.0 with the latest MSVC toolset.
For setup, I created a windows c++ console app, then copied proxy.h to my working directory and enabled c++20 standard.
And I included proxy.h and sstream in my source, also added your example code inside the "quick start" section.

After a while, my editor reported an error of the line
p.Draw(result); // Polymorphic call
with

no instance of function template matches the argument list
argument types are: (std::stringstream)
object type is: pro::proxy<spec::Drawable>

However, the code do compile and run with no error, and there is no error message with the call to Area().
result << ", area = " << p.Area(); // Polymorphic call <-no error reported

I am not sure if you are in charge of this bug here because I believe it is more likely an intellisense bug. Any suggestions or workarounds would be appreciated.

Thanks.