Can a C++20 co-routine implement a synchronous interface without blocking?

Question

I am writing a GRPC service and am trying to use the asynchronous methods with the help from Asio. The service calls into a C++ library that have synchronous methods. The code in that library uses interfaces that must be implemented by the user of the library. These interfaces contains synchronous methods.

I wish to implement these interfaces by using asynchronous GRPC calls to other services. My challenge is that I cannot see how I can implement an adaptor between the synchronous - and asynchronous world. Is this at all possible in C++?

In (my) theory I want this co-routine adaptor to send the GRCP request and then the thread should continue executing other co-routines - and not be blocking - while waiting for the GRPC reply. When the reply is received the synchronous method call is returned to the library. This way I would be able to implement my GRPC service with only one thread and I do not have to worry about multi-threading issues.

When using co_await in a method then the return value is reflecting the async nature of the method, so I cannot use co_await (directly) when implementing a synchronous interface. Instead I can post a lambda containing the co_wait, but then I have to do a blocking wait on a future (or similar) and my single threaded service is deadlocked. I have been thinking of using co_yield and make a type of generator since it seems to be that the consumer of these generators can be synchronous.

Best regards