how to convert Concurrency::streams::istream to std::istream in c++

I was coding with CppRESTsdk and got Concurrency::streams::istream from the response body like the following code:

web::http::http_response localVarResponse;
auto concurrencyStream = localVarResponse.body();

I want to convert the stream to std::istream and use it inside std::shared_ptr so that I can pass it to other classes like this:

std::istream stdStream = concurrencyStream;
auto istream_ptr = std::make_shared<std::istream>(stdStream);   

But I got errors:

 E0312  no suitable user-defined conversion from 
    "Concurrency::streams::istream" to "std::istream" exists

How can I convert to std::istream?

EDIT: indirectly via std::streambuf it works.

std::string str = localVarResponse.content_ready().get().extract_utf8string(true).get();
std::istringstream SStream(str);
std::istream inputStream(SStream.rdbuf());