class InputArchive {

...

template <typename T>
void process ( T && t ) {
    emscripten_yield();
    // deserialize ...
}

template <typename T>
emscripten_coroutine async( T && t ) {
    // create type-erasure proxy that holds T
    return emscripten_coroutine_create( &coro_func, &proxy );
}

static void coro_func( void * arg ) {
    // cast arg to proxy
    proxy.process(); // proxy holds T argument, dispatch it to InputArchive::process
}

};


void deserialize() {
    Document doc;
    auto coroutine = archive.async( doc );

    while( emscripten_coroutine_next( coroutine ) != 0 ) {}
}