我目前正在尝试使用GCC 10.1.0的here所描述的新的C++20特性，称为模块，但是如果我试图构建下面的代码片段，编译器会给我一堆错误。
这是我到目前为止写的片段：

// helloworld.cpp
export module helloworld;  // module declaration
import <iostream>;         // import declaration
 
export void hello() {      // export declaration
    std::cout << "Hello world!\n";
}

// main.cpp
import helloworld;  // import declaration
 
int main() {
    hello();
}

我使用g++ helloworld.cpp main.cpp -std=c++20编译它。
编译器给了我这个错误：

helloworld.cpp:2:1: warning: keyword ‘export’ not implemented, and will be ignored
    2 | export module helloworld;  // module declaration
      | ^~~~~~
helloworld.cpp:2:8: error: ‘module’ does not name a type
    2 | export module helloworld;  // module declaration
      |        ^~~~~~
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
    3 | import <iostream>;         // import declaration
      |         ^~~~~~~~
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:9: error: ‘iostream’ was not declared in this scope
helloworld.cpp:3:1: error: ‘import’ does not name a type
    3 | import <iostream>;         // import declaration
      | ^~~~~~
helloworld.cpp:5:1: warning: keyword ‘export’ not implemented, and will be ignored
    5 | export void hello() {      // export declaration
      | ^~~~~~
helloworld.cpp: In function ‘void hello()’:
helloworld.cpp:6:10: error: ‘cout’ is not a member of ‘std’
    6 |     std::cout << "Hello world!\n";
      |          ^~~~
helloworld.cpp:1:1: note: ‘std::cout’ is defined in header ‘<iostream>’; did you forget to ‘#include <iostream>’?
  +++ |+#include <iostream>
    1 | // helloworld.cpp
main.cpp:2:1: error: ‘import’ does not name a type
    2 | import helloworld;  // import declaration
      | ^~~~~~
main.cpp: In function ‘int main()’:
main.cpp:5:5: error: ‘hello’ was not declared in this scope
    5 |     hello();
      |     ^~~~~

我做错了什么？