C++: When does std::future.set_exception() cause exception? - c++

Question

The following code will print 0 and ends:
// version 1
#include<future>
#include<iostream>
using namespace std;
int main(){
promise<int> p;
auto f = p.get_future();
p.set_exception(current_exception());
try {
cout << f.get() << endl;
} catch (const exception& e) {
cout << "caught future set exception\n";
}
return 0;
}
Well, seems p.set_exception(current_exception()); didn't cause any issue. But when I change it to be:
// version 2
#include<cassert>
#include<future>
#include<iostream>
using namespace std;
int main(){
promise<int> p;
auto f = p.get_future();
string msg = "test promise";
async([&] { // new code which version 1 didn't have
try {
throw runtime_error(msg);
} catch (...) {
cout << "caught exception 1\n";
try {
p.set_exception(current_exception());
} catch (...) {
cout << "caught exception 2\n";
}
}
}).wait(); // end new code
try {
cout << f.get() << endl;
} catch (const exception& e) {
cout << "caught future set exception\n";
assert(e.what() == msg);
}
return 0;
}
It will run and print:
caught exception 1
caught future set exception
Just wonder how/when does a std::promise fall into error/exception state so that its get_future().get() will throw exception?
What's the main reason of my version 2 code that p.set_exception() causes f.get() issue, while version 1 code has no problem?
Thanks.