should stringbuf positioning fail after the sequence is fully read?

[Pavel]

Should the code below print 1 or -1? I think -1, but the standard library
implementations I tried printed 1; who is right and why?

Thanks in advance,
-Pavel

#include <iostream>
#include <sstream>

using namespace std;

int main(int, char*[]) {
const string s(1, 'A');
istringstream iss(s);
char c;
iss >> c;
cout << iss.rdbuf()->pubseekoff(0, ios_base::cur, ios_base::in) << endl;
return 0;
}