This test should really have been a part of the sample tests.

tried it as well,
dont know why i still get the error message:

main.cpp:193:9: error: invalid operands to binary expression ('stringstream' (aka 'basic_stringstream') and 'X')
out << X();
~~~ ^ ~~~
/usr/include/c++/v1/ostream:191:20: note: candidate function not viable: no known conversion from 'X' to 'std::__1::basic_ostream &()(std::__1::basic_ostream &)' for 1st argument
basic_ostream& operator<<(basic_ostream& (__pf)(basic_ostream&));