In C++, << has higher precedence than &&, so in effect what is being done is

    (cout << 1) && 2;

...which is not an error, since the result of operator<< is an std::ostream, which is convertible to bool. Therefore operator&&(bool, bool) is invoked but the result is, of course, discarded.

Disambiguating the expression with parentheses should output 1 in every case.