Question

I have this code and GCC prints “what!?”. How can I avoid that, so that the void cast simply has the C meaning “Ignore the lonely ‘a;'”?

#include <iostream>

struct A {
  template<typename T>
  operator T() { 
    std::cout << "what!?";
  }
};

int main() {
  A a;
  (void)a;
}

Answer 1

As you’ve observed, this is a bug in gcc. The standard reads:

c++11

12.3.2 Conversion functions [class.conv.fct]

(1) A
conversion function is never used to convert a (possibly cv-qualified) object to […] (possibly cv-qualified) void.
116) A conversion to void does not invoke any conversion function (5.2.9).

5.2.9 Static cast [expr.static.cast]

(6) Any expression can be explicitly converted to type cv void, in which case it becomes a discarded-value
expression (Clause 5).

As a workaround, you could write:

a, void();

It’s impossible to overload operator,(void) so there is zero chance of this invoking user-defined behaviour from a conformant implementation.