gistfile1.txt

// [conv.lval]/1
// A glvalue (3.10) of a non-function, non-array type T can be converted to
// a prvalue. If T is an incomplete type, a program that necessitates this
// conversion is ill-formed. ...
//
// [expr.cond]/3, /5 and /6
//
// Note: [class.conv.fct] does not require the type specified in
//       conversion-type-id of conversion-function-id should be complete.

struct incomplete;

struct complete
{
  operator incomplete();
};

template<typename T>
T &&declval();

template<typename T, typename = decltype(false ? declval<complete &>() : declval<T &>())>
constexpr bool test(int)
{
  return true;
}

template<typename T>
constexpr bool test(void *)
{
  return false;
}

int main()
{
  static_assert(test<complete>(0), "");
  static_assert(!test<incomplete>(0), "");
}