[Bug c++/49812] New: strange return type for built-in operator++(int, int)

[frederic.bron at m4x dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49812

           Summary: strange return type for built-in operator++(int, int)
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: frederic.b...@m4x.org


The following code attempts to detect if operator++ returns void or not.
The program writes true (meaning that operator++ returns void) instead of
false.
msvc 10 and icpc 12.0 are right in writing false.

The standard states in 13.6/3 (Built-in operators):
"For every pair (T, VQ), where T is an arithmetic type, and VQ is
either volatile
or empty, there exist candidate operator functions of the form
VQ T& operator++(VQ T&);
T operator++(VQ T&, int);"

That means that for an "int volatile" variable the corresponding built-in
operator should be:
int operator++(int volatile &, int);
which should bind well with operator,(const int&, returns_void_t).

#include <iostream>
#include <iomanip>

namespace detail {
  typedef char yes_type;
  struct no_type { char padding[8]; };

  struct returns_void_t {};
  static yes_type returns_void(returns_void_t);
  static no_type returns_void(int);
}
template <typename T> int operator,(const T&, ::detail::returns_void_t);
template <typename T> int operator,(const volatile T&,
::detail::returns_void_t);

#define RETURNS_VOID(Expr)\
  sizeof(::detail::yes_type)\
  ==\
  sizeof(::detail::returns_void(((Expr), ::detail::returns_void_t())))

int main() {
  int volatile one_int_volatile;
  std::cout<<std::boolalpha<<"RETURNS_VOID:
"<<(RETURNS_VOID((one_int_volatile++)))<<'\n';
  return 0;
}