[Bug c++/49225] New: [C++0x] Weird SFINAE behavior with variadic templates

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49225

           Summary: [C++0x] Weird SFINAE behavior with variadic templates
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: paolo.carlini@oracle.com


I cannot understand why the below compiles, that is why the constructor isn't
disabled (whereas a static_assert in the body with the same expression would
fire if uncommented). Thanks in advance for any clarification!

//////////////////

template<typename _Tp, _Tp __v>
  struct integral_constant
  {
    static constexpr _Tp                  value = __v;
    typedef _Tp                           value_type;
    typedef integral_constant<_Tp, __v>   type;
    constexpr operator value_type() { return value; }
  };

typedef integral_constant<bool, true>     true_type;

typedef integral_constant<bool, false>    false_type;

template<typename _Tp, _Tp __v>
  constexpr _Tp integral_constant<_Tp, __v>::value;

template<bool, typename _Tp = void>
  struct enable_if 
  { };

template<typename _Tp>
  struct enable_if<true, _Tp>
  { typedef _Tp type; };

template<typename, typename>
  struct is_same
  : public false_type { };

template<typename _Tp>
  struct is_same<_Tp, _Tp>
  : public true_type { };

// Just bits of <type_traits> so far...

template<typename...>
  struct __my_and_;

template<typename _B1>
  struct __my_and_<_B1>
  : public _B1
  { };

template<typename... _Args1>
  struct Tt
  {
    template<typename... _Args2, typename = typename
         enable_if<__my_and_<is_same<_Args1, _Args2>...>::value>::type>
      Tt(_Args2...)
      {
    //static_assert(__my_and_<is_same<_Args1, _Args2>...>::value, "Error");
      }
  };

struct A { };

A a;

Tt<int> t(a); // The static_assert would trigger