demo n3651 with macro's illustrating when "duplicate declaration" is needed (at least with clang3.5)

variable_templates.limits.max.cpp

//Purpose:
//  Test compiler implementation of
//  "variable templates" as described in:
/*
http://isocpp.org/files/papers/N3651.pdf
 */
//  and as commented on here:
/*
http://en.cppreference.com/w/Talk:cpp/language/variable_template
 */
//Usage:
//  if defined(USE_DUP_DECL):
//    the "duplicate declaration"
//    (the term used at top of p. 3 of  N3651)
//    outside the numeric_limits class template
//    *is* used.
//  else:
//    the "duplicate declaration"
//    outside the numeric_limits class template
//    *is not* used.
//
//  if defined(USE_REF):
//    the reference of variable in the variable template 
//    *is taken*
//    in the printT template function.
//  else:
//    the reference of variable in the variable template 
//    *is not taken*
//    in the printT template function.
//Result:
//  Using clang3.5 from:
/*
http://llvm.org/releases/3.5.0/clang+llvm-3.5.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz
 */
//  as it was on 2014-11-03:  
//    if !defined(USE_REF):
//      compiles & runs OK (USE_DUP_DECL doesn't matter).
//    else if defined(USE_REF) && defined(USE_DUP_DECL):
//      compiles & runs OK
//    else (defined(USE_REF) && !defined(USE_DUP_DECL)):
//      fails link with error message:
/*
undefined reference to `numeric_limits<int>::min'
 */
//Conclusion:
//  when defined(USE_REF), *must* provide "duplicate declaration".
//=======================================
//#define USE_DUP_DECL
template<typename T>
struct numeric_limits
{
  static const T min
  #if defined(USE_DUP_DECL)
    ;
  #else
    ={};
  #endif
};
#if defined(USE_DUP_DECL)
//The following is the "duplicate declaration"
//mentioned at the top of p. 3 of N3651.
template<typename T>
const T numeric_limits<T>::min = {};// definition of a static data member template
#endif
#include <iostream>
template<typename T>
void printT
( T
#define USE_REF
#ifdef USE_REF
  const&
#endif
  a_t
)
{
  std::cout<<a_t;
}
int main()
{
#if defined(USE_DUP_DECL)
  std::cout<<"defined(USE_DUPL_DECL)\n";
#else
  std::cout<<"!defined(USE_DUPL_DECL)\n";
#endif  
  std::cout<<"numeric_limits<int>::min=";
  printT(numeric_limits<int>::min);
  std::cout<<"\n";
  return 0;
}