numeric

#include <experimental/type_traits>

// N3913 Greatest common divisor and least common multiple.

namespace std
{
namespace experimental
{
inline namespace fundamentals_v2
{

#define __cpp_lib_experimental_gcd_lcm 201411

  //  A constexpr version of abs - this should become normative someday.
  template<typename _Tp>
    constexpr auto
    __abs(_Tp __i)
    -> std::enable_if_t<std::is_integral<_Tp>{}(), _Tp>
    { return __i <_Tp(0) ? -__i : __i; }

  template<typename _Mp, typename _Np = _Mp>
    using __common_int_t = std::enable_if_t<std::is_integral<_Mp>{}()
					 && std::is_integral<_Np>{}(),
					    std::common_type_t<_Mp, _Np>>;

  //  Greatest common divisor.
  template<typename _Mp, typename _Np>
    constexpr __common_int_t<_Mp, _Np>
    gcd(_Mp __m, _Np __n)
    { return __n == 0 ? __abs(__m) : gcd(__n, __abs(__m) % __abs(__n)); }

  //  Least common multiple.
  template<typename _Mp, typename _Np >
    constexpr __common_int_t<_Mp, _Np>
    lcm(_Mp __m, _Np __n)
    { return __m * __n == 0 ? 0 : (__abs(__m) / gcd(__m, __n)) * __abs(__n); }

} // inline namespace fundamentals_v2
} // namespace experimental
} // namespace std

/*

Using
  is_integral_v<_Tp>
in place of
  std::is_integral<_Mp>{}()
causes all sorts of errors:

In file included from test_lcm_gcd.cpp:6:0:
numeric:19:10: error: the value of 'std::experimental::fundamentals_v1::is_integral_v<_Mp>' is not usable in a constant expression
       && is_integral_v<_Np>,
          ^
In file included from numeric:2:0,
                 from test_lcm_gcd.cpp:6:
/home/ed/bin/include/c++/5.0.0/experimental/type_traits:61:18: note: 'std::experimental::fundamentals_v1::is_integral_v<_Mp>' used in its own initializer
   constexpr bool is_integral_v = is_integral<_Tp>::value;
                  ^
In file included from test_lcm_gcd.cpp:6:0:
numeric:20:37: error: the value of 'std::experimental::fundamentals_v1::is_integral_v<_Mp>' is not usable in a constant expression
          std::common_type_t<_Mp, _Np>>;
                                     ^
In file included from numeric:2:0,
                 from test_lcm_gcd.cpp:6:
/home/ed/bin/include/c++/5.0.0/experimental/type_traits:61:18: note: 'std::experimental::fundamentals_v1::is_integral_v<_Mp>' used in its own initializer
   constexpr bool is_integral_v = is_integral<_Tp>::value;
                  ^
In file included from test_lcm_gcd.cpp:6:0:
numeric:24:15: error: '__common_int_t' does not name a type
     constexpr __common_int_t<_Mp, _Np>
               ^
numeric:30:15: error: '__common_int_t' does not name a type
     constexpr __common_int_t<_Mp, _Np>
               ^
test_lcm_gcd.cpp: In function 'int main()':
test_lcm_gcd.cpp:13:55: error: 'gcd' was not declared in this scope
   std::cout << "gcd(234,123) = " << gcd(234ULL, 123ULL) << '\n';
                                                       ^
test_lcm_gcd.cpp:17:55: error: 'lcm' was not declared in this scope
   std::cout << "lcm(234,123) = " << lcm(234ULL, 123ULL) << '\n';

*/