Commit eb9b0635 authored by Mark de Wever's avatar Mark de Wever
Browse files

[libc++] P1645 constexpr for <numeric> 

Implements P1645: constexpr for <numeric> algorithms

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D90569
parent a38d13ed

libcxx/docs/FeatureTestMacroTable.rst

+2 −0
Original line number Diff line number Diff line
@@ -196,6 +196,8 @@ Status 
    ------------------------------------------------- -----------------

    ``__cpp_lib_constexpr_misc``                      *unimplemented*

    ------------------------------------------------- -----------------

    ``__cpp_lib_constexpr_numeric``                   ``201911L``

    ------------------------------------------------- -----------------

    ``__cpp_lib_constexpr_swap_algorithms``           *unimplemented*

    ------------------------------------------------- -----------------

    ``__cpp_lib_constexpr_utility``                   ``201811L``

libcxx/include/numeric

+56 −46
Original line number Diff line number Diff line
@@ -17,115 +17,116 @@ namespace std 
{



template <class InputIterator, class T>

    T

    constexpr T  // constexpr since C++20

    accumulate(InputIterator first, InputIterator last, T init);



template <class InputIterator, class T, class BinaryOperation>

    T

    constexpr T  // constexpr since C++20

    accumulate(InputIterator first, InputIterator last, T init, BinaryOperation binary_op);



template<class InputIterator>

    typename iterator_traits<InputIterator>::value_type

    constexpr typename iterator_traits<InputIterator>::value_type  // constexpr since C++20

    reduce(InputIterator first, InputIterator last);  // C++17



template<class InputIterator, class T>

    T

    constexpr T  // constexpr since C++20

    reduce(InputIterator first, InputIterator last, T init);  // C++17



template<class InputIterator, class T, class BinaryOperation>

    T

    constexpr T  // constexpr since C++20

    reduce(InputIterator first, InputIterator last, T init, BinaryOperation binary_op);  // C++17



template <class InputIterator1, class InputIterator2, class T>

    T

    constexpr T  // constexpr since C++20

    inner_product(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, T init);



template <class InputIterator1, class InputIterator2, class T, class BinaryOperation1, class BinaryOperation2>

    T

    constexpr T  // constexpr since C++20

    inner_product(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2,

                  T init, BinaryOperation1 binary_op1, BinaryOperation2 binary_op2);





template<class InputIterator1, class InputIterator2, class T>

    T

    constexpr T  // constexpr since C++20

    transform_reduce(InputIterator1 first1, InputIterator1 last1,

                     InputIterator2 first2, T init);  // C++17



template<class InputIterator1, class InputIterator2, class T, class BinaryOperation1, class BinaryOperation2>

    T

    constexpr T  // constexpr since C++20

    transform_reduce(InputIterator1 first1, InputIterator1 last1,

                     InputIterator2 first2, T init,

                     BinaryOperation1 binary_op1, BinaryOperation2 binary_op2);  // C++17



template<class InputIterator, class T, class BinaryOperation, class UnaryOperation>

    T

    constexpr T  // constexpr since C++20

    transform_reduce(InputIterator first, InputIterator last, T init,

                     BinaryOperation binary_op, UnaryOperation unary_op);  // C++17



template <class InputIterator, class OutputIterator>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    partial_sum(InputIterator first, InputIterator last, OutputIterator result);



template <class InputIterator, class OutputIterator, class BinaryOperation>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    partial_sum(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op);



template<class InputIterator, class OutputIterator, class T>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    exclusive_scan(InputIterator first, InputIterator last,

                   OutputIterator result, T init); // C++17



template<class InputIterator, class OutputIterator, class T, class BinaryOperation>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    exclusive_scan(InputIterator first, InputIterator last,

                   OutputIterator result, T init, BinaryOperation binary_op); // C++17



template<class InputIterator, class OutputIterator>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    inclusive_scan(InputIterator first, InputIterator last, OutputIterator result);  // C++17



template<class InputIterator, class OutputIterator, class BinaryOperation>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    inclusive_scan(InputIterator first, InputIterator last,

                   OutputIterator result, BinaryOperation binary_op);  // C++17



template<class InputIterator, class OutputIterator, class BinaryOperation, class T>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    inclusive_scan(InputIterator first, InputIterator last,

                   OutputIterator result, BinaryOperation binary_op, T init);  // C++17



template<class InputIterator, class OutputIterator, class T,

         class BinaryOperation, class UnaryOperation>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    transform_exclusive_scan(InputIterator first, InputIterator last,

                             OutputIterator result, T init,

                             BinaryOperation binary_op, UnaryOperation unary_op);  // C++17



template<class InputIterator, class OutputIterator,

         class BinaryOperation, class UnaryOperation>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    transform_inclusive_scan(InputIterator first, InputIterator last,

                             OutputIterator result,

                             BinaryOperation binary_op, UnaryOperation unary_op);  // C++17



template<class InputIterator, class OutputIterator,

         class BinaryOperation, class UnaryOperation, class T>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    transform_inclusive_scan(InputIterator first, InputIterator last,

                             OutputIterator result,

                             BinaryOperation binary_op, UnaryOperation unary_op,

                             T init);  // C++17



template <class InputIterator, class OutputIterator>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    adjacent_difference(InputIterator first, InputIterator last, OutputIterator result);



template <class InputIterator, class OutputIterator, class BinaryOperation>

    OutputIterator

    constexpr OutputIterator  // constexpr since C++20

    adjacent_difference(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op);



template <class ForwardIterator, class T>

    void iota(ForwardIterator first, ForwardIterator last, T value);

    constexpr void  // constexpr since C++20

    iota(ForwardIterator first, ForwardIterator last, T value);



template <class M, class N>

    constexpr common_type_t<M,N> gcd(M m, N n);    // C++17
@@ -133,9 +134,11 @@ template <class M, class N> 
template <class M, class N>

    constexpr common_type_t<M,N> lcm(M m, N n);    // C++17



integer         midpoint(integer a, integer b);                  // C++20

pointer         midpoint(pointer a, pointer b);                  // C++20

floating_point  midpoint(floating_point a, floating_point b);    // C++20

template<class T>

    constexpr T midpoint(T a, T b) noexcept;  // C++20



template<class T>

    constexpr T* midpoint(T* a, T* b);        // C++20



}  // std
@@ -158,7 +161,7 @@ _LIBCPP_PUSH_MACROS 
_LIBCPP_BEGIN_NAMESPACE_STD



template <class _InputIterator, class _Tp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

accumulate(_InputIterator __first, _InputIterator __last, _Tp __init)

{
@@ -168,7 +171,7 @@ accumulate(_InputIterator __first, _InputIterator __last, _Tp __init) 
}



template <class _InputIterator, class _Tp, class _BinaryOperation>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

accumulate(_InputIterator __first, _InputIterator __last, _Tp __init, _BinaryOperation __binary_op)

{
@@ -179,7 +182,7 @@ accumulate(_InputIterator __first, _InputIterator __last, _Tp __init, _BinaryOpe 


#if _LIBCPP_STD_VER > 14

template <class _InputIterator, class _Tp, class _BinaryOp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

reduce(_InputIterator __first, _InputIterator __last, _Tp __init, _BinaryOp __b)

{
@@ -189,7 +192,7 @@ reduce(_InputIterator __first, _InputIterator __last, _Tp __init, _BinaryOp __b) 
}



template <class _InputIterator, class _Tp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

reduce(_InputIterator __first, _InputIterator __last, _Tp __init)

{
@@ -197,7 +200,7 @@ reduce(_InputIterator __first, _InputIterator __last, _Tp __init) 
}



template <class _InputIterator>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

typename iterator_traits<_InputIterator>::value_type

reduce(_InputIterator __first, _InputIterator __last)

{
@@ -207,7 +210,7 @@ reduce(_InputIterator __first, _InputIterator __last) 
#endif



template <class _InputIterator1, class _InputIterator2, class _Tp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

inner_product(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __first2, _Tp __init)

{
@@ -217,7 +220,7 @@ inner_product(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 
}



template <class _InputIterator1, class _InputIterator2, class _Tp, class _BinaryOperation1, class _BinaryOperation2>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

inner_product(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __first2,

              _Tp __init, _BinaryOperation1 __binary_op1, _BinaryOperation2 __binary_op2)
@@ -229,7 +232,7 @@ inner_product(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 


#if _LIBCPP_STD_VER > 14

template <class _InputIterator, class _Tp, class _BinaryOp, class _UnaryOp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

transform_reduce(_InputIterator __first, _InputIterator __last,

           _Tp __init,  _BinaryOp __b, _UnaryOp __u)
@@ -241,7 +244,7 @@ transform_reduce(_InputIterator __first, _InputIterator __last, 


template <class _InputIterator1, class _InputIterator2,

          class _Tp, class _BinaryOp1, class _BinaryOp2>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1,

                 _InputIterator2 __first2, _Tp __init,  _BinaryOp1 __b1, _BinaryOp2 __b2)
@@ -252,7 +255,7 @@ transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1, 
}



template <class _InputIterator1, class _InputIterator2, class _Tp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_Tp

transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1,

                 _InputIterator2 __first2, _Tp __init)
@@ -263,7 +266,7 @@ transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1, 
#endif



template <class _InputIterator, class _OutputIterator>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

partial_sum(_InputIterator __first, _InputIterator __last, _OutputIterator __result)

{
@@ -281,7 +284,7 @@ partial_sum(_InputIterator __first, _InputIterator __last, _OutputIterator __res 
}



template <class _InputIterator, class _OutputIterator, class _BinaryOperation>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

partial_sum(_InputIterator __first, _InputIterator __last, _OutputIterator __result,

              _BinaryOperation __binary_op)
@@ -301,7 +304,7 @@ partial_sum(_InputIterator __first, _InputIterator __last, _OutputIterator __res 


#if _LIBCPP_STD_VER > 14

template <class _InputIterator, class _OutputIterator, class _Tp, class _BinaryOp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

exclusive_scan(_InputIterator __first, _InputIterator __last,

               _OutputIterator __result, _Tp __init, _BinaryOp __b)
@@ -321,7 +324,7 @@ exclusive_scan(_InputIterator __first, _InputIterator __last, 
}



template <class _InputIterator, class _OutputIterator, class _Tp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

exclusive_scan(_InputIterator __first, _InputIterator __last,

               _OutputIterator __result, _Tp __init)
@@ -330,6 +333,7 @@ exclusive_scan(_InputIterator __first, _InputIterator __last, 
}



template <class _InputIterator, class _OutputIterator, class _Tp, class _BinaryOp>

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last,

                               _OutputIterator __result, _BinaryOp __b,  _Tp __init)

{
@@ -341,6 +345,7 @@ _OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last, 
}



template <class _InputIterator, class _OutputIterator, class _BinaryOp>

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last,

                               _OutputIterator __result, _BinaryOp __b)

{
@@ -355,6 +360,7 @@ _OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last, 
}



template <class _InputIterator, class _OutputIterator>

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last,

                               _OutputIterator __result)

{
@@ -363,7 +369,7 @@ _OutputIterator inclusive_scan(_InputIterator __first, _InputIterator __last, 


template <class _InputIterator, class _OutputIterator, class _Tp,

          class _BinaryOp, class _UnaryOp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

transform_exclusive_scan(_InputIterator __first, _InputIterator __last,

                           _OutputIterator __result, _Tp __init,
@@ -384,7 +390,9 @@ transform_exclusive_scan(_InputIterator __first, _InputIterator __last, 
}



template <class _InputIterator, class _OutputIterator, class _Tp, class _BinaryOp, class _UnaryOp>

_OutputIterator transform_inclusive_scan(_InputIterator __first, _InputIterator __last,

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

transform_inclusive_scan(_InputIterator __first, _InputIterator __last,

                           _OutputIterator __result, _BinaryOp __b, _UnaryOp __u, _Tp __init)

{

    for (; __first != __last; ++__first, (void) ++__result) {
@@ -396,7 +404,9 @@ _OutputIterator transform_inclusive_scan(_InputIterator __first, _InputIterator 
}



template <class _InputIterator, class _OutputIterator, class _BinaryOp, class _UnaryOp>

_OutputIterator transform_inclusive_scan(_InputIterator __first, _InputIterator __last,

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

transform_inclusive_scan(_InputIterator __first, _InputIterator __last,

                               _OutputIterator __result, _BinaryOp __b, _UnaryOp __u)

{

    if (__first != __last) {
@@ -411,7 +421,7 @@ _OutputIterator transform_inclusive_scan(_InputIterator __first, _InputIterator 
#endif



template <class _InputIterator, class _OutputIterator>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

adjacent_difference(_InputIterator __first, _InputIterator __last, _OutputIterator __result)

{
@@ -430,7 +440,7 @@ adjacent_difference(_InputIterator __first, _InputIterator __last, _OutputIterat 
}



template <class _InputIterator, class _OutputIterator, class _BinaryOperation>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

_OutputIterator

adjacent_difference(_InputIterator __first, _InputIterator __last, _OutputIterator __result,

                      _BinaryOperation __binary_op)
@@ -450,7 +460,7 @@ adjacent_difference(_InputIterator __first, _InputIterator __last, _OutputIterat 
}



template <class _ForwardIterator, class _Tp>

inline _LIBCPP_INLINE_VISIBILITY

_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17

void

iota(_ForwardIterator __first, _ForwardIterator __last, _Tp __value_)

{

libcxx/include/version

+2 −0
Original line number Diff line number Diff line
@@ -48,6 +48,7 @@ __cpp_lib_concepts 201806L <concepts> 
__cpp_lib_constexpr_dynamic_alloc                       201907L <memory>

__cpp_lib_constexpr_misc                                201811L <array> <functional> <iterator>

                                                                <string_view> <tuple> <utility>

__cpp_lib_constexpr_numeric                             201911L <numeric>

__cpp_lib_constexpr_swap_algorithms                     201806L <algorithm>

__cpp_lib_constexpr_utility                             201811L <utility>

__cpp_lib_destroying_delete                             201806L <new>
@@ -254,6 +255,7 @@ __cpp_lib_void_t 201411L <type_traits> 
// # define __cpp_lib_concepts                             201806L

# define __cpp_lib_constexpr_dynamic_alloc              201907L

// # define __cpp_lib_constexpr_misc                       201811L

# define __cpp_lib_constexpr_numeric                    201911L

// # define __cpp_lib_constexpr_swap_algorithms            201806L

# define __cpp_lib_constexpr_utility                    201811L

# if _LIBCPP_STD_VER > 17 && defined(__cpp_impl_destroying_delete) && __cpp_impl_destroying_delete >= 201806L

libcxx/test/std/language.support/support.limits/support.limits.general/numeric.version.pass.cpp

+20 −0
Original line number Diff line number Diff line
@@ -14,6 +14,7 @@ 
// Test the feature test macros defined by <numeric>



/*  Constant                        Value

    __cpp_lib_constexpr_numeric     201911L [C++2a]

    __cpp_lib_gcd_lcm               201606L [C++17]

    __cpp_lib_interpolate           201902L [C++2a]

    __cpp_lib_parallel_algorithm    201603L [C++17]
@@ -24,6 +25,10 @@ 


#if TEST_STD_VER < 14



# ifdef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should not be defined before c++2a"

# endif



# ifdef __cpp_lib_gcd_lcm

#   error "__cpp_lib_gcd_lcm should not be defined before c++17"

# endif
@@ -38,6 +43,10 @@ 


#elif TEST_STD_VER == 14



# ifdef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should not be defined before c++2a"

# endif



# ifdef __cpp_lib_gcd_lcm

#   error "__cpp_lib_gcd_lcm should not be defined before c++17"

# endif
@@ -52,6 +61,10 @@ 


#elif TEST_STD_VER == 17



# ifdef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should not be defined before c++2a"

# endif



# ifndef __cpp_lib_gcd_lcm

#   error "__cpp_lib_gcd_lcm should be defined in c++17"

# endif
@@ -78,6 +91,13 @@ 


#elif TEST_STD_VER > 17



# ifndef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should be defined in c++2a"

# endif

# if __cpp_lib_constexpr_numeric != 201911L

#   error "__cpp_lib_constexpr_numeric should have the value 201911L in c++2a"

# endif



# ifndef __cpp_lib_gcd_lcm

#   error "__cpp_lib_gcd_lcm should be defined in c++2a"

# endif

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp

+20 −0
Original line number Diff line number Diff line
@@ -42,6 +42,7 @@ 
    __cpp_lib_concepts                             201806L [C++2a]

    __cpp_lib_constexpr_dynamic_alloc              201907L [C++2a]

    __cpp_lib_constexpr_misc                       201811L [C++2a]

    __cpp_lib_constexpr_numeric                    201911L [C++2a]

    __cpp_lib_constexpr_swap_algorithms            201806L [C++2a]

    __cpp_lib_constexpr_utility                    201811L [C++2a]

    __cpp_lib_destroying_delete                    201806L [C++2a]
@@ -227,6 +228,10 @@ 
#   error "__cpp_lib_constexpr_misc should not be defined before c++2a"

# endif



# ifdef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should not be defined before c++2a"

# endif



# ifdef __cpp_lib_constexpr_swap_algorithms

#   error "__cpp_lib_constexpr_swap_algorithms should not be defined before c++2a"

# endif
@@ -619,6 +624,10 @@ 
#   error "__cpp_lib_constexpr_misc should not be defined before c++2a"

# endif



# ifdef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should not be defined before c++2a"

# endif



# ifdef __cpp_lib_constexpr_swap_algorithms

#   error "__cpp_lib_constexpr_swap_algorithms should not be defined before c++2a"

# endif
@@ -1125,6 +1134,10 @@ 
#   error "__cpp_lib_constexpr_misc should not be defined before c++2a"

# endif



# ifdef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should not be defined before c++2a"

# endif



# ifdef __cpp_lib_constexpr_swap_algorithms

#   error "__cpp_lib_constexpr_swap_algorithms should not be defined before c++2a"

# endif
@@ -1910,6 +1923,13 @@ 
#   endif

# endif



# ifndef __cpp_lib_constexpr_numeric

#   error "__cpp_lib_constexpr_numeric should be defined in c++2a"

# endif

# if __cpp_lib_constexpr_numeric != 201911L

#   error "__cpp_lib_constexpr_numeric should have the value 201911L in c++2a"

# endif



# if !defined(_LIBCPP_VERSION)

#   ifndef __cpp_lib_constexpr_swap_algorithms

#     error "__cpp_lib_constexpr_swap_algorithms should be defined in c++2a"