[libc++] Fixed copy/copy_n/copy_backward for compilers that do not support is_constant_evaluated.

libcxx/include/__config

 #endif
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17 constexpr
 #else
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17
 #endif
 
+#if _LIBCPP_STD_VER > 17 && \
+    !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR) && \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+#  define _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED constexpr
+#else
+#  define _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
+#endif
+
 // The _LIBCPP_NODISCARD_ATTRIBUTE should only be used to define other
 // NODISCARD macros to the correct attribute.
 #if __has_cpp_attribute(nodiscard) || defined(_LIBCPP_COMPILER_MSVC)
 #  define _LIBCPP_NODISCARD_ATTRIBUTE [[nodiscard]]
 #elif defined(_LIBCPP_COMPILER_CLANG) && !defined(_LIBCPP_CXX03_LANG)
 #  define _LIBCPP_NODISCARD_ATTRIBUTE [[clang::warn_unused_result]]
 #else
 // We can't use GCC's [[gnu::warn_unused_result]] and

libcxx/include/algorithm

 {
     const size_t __n = static_cast<size_t>(__last - __first);
     if (__n > 0)
         _VSTD::memmove(__result, __first, __n * sizeof(_Up));
     return __result + __n;
 }
 
 template <class _InputIterator, class _OutputIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
 _OutputIterator
 copy(_InputIterator __first, _InputIterator __last, _OutputIterator __result)
 {
     if (__libcpp_is_constant_evaluated()) {
         return _VSTD::__copy_constexpr(
             __unwrap_iter(__first), __unwrap_iter(__last), __unwrap_iter(__result));
     } else {
         return _VSTD::__copy(
__copy_backward(_Tp* __first, _Tp* __last, _Up* __result)
     {
         __result -= __n;
         _VSTD::memmove(__result, __first, __n * sizeof(_Up));
     }
     return __result;
 }
 
 template <class _BidirectionalIterator1, class _BidirectionalIterator2>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
 _BidirectionalIterator2
 copy_backward(_BidirectionalIterator1 __first, _BidirectionalIterator1 __last,
               _BidirectionalIterator2 __result)
 {
     if (__libcpp_is_constant_evaluated()) {
         return _VSTD::__copy_backward_constexpr(__unwrap_iter(__first),
                                                 __unwrap_iter(__last),
                                                 __unwrap_iter(__result));
for (; __first != __last; ++__first)
         }
     }
     return __result;
 }
 
 // copy_n
 
 template<class _InputIterator, class _Size, class _OutputIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
 typename enable_if
 <
     __is_input_iterator<_InputIterator>::value &&
    !__is_random_access_iterator<_InputIterator>::value,
     _OutputIterator
 >::type
 copy_n(_InputIterator __first, _Size __orig_n, _OutputIterator __result)
 {
if (__n > 0)
             *__result = *__first;
             ++__result;
         }
     }
     return __result;
 }
 
 template<class _InputIterator, class _Size, class _OutputIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
 typename enable_if
 <
     __is_random_access_iterator<_InputIterator>::value,
     _OutputIterator
 >::type
 copy_n(_InputIterator __first, _Size __orig_n, _OutputIterator __result)
 {
     typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy.pass.cpp

test()
 
   return true;
 }
 
 int main(int, char**)
 {
     test();
 
-#if TEST_STD_VER > 17
+#if TEST_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
     static_assert(test());
 #endif
 
   return 0;
 }

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_backward.pass.cpp

test()
 
     return true;
 }
 
 int main(int, char**)
 {
     test();
 
-#if TEST_STD_VER > 17
+#if TEST_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
     static_assert(test());
 #endif
 
   return 0;
 }

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_n.pass.cpp

test()
 
   return true;
 }
 
 int main(int, char**)
 {
     test();
 
-#if TEST_STD_VER > 17
+#if TEST_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
     static_assert(test());
 #endif
 
   return 0;
 }