Add (commented out) constexpr tests for copy/copy_backwards/copy_if/copy_n. These will be enabled when that part of P0202 is implemented. NFC at this time.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323137 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -10,14 +10,27 @@
 // <algorithm>
 
 // template<InputIterator InIter, OutputIterator<auto, InIter::reference> OutIter>
-//   OutIter
+//   constexpr OutIter   // constexpr after C++17
 //   copy(InIter first, InIter last, OutIter result);
 
 #include <algorithm>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
+// #if TEST_STD_VER > 17
+// TEST_CONSTEXPR bool test_constexpr() {
+//     int ia[] = {1, 2, 3, 4, 5};
+//     int ic[] = {6, 6, 6, 6, 6, 6, 6};
+// 
+// 	auto p = std::copy(std::begin(ia), std::end(ia), std::begin(ic));
+// 	return std::equal(std::begin(ia), std::end(ia), std::begin(ic), p)
+// 	    && std::all_of(p, std::end(ic), [](int a){return a == 6;})
+// 	    ;	    
+//     }
+// #endif
+
 template <class InIter, class OutIter>
 void
 test()
@@ -70,4 +83,8 @@ int main()
     test<const int*, bidirectional_iterator<int*> >();
     test<const int*, random_access_iterator<int*> >();
     test<const int*, int*>();
+
+// #if TEST_STD_VER > 17
+//     static_assert(test_constexpr());
+// #endif
 }