Another batch of P0202 constepr algirithms. remove/remove_if/remove_copy/remove_copy_if/reverse_copy, and tests (commented out) for rotate_copy, because that depends on std::copy

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323152 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/algorithms/alg.modifying.operations/alg.remove/remove_copy_if.pass.cpp

@@ -12,16 +12,31 @@
 // template<InputIterator InIter, OutputIterator<auto, InIter::reference> OutIter,
 //          Predicate<auto, InIter::value_type> Pred>
 //   requires CopyConstructible<Pred>
-//   OutIter
+//   constexpr OutIter         // constexpr after C++17
 //   remove_copy_if(InIter first, InIter last, OutIter result, Pred pred);
 
 #include <algorithm>
 #include <functional>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
-bool equalToTwo(int v) { return v == 2; }
+TEST_CONSTEXPR bool equalToTwo(int v) { return v == 2; }
+
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+    int ia[] = {1, 3, 5, 2, 5, 6};
+    int ib[std::size(ia)] = {0};
+    
+    auto it = std::remove_copy_if(std::begin(ia), std::end(ia), std::begin(ib), equalToTwo);
+
+    return std::distance(std::begin(ib), it) == (std::size(ia) - 1)   // we removed one element
+        && std::none_of(std::begin(ib), it, equalToTwo)
+        && std::all_of (it, std::end(ib), [](int a) {return a == 0;})
+           ;
+    }
+#endif
 
 template <class InIter, class OutIter>
 void
@@ -72,4 +87,8 @@ 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
 }