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.pass.cpp

@@ -12,7 +12,7 @@
 // template<ForwardIterator Iter, class T>
 //   requires OutputIterator<Iter, RvalueOf<Iter::reference>::type>
 //         && HasEqualTo<Iter::value_type, T>
-//   Iter
+//   constexpr Iter         // constexpr after C++17
 //   remove(Iter first, Iter last, const T& value);
 
 #include <algorithm>
@@ -22,6 +22,18 @@
 #include "test_macros.h"
 #include "test_iterators.h"
 
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+    int ia[] = {1, 3, 5, 2, 5, 6};
+    
+    auto it = std::remove(std::begin(ia), std::end(ia), 5);
+
+    return (std::begin(ia) + std::size(ia) - 2) == it  // we removed two elements
+        && std::none_of(std::begin(ia), it, [](int a) {return a == 5; })
+           ;
+    }
+#endif
+
 template <class Iter>
 void
 test()
@@ -75,4 +87,8 @@ int main()
     test1<random_access_iterator<std::unique_ptr<int>*> >();
     test1<std::unique_ptr<int>*>();
 #endif // TEST_STD_VER >= 11
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }