[libc++] Fix several double-moves in the code base

This patch hardens the "test iterators" we use to test algorithms by
ensuring that they don't get double-moved. As a result of this hardening,
the tests started reporting multiple failures where we would double-move
iterators, which are being fixed in this patch.

In particular:
- Fixed a double-move in pstl.partition
- Add coverage for begin()/end() in subrange tests
- Fix tests for ranges::ends_with and ranges::contains, which were incorrectly
  calling begin() twice on the same subrange containing non-copyable input
  iterators.

Fixes #100709

Author: ldionne

libcxx/include/__algorithm/partition.h

@@ -29,7 +29,7 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_ForwardIterator, _Forw
 __partition_impl(_ForwardIterator __first, _Sentinel __last, _Predicate __pred, forward_iterator_tag) {
   while (true) {
     if (__first == __last)
-      return std::make_pair(std::move(__first), std::move(__first));
+      return std::make_pair(__first, __first);
     if (!__pred(*__first))
       break;
     ++__first;

libcxx/include/__pstl/backends/default.h

@@ -163,7 +163,7 @@ struct __is_partitioned<__default_backend_tag, _ExecutionPolicy> {
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred) const noexcept {
     using _FindIfNot   = __dispatch<__find_if_not, __current_configuration, _ExecutionPolicy>;
-    auto __maybe_first = _FindIfNot()(__policy, std::move(__first), std::move(__last), __pred);
+    auto __maybe_first = _FindIfNot()(__policy, std::move(__first), __last, __pred);
     if (__maybe_first == nullopt)
       return nullopt;
 

libcxx/test/std/algorithms/alg.modifying.operations/alg.partitions/partition.pass.cpp

@@ -98,11 +98,13 @@ test()
 
 int main(int, char**)
 {
+    test<forward_iterator<int*> >();
     test<bidirectional_iterator<int*> >();
     test<random_access_iterator<int*> >();
     test<int*>();
 
 #if TEST_STD_VER >= 20
+    static_assert(test<forward_iterator<int*>>());
     static_assert(test<bidirectional_iterator<int*>>());
     static_assert(test<random_access_iterator<int*>>());
     static_assert(test<int*>());

libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains.pass.cpp

@@ -65,78 +65,84 @@ constexpr void test_iterators() {
   using ValueT = std::iter_value_t<Iter>;
   { // simple tests
     ValueT a[] = {1, 2, 3, 4, 5, 6};
-    auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 6)));
     {
+      auto whole                            = std::ranges::subrange(Iter(a), Sent(Iter(a + 6)));
       std::same_as<bool> decltype(auto) ret = std::ranges::contains(whole.begin(), whole.end(), 3);
       assert(ret);
     }
     {
+      auto whole                            = std::ranges::subrange(Iter(a), Sent(Iter(a + 6)));
       std::same_as<bool> decltype(auto) ret = std::ranges::contains(whole, 3);
       assert(ret);
     }
   }
 
   { // check that a range with a single element works
     ValueT a[] = {32};
-    auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 1)));
     {
-      bool ret = std::ranges::contains(whole.begin(), whole.end(), 32);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 1)));
+      bool ret   = std::ranges::contains(whole.begin(), whole.end(), 32);
       assert(ret);
     }
     {
-      bool ret = std::ranges::contains(whole, 32);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 1)));
+      bool ret   = std::ranges::contains(whole, 32);
       assert(ret);
     }
   }
 
   { // check that an empty range works
     std::array<ValueT, 0> a = {};
-    auto whole              = std::ranges::subrange(Iter(a.data()), Sent(Iter(a.data())));
     {
-      bool ret = std::ranges::contains(whole.begin(), whole.end(), 1);
+      auto whole = std::ranges::subrange(Iter(a.data()), Sent(Iter(a.data())));
+      bool ret   = std::ranges::contains(whole.begin(), whole.end(), 1);
       assert(!ret);
     }
     {
-      bool ret = std::ranges::contains(whole, 1);
+      auto whole = std::ranges::subrange(Iter(a.data()), Sent(Iter(a.data())));
+      bool ret   = std::ranges::contains(whole, 1);
       assert(!ret);
     }
   }
 
   { // check that the first element matches
     ValueT a[] = {32, 3, 2, 1, 0, 23, 21, 9, 40, 100};
-    auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 10)));
     {
-      bool ret = std::ranges::contains(whole.begin(), whole.end(), 32);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 10)));
+      bool ret   = std::ranges::contains(whole.begin(), whole.end(), 32);
       assert(ret);
     }
     {
-      bool ret = std::ranges::contains(whole, 32);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 10)));
+      bool ret   = std::ranges::contains(whole, 32);
       assert(ret);
     }
   }
 
   { // check that the last element matches
     ValueT a[] = {3, 22, 1, 43, 99, 0, 56, 100, 32};
-    auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 9)));
     {
-      bool ret = std::ranges::contains(whole.begin(), whole.end(), 32);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 9)));
+      bool ret   = std::ranges::contains(whole.begin(), whole.end(), 32);
       assert(ret);
     }
     {
-      bool ret = std::ranges::contains(whole, 32);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 9)));
+      bool ret   = std::ranges::contains(whole, 32);
       assert(ret);
     }
   }
 
   { // no match
     ValueT a[] = {13, 1, 21, 4, 5};
-    auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
     {
-      bool ret = std::ranges::contains(whole.begin(), whole.end(), 10);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      bool ret   = std::ranges::contains(whole.begin(), whole.end(), 10);
       assert(!ret);
     }
     {
-      bool ret = std::ranges::contains(whole, 10);
+      auto whole = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      bool ret   = std::ranges::contains(whole, 10);
       assert(!ret);
     }
   }