[test] std::nth_element regression

[rupprecht]

Summary

This shows a test case that should pass, but fails, due to a regression in std::nth_element after D93557. The last two elements should be 6 and 7, but they are 7 and 6.

[ldionne]

Thanks a lot for finding this regression and reporting it. How did you find it?

I think this can be closed since we're going to go with D96084.

[rupprecht]

In D96074#2545092, @ldionne wrote:

Thanks a lot for finding this regression and reporting it. How did you find it?

We had 10-20 test failures internally that all bisected to this patch, and I logged the before/after of std::nth_element near the test failures. The values here are the relative ordering of the case that failed -- not the actual values, of course :)

I think this can be closed since we're going to go with D96084.

libcxx/test/std/algorithms/alg.sorting/alg.nth.element/nth_element.pass.cpp

TEST_CONSTEXPR_CXX20 bool test()
     }
 
     {
         T input[] = {3,1,4,1,5,9,2};
         std::nth_element(Iter(input), Iter(input+4), Iter(input+7));
         assert(input[4] == 4);
         assert(input[5] + input[6] == 5 + 9);
     }
+
+    {
+      T input[] = {0, 1, 2, 3, 4, 5, 7, 6};
+      std::nth_element(Iter(input), Iter(input + 6), Iter(input + 8));
+      assert(input[6] == 6);
+      assert(input[7] == 7);
+    }
     return true;
 }
 
 int main(int, char**)
 {
     test<int, random_access_iterator<int*> >();
     test<int, int*>();