[libc++][test] Disable test for extension that's unsupportable in C++20

[CaseyCarter]

Summary

Defining the nested types reference and iterator_concept of reverse_iterator<I> necessarily requires I to be complete in C++20. These tests that verify that std::map<int, X>::reverse_iterator can be instantiated when X is incomplete are going to have a bad time.

[ldionne]

Could you add a short comment explaining the #if?

[CaseyCarter]

Review comments.

libcxx/test/std/containers/associative/map/map.cons/default_recursive.pass.cpp

 
 #include "test_macros.h"
 
 struct X
 {
     std::map<int, X> m;
     std::map<int, X>::iterator i;
     std::map<int, X>::const_iterator ci;
+#if TEST_STD_VER <= 17
+    // These reverse_iterator specializations require X to be complete in C++20.
     std::map<int, X>::reverse_iterator ri;
     std::map<int, X>::const_reverse_iterator cri;
+#endif // TEST_STD_VER <= 17
 };
 
-int main(int, char**)
-{
-
-  return 0;
-}
+int main(int, char**) { return 0; }

libcxx/test/std/containers/associative/multimap/multimap.cons/default_recursive.pass.cpp

 
 #include "test_macros.h"
 
 struct X
 {
     std::multimap<int, X> m;
     std::multimap<int, X>::iterator i;
     std::multimap<int, X>::const_iterator ci;
+#if TEST_STD_VER <= 17
+    // These reverse_iterator specializations require X to be complete in C++20.
     std::multimap<int, X>::reverse_iterator ri;
     std::multimap<int, X>::const_reverse_iterator cri;
+#endif // TEST_STD_VER <= 17
 };
 
-int main(int, char**)
-{
-
-  return 0;
-}
+int main(int, char**) { return 0; }