Implement N3644 "Null Forward Pointers" in _LIBCPP_DEBUG mode.

[• Quuxplusone]

Summary

This functionality is already tested in std/containers/sequences/vector/iterators.pass.cpp.
It was not tested in string.iter/iterators.pass.cpp, so I've also added a test there (and refactored the original to reduce tedium). The added test fails before this patch.

list has debug iterators, but it's not random-access (so it has no </<= operators to be affected by this patch).
deque is random-access, but it doesn't have debug iterators (so it is unaffected by this patch).

This came, obliquely, out of D100866. Fixing this test now means I won't have to mark it XFAILed in debug mode because it no longer fails.

[Mordante]

LGTM!

[• Quuxplusone]

@ldionne ping for a second libc++ approval!

[ldionne]

Do you know why the test in std/containers/sequences/vector/iterators.pass.cpp didn't fail before this patch? Thanks for fixing.

[• Quuxplusone]

Do you know why the test in std/containers/sequences/vector/iterators.pass.cpp didn't fail before this patch?

IIUYQC, @ldionne, that test did fail... but only in debug_level=1 mode, which we don't buildkite yet. It is one of the tests I had to annotate in D100866 (but now I won't have to).

libcxx/src/debug.cpp

 {
 #ifndef _LIBCPP_HAS_NO_THREADS
     RLock _(mut());
 #endif
     __i_node* i = __find_iterator(__i);
     __i_node* j = __find_iterator(__j);
     __c_node* ci = i != nullptr ? i->__c_ : nullptr;
     __c_node* cj = j != nullptr ? j->__c_ : nullptr;
-    return ci != nullptr && ci == cj;
+    return ci == cj;
 }
 
 void
 __libcpp_db::swap(void* c1, void* c2)
 {
 #ifndef _LIBCPP_HAS_NO_THREADS
     WLock _(mut());
 #endif

libcxx/test/std/strings/basic.string/string.iterators/iterators.pass.cpp

 // const_iterator cbegin() const;
 // const_iterator cend()   const;
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+template<class C>
+void test()
 {
     { // N3644 testing
-        typedef std::string C;
-        C::iterator ii1{}, ii2{};
-        C::iterator ii4 = ii1;
-        C::const_iterator cii{};
+        typename C::iterator ii1{}, ii2{};
+        typename C::iterator ii4 = ii1;
+        typename C::const_iterator cii{};
         assert ( ii1 == ii2 );
         assert ( ii1 == ii4 );
-        assert ( ii1 == cii );
-        assert ( !(ii1 != ii2 ));
-        assert ( !(ii1 != cii ));
-    }
 
-    { // N3644 testing
-        typedef std::wstring C;
-        C::iterator ii1{}, ii2{};
-        C::iterator ii4 = ii1;
-        C::const_iterator cii{};
-        assert ( ii1 == ii2 );
-        assert ( ii1 == ii4 );
-        assert ( ii1 == cii );
-        assert ( !(ii1 != ii2 ));
-        assert ( !(ii1 != cii ));
-    }
+        assert (!(ii1 != ii2 ));
+
+        assert ( (ii1 == cii ));
+        assert ( (cii == ii1 ));
+        assert (!(ii1 != cii ));
+        assert (!(cii != ii1 ));
+        assert (!(ii1 <  cii ));
+        assert (!(cii <  ii1 ));
+        assert ( (ii1 <= cii ));
+        assert ( (cii <= ii1 ));
+        assert (!(ii1 >  cii ));
+        assert (!(cii >  ii1 ));
+        assert ( (ii1 >= cii ));
+        assert ( (cii >= ii1 ));
+        assert (cii - ii1 == 0);
+        assert (ii1 - cii == 0);
+    }
 
-#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
-    {
-        typedef std::u8string C;
-        C::iterator ii1{}, ii2{};
-        C::iterator ii4 = ii1;
-        C::const_iterator cii{};
-        assert ( ii1 == ii2 );
-        assert ( ii1 == ii4 );
-        assert ( ii1 == cii );
-        assert ( !(ii1 != ii2 ));
-        assert ( !(ii1 != cii ));
-    }
-#endif
+int main(int, char**)
+    test<std::string>();
+    test<std::wstring>();
 
-    { // N3644 testing
-        typedef std::u16string C;
-        C::iterator ii1{}, ii2{};
-        C::iterator ii4 = ii1;
-        C::const_iterator cii{};
-        assert ( ii1 == ii2 );
-        assert ( ii1 == ii4 );
-        assert ( ii1 == cii );
-        assert ( !(ii1 != ii2 ));
-        assert ( !(ii1 != cii ));
-    }
+#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
+    test<std::u8string>();
+#endif
 
-    { // N3644 testing
-        typedef std::u32string C;
-        C::iterator ii1{}, ii2{};
-        C::iterator ii4 = ii1;
-        C::const_iterator cii{};
-        assert ( ii1 == ii2 );
-        assert ( ii1 == ii4 );
-        assert ( ii1 == cii );
-        assert ( !(ii1 != ii2 ));
-        assert ( !(ii1 != cii ));
-    }
+    test<std::u16string>();
+    test<std::u32string>();
 
     return 0;
 }