diff --git a/libcxx/include/__bit_reference b/libcxx/include/__bit_reference
--- a/libcxx/include/__bit_reference
+++ b/libcxx/include/__bit_reference
@@ -1238,7 +1238,7 @@
 
 private:
     _LIBCPP_INLINE_VISIBILITY
-    __bit_iterator(__storage_pointer __s, unsigned __ctz) _NOEXCEPT
+    explicit __bit_iterator(__storage_pointer __s, unsigned __ctz) _NOEXCEPT
         : __seg_(__s), __ctz_(__ctz) {}
 
     friend typename _Cp::__self;
diff --git a/libcxx/include/iterator b/libcxx/include/iterator
--- a/libcxx/include/iterator
+++ b/libcxx/include/iterator
@@ -1737,14 +1737,14 @@
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_IF_NODEBUG iterator_type base() const _NOEXCEPT {return __i;}
 
 private:
-#if _LIBCPP_DEBUG_LEVEL == 2
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_IF_NODEBUG __wrap_iter(const void* __p, iterator_type __x) : __i(__x)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_IF_NODEBUG explicit __wrap_iter(iterator_type __x, const void* __c) : __i(__x)
     {
-        __get_db()->__insert_ic(this, __p);
-    }
+#if _LIBCPP_DEBUG_LEVEL == 2
+        __get_db()->__insert_ic(this, __c);
 #else
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_IF_NODEBUG __wrap_iter(iterator_type __x) _NOEXCEPT : __i(__x) {}
+        (void)__c;
 #endif
+    }
 
     template <class _Up> friend class __wrap_iter;
     template <class _CharT, class _Traits, class _Alloc> friend class basic_string;
diff --git a/libcxx/include/list b/libcxx/include/list
--- a/libcxx/include/list
+++ b/libcxx/include/list
@@ -293,19 +293,16 @@
 
     __link_pointer __ptr_;
 
-#if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_INLINE_VISIBILITY
     explicit __list_iterator(__link_pointer __p, const void* __c) _NOEXCEPT
         : __ptr_(__p)
     {
+#if _LIBCPP_DEBUG_LEVEL == 2
         __get_db()->__insert_ic(this, __c);
-    }
 #else
-    _LIBCPP_INLINE_VISIBILITY
-    explicit __list_iterator(__link_pointer __p) _NOEXCEPT : __ptr_(__p) {}
+        (void)__c;
 #endif
-
-
+    }
 
     template<class, class> friend class list;
     template<class, class> friend class __list_imp;
@@ -416,17 +413,16 @@
 
     __link_pointer __ptr_;
 
-#if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_INLINE_VISIBILITY
     explicit __list_const_iterator(__link_pointer __p, const void* __c) _NOEXCEPT
         : __ptr_(__p)
     {
+#if _LIBCPP_DEBUG_LEVEL == 2
         __get_db()->__insert_ic(this, __c);
-    }
 #else
-    _LIBCPP_INLINE_VISIBILITY
-    explicit __list_const_iterator(__link_pointer __p) _NOEXCEPT : __ptr_(__p) {}
+        (void)__c;
 #endif
+    }
 
     template<class, class> friend class list;
     template<class, class> friend class __list_imp;
@@ -614,38 +610,22 @@
     _LIBCPP_INLINE_VISIBILITY
     iterator begin() _NOEXCEPT
     {
-#if _LIBCPP_DEBUG_LEVEL == 2
         return iterator(__end_.__next_, this);
-#else
-        return iterator(__end_.__next_);
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY
     const_iterator begin() const  _NOEXCEPT
     {
-#if _LIBCPP_DEBUG_LEVEL == 2
         return const_iterator(__end_.__next_, this);
-#else
-        return const_iterator(__end_.__next_);
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY
     iterator end() _NOEXCEPT
     {
-#if _LIBCPP_DEBUG_LEVEL == 2
         return iterator(__end_as_link(), this);
-#else
-        return iterator(__end_as_link());
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY
     const_iterator end() const _NOEXCEPT
     {
-#if _LIBCPP_DEBUG_LEVEL == 2
         return const_iterator(__end_as_link(), this);
-#else
-        return const_iterator(__end_as_link());
-#endif
     }
 
     void swap(__list_imp& __c)
@@ -1459,11 +1439,7 @@
     __node_alloc_traits::construct(__na, _VSTD::addressof(__hold->__value_), __x);
     __link_nodes(__p.__ptr_, __hold->__as_link(), __hold->__as_link());
     ++base::__sz();
-#if _LIBCPP_DEBUG_LEVEL == 2
     return iterator(__hold.release()->__as_link(), this);
-#else
-    return iterator(__hold.release()->__as_link());
-#endif
 }
 
 template <class _Tp, class _Alloc>
@@ -1474,10 +1450,8 @@
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__p) == this,
         "list::insert(iterator, n, x) called with an iterator not"
         " referring to this list");
-    iterator __r(__p.__ptr_, this);
-#else
-    iterator __r(__p.__ptr_);
 #endif
+    iterator __r(__p.__ptr_, this);
     if (__n > 0)
     {
         size_type __ds = 0;
@@ -1485,11 +1459,7 @@
         __hold_pointer __hold = __allocate_node(__na);
         __node_alloc_traits::construct(__na, _VSTD::addressof(__hold->__value_), __x);
         ++__ds;
-#if _LIBCPP_DEBUG_LEVEL == 2
         __r = iterator(__hold->__as_link(), this);
-#else
-        __r = iterator(__hold->__as_link());
-#endif
         __hold.release();
         iterator __e = __r;
 #ifndef _LIBCPP_NO_EXCEPTIONS
@@ -1515,11 +1485,7 @@
                 __node_alloc_traits::deallocate(__na, __e.__ptr_->__as_node(), 1);
                 if (__prev == 0)
                     break;
-#if _LIBCPP_DEBUG_LEVEL == 2
                 __e = iterator(__prev, this);
-#else
-                __e = iterator(__prev);
-#endif
             }
             throw;
         }
@@ -1540,10 +1506,8 @@
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__p) == this,
         "list::insert(iterator, range) called with an iterator not"
         " referring to this list");
-    iterator __r(__p.__ptr_, this);
-#else
-    iterator __r(__p.__ptr_);
 #endif
+    iterator __r(__p.__ptr_, this);
     if (__f != __l)
     {
         size_type __ds = 0;
@@ -1551,11 +1515,7 @@
         __hold_pointer __hold = __allocate_node(__na);
         __node_alloc_traits::construct(__na, _VSTD::addressof(__hold->__value_), *__f);
         ++__ds;
-#if _LIBCPP_DEBUG_LEVEL == 2
         __r = iterator(__hold.get()->__as_link(), this);
-#else
-        __r = iterator(__hold.get()->__as_link());
-#endif
         __hold.release();
         iterator __e = __r;
 #ifndef _LIBCPP_NO_EXCEPTIONS
@@ -1581,11 +1541,7 @@
                 __node_alloc_traits::deallocate(__na, __e.__ptr_->__as_node(), 1);
                 if (__prev == 0)
                     break;
-#if _LIBCPP_DEBUG_LEVEL == 2
                 __e = iterator(__prev, this);
-#else
-                __e = iterator(__prev);
-#endif
             }
             throw;
         }
@@ -1707,11 +1663,7 @@
     __link_nodes(__p.__ptr_, __nl, __nl);
     ++base::__sz();
     __hold.release();
-#if _LIBCPP_DEBUG_LEVEL == 2
     return iterator(__nl, this);
-#else
-    return iterator(__nl);
-#endif
 }
 
 template <class _Tp, class _Alloc>
@@ -1730,11 +1682,7 @@
     __link_nodes(__p.__ptr_, __nl, __nl);
     ++base::__sz();
     __hold.release();
-#if _LIBCPP_DEBUG_LEVEL == 2
     return iterator(__nl, this);
-#else
-    return iterator(__nl);
-#endif
 }
 
 #endif  // _LIBCPP_CXX03_LANG
@@ -1831,11 +1779,7 @@
     __node_pointer __np = __n->__as_node();
     __node_alloc_traits::destroy(__na, _VSTD::addressof(__np->__value_));
     __node_alloc_traits::deallocate(__na, __np, 1);
-#if _LIBCPP_DEBUG_LEVEL == 2
     return iterator(__r, this);
-#else
-    return iterator(__r);
-#endif
 }
 
 template <class _Tp, class _Alloc>
@@ -1879,11 +1823,7 @@
             __node_alloc_traits::deallocate(__na, __np, 1);
         }
     }
-#if _LIBCPP_DEBUG_LEVEL == 2
     return iterator(__l.__ptr_, this);
-#else
-    return iterator(__l.__ptr_);
-#endif
 }
 
 template <class _Tp, class _Alloc>
@@ -1900,11 +1840,7 @@
         __hold_pointer __hold = __allocate_node(__na);
         __node_alloc_traits::construct(__na, _VSTD::addressof(__hold->__value_));
         ++__ds;
-#if _LIBCPP_DEBUG_LEVEL == 2
         iterator __r = iterator(__hold.release()->__as_link(), this);
-#else
-        iterator __r = iterator(__hold.release()->__as_link());
-#endif
         iterator __e = __r;
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
@@ -1929,11 +1865,7 @@
                 __node_alloc_traits::deallocate(__na, __e.__ptr_->__as_node(), 1);
                 if (__prev == 0)
                     break;
-#if _LIBCPP_DEBUG_LEVEL == 2
                 __e = iterator(__prev, this);
-#else
-                __e = iterator(__prev);
-#endif
             }
             throw;
         }
@@ -1958,11 +1890,7 @@
         __node_alloc_traits::construct(__na, _VSTD::addressof(__hold->__value_), __x);
         ++__ds;
         __link_pointer __nl = __hold.release()->__as_link();
-#if _LIBCPP_DEBUG_LEVEL == 2
         iterator __r = iterator(__nl, this);
-#else
-        iterator __r = iterator(__nl);
-#endif
         iterator __e = __r;
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
@@ -1987,11 +1915,7 @@
                 __node_alloc_traits::deallocate(__na, __e.__ptr_->__as_node(), 1);
                 if (__prev == 0)
                     break;
-#if _LIBCPP_DEBUG_LEVEL == 2
                 __e = iterator(__prev, this);
-#else
-                __e = iterator(__prev);
-#endif
             }
             throw;
         }
diff --git a/libcxx/include/span b/libcxx/include/span
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -344,8 +344,8 @@
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
 // [span.iter], span iterator support
-    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
-    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data(), this); }
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size(), this); }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
 
@@ -357,7 +357,6 @@
 
 private:
     pointer    __data;
-
 };
 
 
@@ -503,8 +502,8 @@
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
 // [span.iter], span iterator support
-    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
-    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data(), this); }
+    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size(), this); }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
 
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -920,33 +920,18 @@
     _LIBCPP_INLINE_VISIBILITY basic_string& operator=(const value_type* __s) {return assign(__s);}
     basic_string& operator=(value_type __c);
 
-#if _LIBCPP_DEBUG_LEVEL == 2
-    _LIBCPP_INLINE_VISIBILITY
-    iterator begin() _NOEXCEPT
-        {return iterator(this, __get_pointer());}
-    _LIBCPP_INLINE_VISIBILITY
-    const_iterator begin() const _NOEXCEPT
-        {return const_iterator(this, __get_pointer());}
-    _LIBCPP_INLINE_VISIBILITY
-    iterator end() _NOEXCEPT
-        {return iterator(this, __get_pointer() + size());}
-    _LIBCPP_INLINE_VISIBILITY
-    const_iterator end() const _NOEXCEPT
-        {return const_iterator(this, __get_pointer() + size());}
-#else
     _LIBCPP_INLINE_VISIBILITY
     iterator begin() _NOEXCEPT
-        {return iterator(__get_pointer());}
+        {return iterator(__get_pointer(), this);}
     _LIBCPP_INLINE_VISIBILITY
     const_iterator begin() const _NOEXCEPT
-        {return const_iterator(__get_pointer());}
+        {return const_iterator(__get_pointer(), this);}
     _LIBCPP_INLINE_VISIBILITY
     iterator end() _NOEXCEPT
-        {return iterator(__get_pointer() + size());}
+        {return iterator(__get_pointer() + size(), this);}
     _LIBCPP_INLINE_VISIBILITY
     const_iterator end() const _NOEXCEPT
-        {return const_iterator(__get_pointer() + size());}
-#endif  // _LIBCPP_DEBUG_LEVEL == 2
+        {return const_iterator(__get_pointer() + size(), this);}
     _LIBCPP_INLINE_VISIBILITY
     reverse_iterator rbegin() _NOEXCEPT
         {return reverse_iterator(end());}
@@ -1780,11 +1765,7 @@
 template <class _CharT, class _Traits, class _Allocator>
 inline
 void
-basic_string<_CharT, _Traits, _Allocator>::__invalidate_iterators_past(size_type
-#if _LIBCPP_DEBUG_LEVEL == 2
-                                                                        __pos
-#endif
-                                                                      )
+basic_string<_CharT, _Traits, _Allocator>::__invalidate_iterators_past(size_type __pos)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     __c_node* __c = __get_db()->__find_c_and_lock(this);
@@ -1804,6 +1785,8 @@
         }
         __get_db()->unlock();
     }
+#else
+    (void)__pos;
 #endif  // _LIBCPP_DEBUG_LEVEL == 2
 }
 
diff --git a/libcxx/include/vector b/libcxx/include/vector
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -1491,11 +1491,7 @@
 typename vector<_Tp, _Allocator>::iterator
 vector<_Tp, _Allocator>::__make_iter(pointer __p) _NOEXCEPT
 {
-#if _LIBCPP_DEBUG_LEVEL == 2
-    return iterator(this, __p);
-#else
-    return iterator(__p);
-#endif
+    return iterator(__p, this);
 }
 
 template <class _Tp, class _Allocator>
@@ -1503,11 +1499,7 @@
 typename vector<_Tp, _Allocator>::const_iterator
 vector<_Tp, _Allocator>::__make_iter(const_pointer __p) const _NOEXCEPT
 {
-#if _LIBCPP_DEBUG_LEVEL == 2
-    return const_iterator(this, __p);
-#else
-    return const_iterator(__p);
-#endif
+    return const_iterator(__p, this);
 }
 
 template <class _Tp, class _Allocator>