diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -757,6 +757,20 @@
 // Just so we can migrate to the new macros gradually.
 #define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
 
+// Selectively hide functions from ABI for either STABLE or UNSTABLE ABI.
+// These macros allow ABI breakage for UNSTABLE while preserving STABLE ABI.
+#ifdef _LIBCPP_ABI_UNSTABLE
+#define _LIBCPP_HIDE_FROM_ABI_IN_STABLE
+#define _LIBCPP_INLINE_IN_STABLE
+#define _LIBCPP_HIDE_FROM_ABI_IN_UNSTABLE _LIBCPP_HIDE_FROM_ABI
+#define _LIBCPP_INLINE_IN_UNSTABLE inline
+#else
+#define _LIBCPP_HIDE_FROM_ABI_IN_STABLE _LIBCPP_HIDE_FROM_ABI
+#define _LIBCPP_INLINE_IN_STABLE inline
+#define _LIBCPP_HIDE_FROM_ABI_IN_UNSTABLE
+#define _LIBCPP_INLINE_IN_UNSTABLE
+#endif
+
 // Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
 #define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_ABI_NAMESPACE {
 #define _LIBCPP_END_NAMESPACE_STD  } }
diff --git a/libcxx/include/memory b/libcxx/include/memory
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -2167,6 +2167,9 @@
 };
 #endif
 
+// Tag used to default initialize one or both of the pair's elements.
+struct __default_value_tag {};
+
 template <class _Tp, int _Idx,
           bool _CanBeEmptyBase =
               is_empty<_Tp>::value && !__libcpp_is_final<_Tp>::value>
@@ -2177,6 +2180,7 @@
 
 #ifndef _LIBCPP_CXX03_LANG
   _LIBCPP_INLINE_VISIBILITY constexpr __compressed_pair_elem() : __value_() {}
+  _LIBCPP_INLINE_VISIBILITY constexpr __compressed_pair_elem(__default_value_tag) {}
 
   template <class _Up, class = typename enable_if<
       !is_same<__compressed_pair_elem, typename decay<_Up>::type>::value
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -796,7 +796,9 @@
         _NOEXCEPT;
 #endif
 
-    basic_string(const basic_string& __str);
+    _LIBCPP_HIDE_FROM_ABI_IN_UNSTABLE
+    basic_string(const basic_UNstring& __str);
+    _LIBCPP_HIDE_FROM_ABI_IN_UNSTABLE
     basic_string(const basic_string& __str, const allocator_type& __a);
 
 #ifndef _LIBCPP_CXX03_LANG
@@ -1543,6 +1545,9 @@
     inline
     void __init(size_type __n, value_type __c);
 
+    _LIBCPP_HIDE_FROM_ABI_IN_STABLE
+    void __init_long(const basic_string& __str);
+
     template <class _InputIterator>
     inline
     typename enable_if
@@ -1793,6 +1798,21 @@
     traits_type::assign(__p[__sz], value_type());
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_INLINE_IN_STABLE
+void basic_string<_CharT, _Traits, _Allocator>::__init_long(
+    const basic_string& __str) {
+  const value_type* __s = _VSTD::__to_raw_pointer(__str.__get_long_pointer());
+  size_type __sz = __str.__get_long_size();
+  size_type __cap = __recommend(__sz);
+  pointer __p = __alloc_traits::allocate(__alloc(), __cap + 1);
+  __set_long_pointer(__p);
+  __set_long_cap(__cap + 1);
+  __set_long_size(__sz);
+  traits_type::copy(_VSTD::__to_raw_pointer(__p), __s, __sz);
+  traits_type::assign(__p[__sz], value_type());
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 template <class>
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, const _Allocator& __a)
@@ -1829,27 +1849,29 @@
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_INLINE_IN_UNSTABLE
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str)
-    : __r_(__second_tag(), __alloc_traits::select_on_container_copy_construction(__str.__alloc()))
+    : __r_(__default_value_tag(), __alloc_traits::select_on_container_copy_construction(__str.__alloc()))
 {
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
-        __init(_VSTD::__to_raw_pointer(__str.__get_long_pointer()), __str.__get_long_size());
+        __init_long(__str);
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_INLINE_IN_UNSTABLE
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     const basic_string& __str, const allocator_type& __a)
-    : __r_(__second_tag(), __a)
+    : __r_(__default_value_tag(), __a)
 {
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
-        __init(_VSTD::__to_raw_pointer(__str.__get_long_pointer()), __str.__get_long_size());
+        __init_long(__str);
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif