Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -23,12 +23,22 @@
 
 #define _LIBCPP_VERSION 3800
 
-#define _LIBCPP_ABI_VERSION 1
+#ifndef _LIBCPP_ABI_MAJOR_VERSION
+#define _LIBCPP_ABI_MAJOR_VERSION 1
+#endif
+
+#ifndef _LIBCPP_ABI_MINOR_VERSION
+#define _LIBCPP_ABI_MINOR_VERSION 0
+#endif
+
+#if _LIBCPP_ABI_MAJOR_VERSION >= 2
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#endif
 
 #define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y
 #define _LIBCPP_CONCAT(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y)
 
-#define _LIBCPP_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_VERSION)
+#define _LIBCPP_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_MAJOR_VERSION)
 
 
 #ifndef __has_attribute
@@ -227,9 +237,10 @@
 
 #if defined(__clang__)
 
-#if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&        \
-    !defined(__arm__)
-#define _LIBCPP_ALTERNATE_STRING_LAYOUT
+#if (defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&       \
+     !defined(__arm__)) ||                                                     \
+    defined(_LIBCPP_ALTERNATE_STRING_LAYOUT)
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 #endif
 
 #if __has_feature(cxx_alignas)
Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -1185,7 +1185,7 @@
 #pragma warning( pop )
 #endif // _LIBCPP_MSVC
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
 template <class _CharT, size_t = sizeof(_CharT)>
 struct __padding
@@ -1198,7 +1198,7 @@
 {
 };
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
 template<class _CharT, class _Traits, class _Allocator>
 class _LIBCPP_TYPE_VIS_ONLY basic_string
@@ -1234,7 +1234,7 @@
 
 private:
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     struct __long
     {
@@ -1294,7 +1294,7 @@
         value_type __data_[__min_cap];
     };
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     union __ulx{__long __lx; __short __lxx;};
 
@@ -1697,7 +1697,7 @@
     const allocator_type& __alloc() const _NOEXCEPT
         {return __r_.second();}
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT
@@ -1715,7 +1715,7 @@
         {return __r_.first().__s.__size_;}
 #   endif
 
-#else  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#else  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT
@@ -1733,7 +1733,7 @@
         {return __r_.first().__s.__size_ >> 1;}
 #   endif
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_long_size(size_type __s) _NOEXCEPT