diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -931,9 +931,11 @@
         {return __is_long() ? __get_long_size() : __get_short_size();}
     _LIBCPP_INLINE_VISIBILITY size_type length() const _NOEXCEPT {return size();}
     _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY size_type capacity() const _NOEXCEPT
-        {return (__is_long() ? __get_long_cap()
-                             : static_cast<size_type>(__min_cap)) - 1;}
+
+    _LIBCPP_HIDE_FROM_ABI size_type capacity() const _NOEXCEPT {
+        // We store the capacity including the null-terminator, but capacity() does not include the null-terminator.
+        return (__is_long() ? __get_long_cap() : static_cast<size_type>(__min_cap)) - 1;
+    }
 
     void resize(size_type __n, value_type __c);
     _LIBCPP_INLINE_VISIBILITY void resize(size_type __n) {resize(__n, value_type());}
@@ -1491,6 +1493,7 @@
     void __set_size(size_type __s) _NOEXCEPT
         {if (__is_long()) __set_long_size(__s); else __set_short_size(__s);}
 
+    // Note that the capacity we store always includes the null-terminator.
     _LIBCPP_INLINE_VISIBILITY
     void __set_long_cap(size_type __s) _NOEXCEPT
         {__r_.first().__l.__cap_  = __long_mask | __s;}
@@ -1533,15 +1536,19 @@
         size_type __align_it(size_type __s) _NOEXCEPT
             {return (__s + (__a-1)) & ~(__a-1);}
     enum {__alignment = 16};
+
+    // Returns the recommended capacity to store a string of size __s, *including* the null-terminator
+    // Note that, unlike __recommend(), basic_string::capacity() returns the capacity *excluding* the null-terminator
     static _LIBCPP_INLINE_VISIBILITY
     size_type __recommend(size_type __s) _NOEXCEPT
-        {
-        if (__s < __min_cap) return static_cast<size_type>(__min_cap) - 1;
-        size_type __guess = __align_it<sizeof(value_type) < __alignment ?
-                     __alignment/sizeof(value_type) : 1 > (__s+1) - 1;
-        if (__guess == __min_cap) ++__guess;
+    {
+        if (__s < __min_cap)
+            return static_cast<size_type>(__min_cap);
+        size_type __guess = __align_it<sizeof(value_type) < __alignment ? __alignment / sizeof(value_type) : 1>(__s + 1);
+        if (__guess == __min_cap + 1)
+            ++__guess;
         return __guess;
-        }
+    }
 
     inline
     void __init(const value_type* __s, size_type __sz, size_type __reserve);
@@ -1834,9 +1841,9 @@
     else
     {
         size_type __cap = __recommend(__reserve);
-        __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __p = __alloc_traits::allocate(__alloc(), __cap);
         __set_long_pointer(__p);
-        __set_long_cap(__cap+1);
+        __set_long_cap(__cap);
         __set_long_size(__sz);
     }
     traits_type::copy(_VSTD::__to_address(__p), __s, __sz);
@@ -1858,9 +1865,9 @@
     else
     {
         size_type __cap = __recommend(__sz);
-        __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __p = __alloc_traits::allocate(__alloc(), __cap);
         __set_long_pointer(__p);
-        __set_long_cap(__cap+1);
+        __set_long_cap(__cap);
         __set_long_size(__sz);
     }
     traits_type::copy(_VSTD::__to_address(__p), __s, __sz);
@@ -1933,9 +1940,9 @@
     if (__sz > max_size())
       __throw_length_error();
     size_t __cap = __recommend(__sz);
-    __p = __alloc_traits::allocate(__alloc(), __cap + 1);
+    __p = __alloc_traits::allocate(__alloc(), __cap);
     __set_long_pointer(__p);
-    __set_long_cap(__cap + 1);
+    __set_long_cap(__cap);
     __set_long_size(__sz);
   }
   traits_type::copy(_VSTD::__to_address(__p), __s, __sz + 1);
@@ -1997,9 +2004,9 @@
     else
     {
         size_type __cap = __recommend(__n);
-        __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __p = __alloc_traits::allocate(__alloc(), __cap);
         __set_long_pointer(__p);
-        __set_long_cap(__cap+1);
+        __set_long_cap(__cap);
         __set_long_size(__n);
     }
     traits_type::assign(_VSTD::__to_address(__p), __n, __c);
@@ -2128,9 +2135,9 @@
     else
     {
         size_type __cap = __recommend(__sz);
-        __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __p = __alloc_traits::allocate(__alloc(), __cap);
         __set_long_pointer(__p);
-        __set_long_cap(__cap+1);
+        __set_long_cap(__cap);
         __set_long_size(__sz);
     }
 
@@ -2222,7 +2229,7 @@
     size_type __cap = __old_cap < __ms / 2 - __alignment ?
                           __recommend(_VSTD::max(__old_cap + __delta_cap, 2 * __old_cap)) :
                           __ms - 1;
-    pointer __p = __alloc_traits::allocate(__alloc(), __cap+1);
+    pointer __p = __alloc_traits::allocate(__alloc(), __cap);
     __invalidate_all_iterators();
     if (__n_copy != 0)
         traits_type::copy(_VSTD::__to_address(__p),
@@ -2236,7 +2243,7 @@
     if (__old_cap+1 != __min_cap)
         __alloc_traits::deallocate(__alloc(), __old_p, __old_cap+1);
     __set_long_pointer(__p);
-    __set_long_cap(__cap+1);
+    __set_long_cap(__cap);
     __old_sz = __n_copy + __n_add + __sec_cp_sz;
     __set_long_size(__old_sz);
     traits_type::assign(__p[__old_sz], value_type());
@@ -2254,7 +2261,7 @@
     size_type __cap = __old_cap < __ms / 2 - __alignment ?
                           __recommend(_VSTD::max(__old_cap + __delta_cap, 2 * __old_cap)) :
                           __ms - 1;
-    pointer __p = __alloc_traits::allocate(__alloc(), __cap+1);
+    pointer __p = __alloc_traits::allocate(__alloc(), __cap);
     __invalidate_all_iterators();
     if (__n_copy != 0)
         traits_type::copy(_VSTD::__to_address(__p),
@@ -2267,7 +2274,7 @@
     if (__old_cap+1 != __min_cap)
         __alloc_traits::deallocate(__alloc(), __old_p, __old_cap+1);
     __set_long_pointer(__p);
-    __set_long_cap(__cap+1);
+    __set_long_cap(__cap);
 }
 
 // assign
@@ -3236,7 +3243,8 @@
 
     size_type __target_capacity = _VSTD::max(__requested_capacity, size());
     __target_capacity = __recommend(__target_capacity);
-    if (__target_capacity == capacity()) return;
+    if (__target_capacity == capacity() + 1) // __recommend() includes the null-terminator, while capacity() doesn't
+        return;
 
     __shrink_or_extend(__target_capacity);
 }
@@ -3247,7 +3255,8 @@
 basic_string<_CharT, _Traits, _Allocator>::shrink_to_fit() _NOEXCEPT
 {
     size_type __target_capacity = __recommend(size());
-    if (__target_capacity == capacity()) return;
+    if (__target_capacity == capacity() + 1) // __recommend() includes the null-terminator, while capacity() doesn't
+        return;
 
     __shrink_or_extend(__target_capacity);
 }
@@ -3262,7 +3271,7 @@
 
     pointer __new_data, __p;
     bool __was_long, __now_long;
-    if (__target_capacity == __min_cap - 1)
+    if (__target_capacity == __min_cap)
     {
         __was_long = true;
         __now_long = false;
@@ -3272,14 +3281,14 @@
     else
     {
         if (__target_capacity > __cap)
-            __new_data = __alloc_traits::allocate(__alloc(), __target_capacity+1);
+            __new_data = __alloc_traits::allocate(__alloc(), __target_capacity);
         else
         {
         #ifndef _LIBCPP_NO_EXCEPTIONS
             try
             {
         #endif // _LIBCPP_NO_EXCEPTIONS
-                __new_data = __alloc_traits::allocate(__alloc(), __target_capacity+1);
+                __new_data = __alloc_traits::allocate(__alloc(), __target_capacity);
         #ifndef _LIBCPP_NO_EXCEPTIONS
             }
             catch (...)
@@ -3301,7 +3310,7 @@
         __alloc_traits::deallocate(__alloc(), __p, __cap+1);
     if (__now_long)
     {
-        __set_long_cap(__target_capacity+1);
+        __set_long_cap(__target_capacity);
         __set_long_size(__sz);
         __set_long_pointer(__new_data);
     }