diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -638,6 +638,8 @@
 typedef basic_string<char32_t> u32string;
 #endif
 
+struct __capacity_tag {};
+
 template<class _CharT, class _Traits, class _Allocator>
 class
     _LIBCPP_TEMPLATE_VIS
@@ -762,6 +764,19 @@
 
     __compressed_pair<__rep, allocator_type> __r_;
 
+    _LIBCPP_HIDE_FROM_ABI basic_string(__capacity_tag, size_type __alloc_capacity, const allocator_type& __a)
+        : __r_(__default_init_tag(), __a) {
+        if (__fits_in_sso(__alloc_capacity)) {
+            __zero();
+        } else {
+            auto __capacity = __recommend(__alloc_capacity);
+            auto __allocation = __alloc_traits::allocate(__alloc(), __capacity + 1);
+            __set_long_cap(__capacity);
+            __set_long_pointer(__allocation);
+            __set_long_size(0);
+        }
+    }
+
 public:
     _LIBCPP_TEMPLATE_DATA_VIS
     static const size_type npos = -1;
@@ -4165,10 +4180,12 @@
           const basic_string<_CharT, _Traits, _Allocator>& __rhs)
 {
     using _String = basic_string<_CharT, _Traits, _Allocator>;
-    _String __r(_String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
     typename _String::size_type __lhs_sz = __lhs.size();
     typename _String::size_type __rhs_sz = __rhs.size();
-    __r.__init(__lhs.data(), __lhs_sz, __lhs_sz + __rhs_sz);
+    _String __r(__capacity_tag(),
+                __lhs_sz + __rhs_sz,
+                _String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
+    __r.append(__lhs.data(), __lhs_sz);
     __r.append(__rhs.data(), __rhs_sz);
     return __r;
 }
@@ -4178,10 +4195,12 @@
 operator+(const _CharT* __lhs , const basic_string<_CharT,_Traits,_Allocator>& __rhs)
 {
     using _String = basic_string<_CharT, _Traits, _Allocator>;
-    _String __r(_String::__alloc_traits::select_on_container_copy_construction(__rhs.get_allocator()));
     typename _String::size_type __lhs_sz = _Traits::length(__lhs);
     typename _String::size_type __rhs_sz = __rhs.size();
-    __r.__init(__lhs, __lhs_sz, __lhs_sz + __rhs_sz);
+    _String __r(__capacity_tag(),
+                __lhs_sz + __rhs_sz,
+                _String::__alloc_traits::select_on_container_copy_construction(__rhs.get_allocator()));
+    __r.append(__lhs, __lhs_sz);
     __r.append(__rhs.data(), __rhs_sz);
     return __r;
 }
@@ -4191,9 +4210,11 @@
 operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Allocator>& __rhs)
 {
     using _String = basic_string<_CharT, _Traits, _Allocator>;
-    _String __r(_String::__alloc_traits::select_on_container_copy_construction(__rhs.get_allocator()));
     typename _String::size_type __rhs_sz = __rhs.size();
-    __r.__init(&__lhs, 1, 1 + __rhs_sz);
+    _String __r(__capacity_tag(),
+                __rhs_sz + 1,
+                _String::__alloc_traits::select_on_container_copy_construction(__rhs.get_allocator()));
+    __r.push_back(__lhs);
     __r.append(__rhs.data(), __rhs_sz);
     return __r;
 }
@@ -4204,10 +4225,12 @@
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, const _CharT* __rhs)
 {
     using _String = basic_string<_CharT, _Traits, _Allocator>;
-    _String __r(_String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
     typename _String::size_type __lhs_sz = __lhs.size();
     typename _String::size_type __rhs_sz = _Traits::length(__rhs);
-    __r.__init(__lhs.data(), __lhs_sz, __lhs_sz + __rhs_sz);
+    _String __r(__capacity_tag(),
+                __lhs_sz + __rhs_sz,
+                _String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
+    __r.append(__lhs.data(), __lhs_sz);
     __r.append(__rhs, __rhs_sz);
     return __r;
 }
@@ -4217,9 +4240,11 @@
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, _CharT __rhs)
 {
     using _String = basic_string<_CharT, _Traits, _Allocator>;
-    _String __r(_String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
     typename _String::size_type __lhs_sz = __lhs.size();
-    __r.__init(__lhs.data(), __lhs_sz, __lhs_sz + 1);
+    _String __r(__capacity_tag(),
+                __lhs_sz + 1,
+                _String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()));
+    __r.append(__lhs.data(), __lhs_sz);
     __r.push_back(__rhs);
     return __r;
 }