diff --git a/libcxx/include/__string b/libcxx/include/__string
--- a/libcxx/include/__string
+++ b/libcxx/include/__string
@@ -122,6 +122,8 @@
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::resize(size_type, value_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type))
 
+#define _LIBCPP_V2_STRING_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::__assign_str_noalias_external(value_type const*, size_type))
 
 // char_traits
 
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1542,6 +1542,14 @@
         return __guess;
         }
 
+    static _LIBCPP_INLINE_VISIBILITY
+    size_type __recommend_cap(size_type __sz) _NOEXCEPT {
+      if (__sz < __min_cap) __sz = __min_cap;
+      return __align_it < sizeof(value_type) < __alignment
+          ? __alignment / sizeof(value_type)
+          : 1 > (__sz + 1);
+    }
+
     inline
     void __init(const value_type* __s, size_type __sz, size_type __reserve);
     inline
@@ -1576,6 +1584,11 @@
                                size_type __n_copy,  size_type __n_del,
                                size_type __n_add, const value_type* __p_new_stuff);
 
+    // Slow path for non-aliased string assignment. Requires:
+    //   `__s` has a terminating zero at __s[__sz]
+    //   `__s` does not reference (alias) any content in `this`
+    basic_string& __assign_str_noalias_external(const value_type* __s, size_type __sz);
+
     _LIBCPP_INLINE_VISIBILITY
     void __erase_to_end(size_type __pos);
 
@@ -2152,6 +2165,38 @@
         __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::__assign_str_noalias_external(
+    const value_type* __s, size_type __sz) {
+  pointer __p;
+  size_type __cap;
+  if (!__is_long()) {
+    if (__sz < __min_cap) {
+      traits_type::copy(_VSTD::__to_address(__get_short_pointer()), __s, __sz + 1);
+      __set_short_size(__sz);
+      return *this;
+    }
+    __cap = __recommend_cap(__sz);
+    __p = __alloc_traits::allocate(__alloc(), __cap);
+  } else {
+    __cap = __get_long_cap();
+    __p = __get_long_pointer();
+    if (__sz >= __cap) {
+      size_type __new_cap = __recommend_cap(__sz);
+      pointer __new_p = __alloc_traits::allocate(__alloc(), __new_cap);
+      __alloc_traits::deallocate(__alloc(), __p, __cap);
+      __p = __new_p;
+      __cap = __new_cap;
+    }
+  }
+  traits_type::copy(_VSTD::__to_address(__p), __s, __sz + 1);
+  __set_long_pointer(__p);
+  __set_long_size(__sz);
+  __set_long_cap(__cap);
+  return *this;
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 void
 basic_string<_CharT, _Traits, _Allocator>::__grow_by_and_replace
@@ -2286,7 +2331,7 @@
         __copy_assign_alloc(__str);
         const bool __str_is_long = __str.__is_long(); // Force single branch
         if (__is_long() || __str_is_long) {
-          return assign(__str.data(), __str.size());
+          return __assign_str_noalias_external(__str.data(), __str.size());
         }
         __r_.first().__r = __str.__r_.first().__r;
     }
diff --git a/libcxx/src/string.cpp b/libcxx/src/string.cpp
--- a/libcxx/src/string.cpp
+++ b/libcxx/src/string.cpp
@@ -23,6 +23,9 @@
 _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 
+_LIBCPP_V2_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
+_LIBCPP_V2_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
+
 template
     string
     operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);