Revert r263036, it's ABI-breaking.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@263246 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index b937d29..06cbf9d 100644
--- a/include/string
+++ b/include/string
@@ -98,10 +98,8 @@
     basic_string(const basic_string& str);
     basic_string(basic_string&& str)
         noexcept(is_nothrow_move_constructible<allocator_type>::value);
-    basic_string(const basic_string& str, size_type pos,                 // LWG#2583
+    basic_string(const basic_string& str, size_type pos, size_type n = npos,
                  const allocator_type& a = allocator_type());
-    basic_string(const basic_string& str, size_type pos, size_type n,    // LWG#2583
-                 const Allocator& a = Allocator());             
     basic_string(const value_type* s, const allocator_type& a = allocator_type());
     basic_string(const value_type* s, size_type n, const allocator_type& a = allocator_type());
     basic_string(size_type n, value_type c, const allocator_type& a = allocator_type());
@@ -1399,9 +1397,7 @@
     basic_string(size_type __n, value_type __c);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(size_type __n, value_type __c, const allocator_type& __a);
-    basic_string(const basic_string& __str, size_type __pos, size_type __n,
-                 const allocator_type& __a = allocator_type());
-    basic_string(const basic_string& __str, size_type __pos,
+    basic_string(const basic_string& __str, size_type __pos, size_type __n = npos,
                  const allocator_type& __a = allocator_type());
     template<class _InputIterator>
         _LIBCPP_INLINE_VISIBILITY
@@ -2227,20 +2223,6 @@
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str, size_type __pos,
-                                                        const allocator_type& __a)
-    : __r_(__a)
-{
-    size_type __str_sz = __str.size();
-    if (__pos > __str_sz)
-        this->__throw_out_of_range();
-    __init(__str.data() + __pos, __str_sz - __pos);
-#if _LIBCPP_DEBUG_LEVEL >= 2
-    __get_db()->__insert_c(this);
-#endif
-}
-
-template <class _CharT, class _Traits, class _Allocator>
 template <class _InputIterator>
 typename enable_if
 <