diff --git a/include/string b/include/string
index b56ab38..95fbce0 100644
--- a/include/string
+++ b/include/string
@@ -926,6 +926,8 @@
     basic_string& append(const value_type* __s, size_type __n);
     basic_string& append(const value_type* __s);
     basic_string& append(size_type __n, value_type __c);
+    template <class _ForwardIterator>
+    basic_string& __append_forward_unsafe(_ForwardIterator, _ForwardIterator);
     template<class _InputIterator>
         typename enable_if
         <
@@ -933,7 +935,12 @@
                 || !__libcpp_string_gets_noexcept_iterator<_InputIterator>::value,
             basic_string&
         >::type
-        append(_InputIterator __first, _InputIterator __last);
+    _LIBCPP_INLINE_VISIBILITY
+    append(_InputIterator __first, _InputIterator __last) {
+      const basic_string __temp (__first, __last, __alloc());
+      append(__temp.data(), __temp.size());
+      return *this;
+    }
     template<class _ForwardIterator>
         typename enable_if
         <
@@ -941,7 +948,11 @@
                 && __libcpp_string_gets_noexcept_iterator<_ForwardIterator>::value,
             basic_string&
         >::type
-        append(_ForwardIterator __first, _ForwardIterator __last);
+    _LIBCPP_INLINE_VISIBILITY
+    append(_ForwardIterator __first, _ForwardIterator __last) {
+      return __append_forward_unsafe(__first, __last);
+    }
+
 #ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
     _LIBCPP_INLINE_VISIBILITY
     basic_string& append(initializer_list<value_type> __il) {return append(__il.begin(), __il.size());}
@@ -2182,21 +2193,6 @@
     traits_type::assign(*++__p, value_type());
 }
 
-template <class _CharT, class _Traits, class _Allocator>
-template<class _InputIterator>
-typename enable_if
-<
-    __is_exactly_input_iterator<_InputIterator>::value
-             || !__libcpp_string_gets_noexcept_iterator<_InputIterator>::value,
-    basic_string<_CharT, _Traits, _Allocator>&
->::type
-basic_string<_CharT, _Traits, _Allocator>::append(_InputIterator __first, _InputIterator __last)
-{
-    const basic_string __temp (__first, __last, __alloc());
-    append(__temp.data(), __temp.size());
-    return *this;
-}
-
 template <class _Tp>
 bool __ptr_in_range (const _Tp* __p, const _Tp* __first, const _Tp* __last)
 {
@@ -2211,14 +2207,12 @@
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
-typename enable_if
-<
-    __is_forward_iterator<_ForwardIterator>::value
-          && __libcpp_string_gets_noexcept_iterator<_ForwardIterator>::value,
-    basic_string<_CharT, _Traits, _Allocator>&
->::type
-basic_string<_CharT, _Traits, _Allocator>::append(_ForwardIterator __first, _ForwardIterator __last)
+basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::__append_forward_unsafe(
+    _ForwardIterator __first, _ForwardIterator __last)
 {
+    static_assert(__is_forward_iterator<_ForwardIterator>::value,
+                  "function requires a ForwardIterator");
     size_type __sz = size();
     size_type __cap = capacity();
     size_type __n = static_cast<size_type>(_VSTD::distance(__first, __last));