diff --git a/include/boost/static_string/config.hpp b/include/boost/static_string/config.hpp
index 7e3fc16..c46ed57 100644
--- a/include/boost/static_string/config.hpp
+++ b/include/boost/static_string/config.hpp
@@ -119,11 +119,15 @@
 
 #if (__cplusplus >= 201402L) && \
 (__cplusplus < 201703L) && \
-defined(BOOST_STATIC_STRING_NULL_OPTIMIZATION) && \
 defined(__clang__) && \
 ((__clang_major__ == 4) || (__clang_major__ == 5))
-#error The null terminator optimization is not supported for clang 4.x and clang 5.x
-#undef BOOST_STATIC_STRING_NULL_OPTIMIZATION
+// This directive works on clang
+#warning C++14 constexpr is not supported in clang 4.x and 5.x due to a compiler bug.
+#ifdef BOOST_STATIC_STRING_CPP14_CONSTEXPR_USED
+#undef BOOST_STATIC_STRING_CPP14_CONSTEXPR_USED
+#endif
+#undef BOOST_STATIC_STRING_CPP14_CONSTEXPR
+#define BOOST_STATIC_STRING_CPP14_CONSTEXPR
 #endif
 
 namespace boost {
diff --git a/include/boost/static_string/detail/static_string.hpp b/include/boost/static_string/detail/static_string.hpp
index 958e45f..3c699a1 100644
--- a/include/boost/static_string/detail/static_string.hpp
+++ b/include/boost/static_string/detail/static_string.hpp
@@ -31,7 +31,8 @@ using static_string = basic_static_string<N, char, std::char_traits<char>>;
 template<std::size_t N>
 using static_wstring = basic_static_string<N, wchar_t, std::char_traits<wchar_t>>;
 
-// At minimum an integral type shall not qualify as an iterator (Agustin Berge)
+// At minimum an integral type shall not qualify as an iterator (Agustin Berge) 
+// Applicable standardese: http://eel.is/c++draft/containers#container.requirements.general-17
 template<class T>
 using is_input_iterator =
     std::integral_constant<bool,
@@ -69,6 +70,46 @@ struct is_nothrow_convertible<From, To, typename std::enable_if<
     is_nothrow_convertible_msvc_helper<From, To>::value>::type>
         : std::true_type { };
 
+// void_t for c++11
+template<typename...>
+using void_t = void;
+
+// Check if iterator can do subtraction, meaning its random access
+template<typename T, typename = void>
+struct is_difference_iter : std::false_type { };
+
+template<typename T>
+struct is_difference_iter<T, void_t<decltype(std::declval<T>() - std::declval<T>())>> : std::true_type { };
+
+// constexpr distance for c++14
+template<typename InputIt, typename std::enable_if<!is_difference_iter<InputIt>::value>::type* = nullptr>
+BOOST_STATIC_STRING_CPP14_CONSTEXPR 
+typename std::iterator_traits<InputIt>::difference_type
+distance(InputIt first, InputIt last)
+{
+  std::size_t dist = 0;
+  for (; first != last; ++first, ++dist);
+  return dist;
+}
+
+template<typename RandomIt, typename std::enable_if<is_difference_iter<RandomIt>::value>::type* = nullptr>
+BOOST_STATIC_STRING_CPP14_CONSTEXPR 
+typename std::iterator_traits<RandomIt>::difference_type
+distance(RandomIt first, RandomIt last)
+{
+  return last - first;
+}
+
+// Copy using traits, respecting iterator rules
+template<typename Traits, typename InputIt, typename CharT>
+BOOST_STATIC_STRING_CPP14_CONSTEXPR
+void
+copy_with_traits(InputIt first, InputIt last, CharT* out)
+{
+  for (; first != last; ++first, ++out)
+    Traits::assign(*out, *first);
+}
+
 // Optimization for using the smallest possible type
 template<std::size_t N, typename CharT, typename Traits>
 class static_string_base_zero
@@ -429,10 +470,14 @@ template<
     typename Traits,
     typename CharT,
     typename ForwardIterator>
+BOOST_STATIC_STRING_CPP14_CONSTEXPR
 inline
 ForwardIterator
 find_not_of(
-  ForwardIterator first, ForwardIterator last, const CharT* str, std::size_t n) noexcept
+  ForwardIterator first, 
+  ForwardIterator last, 
+  const CharT* str, 
+  std::size_t n) noexcept
 {
   for (; first != last; ++first)
     if (!Traits::find(str, n, *first))
@@ -445,3 +490,4 @@ find_not_of(
 } // boost
 
 #endif
+
diff --git a/include/boost/static_string/impl/static_string.hpp b/include/boost/static_string/impl/static_string.hpp
index fa0e141..d403528 100644
--- a/include/boost/static_string/impl/static_string.hpp
+++ b/include/boost/static_string/impl/static_string.hpp
@@ -224,7 +224,7 @@ assign(
             detail::is_input_iterator<InputIterator>::value,
                 basic_static_string&>::type
 {
-    std::size_t const n = std::distance(first, last);
+    std::size_t const n = detail::distance(first, last);
     BOOST_STATIC_STRING_THROW_IF(n > max_size(),
                                  std::length_error{"n > max_size()"});
     this->set_size(n);
@@ -298,22 +298,6 @@ clear() noexcept
 
 //------------------------------------------------------------------------------
 
-template<std::size_t N, typename CharT, typename Traits>
-BOOST_STATIC_STRING_CPP14_CONSTEXPR
-auto
-basic_static_string<N, CharT, Traits>::
-insert(
-    size_type index,
-    size_type count,
-    CharT ch) BOOST_STATIC_STRING_COND_NOEXCEPT ->
-        basic_static_string&
-{
-    BOOST_STATIC_STRING_THROW_IF(
-        index > size(), std::out_of_range{"index > size()"});
-    insert(begin() + index, count, ch);
-    return *this;
-}
-
 template<std::size_t N, typename CharT, typename Traits>
 BOOST_STATIC_STRING_CPP14_CONSTEXPR
 auto
@@ -390,48 +374,60 @@ insert(
             detail::is_input_iterator<
                 InputIterator>::value, iterator>::type
 {
-  const size_type index = pos - begin();
-  return insert(index, &*first, std::distance(first, last)).begin() + index;
+    const auto curr_size = size();
+    const auto curr_data = data();
+    const auto count = detail::distance(first, last);
+    const auto index = pos - begin();
+    const auto s = &*first;
+    BOOST_STATIC_STRING_THROW_IF(
+      index > curr_size, std::out_of_range{"index > size()"});
+    BOOST_STATIC_STRING_THROW_IF(
+      count > max_size() - curr_size, std::length_error{"count > max_size() - size()"});
+    const bool inside = s <= &curr_data[curr_size] && s >= curr_data;
+    if (!inside || (inside && ((s - curr_data) + count <= index)))
+    {
+      Traits::move(&curr_data[index + count], &curr_data[index], curr_size - index + 1);
+      detail::copy_with_traits<Traits>(first, last, &curr_data[index]);
+    }
+    else
+    {
+      const size_type offset = s - curr_data;
+      Traits::move(&curr_data[index + count], &curr_data[index], curr_size - index + 1);
+      if (offset < index)
+      {
+        const size_type diff = index - offset;
+        Traits::copy(&curr_data[index], &curr_data[offset], diff);
+        Traits::copy(&curr_data[index + diff], &curr_data[index + count], count - diff);
+      }
+      else
+      {
+        Traits::copy(&curr_data[index], &curr_data[offset + count], count);
+      }
+    }
+    this->set_size(curr_size + count);
+    return begin() + index;
 }
 
 template<std::size_t N, typename CharT, typename Traits>
-template<class T>
 BOOST_STATIC_STRING_CPP14_CONSTEXPR
 auto
 basic_static_string<N, CharT, Traits>::
 insert(
-    size_type index,
-    T const & t) BOOST_STATIC_STRING_COND_NOEXCEPT ->
-        typename std::enable_if<
-            std::is_convertible<
-                T const&, string_view_type>::value &&
-            ! std::is_convertible<
-                T const&, CharT const*>::value, basic_static_string&
-                    >::type
+    const_iterator pos,
+    std::initializer_list<CharT> ilist) BOOST_STATIC_STRING_COND_NOEXCEPT ->
+        iterator
 {
-    return insert(index, t, 0, npos);
-}
-
-template<std::size_t N, typename CharT, typename Traits>
-template<class T>
-BOOST_STATIC_STRING_CPP14_CONSTEXPR
-auto
-basic_static_string<N, CharT, Traits>::
-insert(
-    size_type index,
-    T const & t,
-    size_type index_str,
-    size_type count) BOOST_STATIC_STRING_COND_NOEXCEPT ->
-        typename std::enable_if<
-            std::is_convertible<
-                T const&, string_view_type>::value &&
-            ! std::is_convertible<
-                T const&, CharT const*>::value, basic_static_string&
-                    >::type
-{
-    auto const s =
-        string_view_type(t).substr(index_str, count);
-    return insert(index, s.data(), s.size());
+    const auto curr_size = size();
+    const auto curr_data = data();
+    const auto index = pos - begin();
+    BOOST_STATIC_STRING_THROW_IF(
+      index > curr_size, std::out_of_range{"index > size()"});
+    BOOST_STATIC_STRING_THROW_IF(
+      ilist.size() > max_size() - curr_size, std::length_error{"count > max_size() - size()"});
+    Traits::move(&curr_data[index + ilist.size()], &curr_data[index], curr_size - index + 1);
+    detail::copy_with_traits<Traits>(ilist.begin(), ilist.end(), &curr_data[index]);
+    this->set_size(curr_size + ilist.size());
+    return curr_data + index;
 }
 
 //------------------------------------------------------------------------------
@@ -477,7 +473,7 @@ erase(
         iterator
 {
     erase(first - begin(),
-        std::distance(first, last));
+        detail::distance(first, last));
     return begin() + (first - begin());
 }
 
@@ -694,6 +690,97 @@ replace(
     return *this;
 }
 
+template<std::size_t N, typename CharT, typename Traits>
+template<typename InputIterator>
+BOOST_STATIC_STRING_CPP14_CONSTEXPR
+auto
+basic_static_string<N, CharT, Traits>::
+replace(
+      const_iterator i1,
+      const_iterator i2,
+      InputIterator j1,
+      InputIterator j2) BOOST_STATIC_STRING_COND_NOEXCEPT ->
+          typename std::enable_if<
+              detail::is_input_iterator<InputIterator>::value,
+                  basic_static_string<N, CharT, Traits>&>::type
+{
+    const auto curr_size = size();
+    const auto curr_data = data();
+    std::size_t n1 = std::distance(i1, i2);
+    const std::size_t n2 = detail::distance(j1, j2);
+    const std::size_t pos = i1 - begin();
+    const auto s = &*j1;
+    BOOST_STATIC_STRING_THROW_IF(
+      pos > curr_size, std::out_of_range{"pos > size()"});
+    BOOST_STATIC_STRING_THROW_IF(
+      curr_size - (std::min)(n1, curr_size - pos) >= max_size() - n2,
+      std::length_error{"replaced string exceeds max_size()"});
+    if (pos + n1 >= curr_size)
+      n1 = curr_size - pos;
+    const bool inside = s <= &curr_data[curr_size] && s >= curr_data;
+    if (inside && size_type(s - curr_data) == pos && n1 == n2)
+      return *this;
+    if (!inside || (inside && ((s - curr_data) + n2 <= pos)))
+    {
+      // source outside
+      Traits::move(&curr_data[pos + n2], &curr_data[pos + n1], curr_size - pos - n1 + 1);
+      detail::copy_with_traits<Traits>(j1, j2, &curr_data[pos]);
+    }
+    else
+    {
+      // source inside
+      const size_type offset = s - curr_data;
+      if (n2 >= n1)
+      {
+        // grow/unchanged
+        // shift all right of splice point by n2 - n1 to the right
+        Traits::move(&curr_data[pos + n2], &curr_data[pos + n1], curr_size - pos - n1 + 1);
+        const size_type diff = offset <= pos + n1 ? (std::min)((pos + n1) - offset, n2) : 0;
+        // copy all before splice point
+        Traits::move(&curr_data[pos], &curr_data[offset], diff);
+        // copy all after splice point
+        Traits::move(&curr_data[pos + diff], &curr_data[offset + (n2 - n1) + diff], n2 - diff);
+      }
+      else
+      {
+        // shrink
+        // copy all elements into place
+        Traits::move(&curr_data[pos], &curr_data[offset], n2);
+        // shift all elements after splice point left
+        Traits::move(&curr_data[pos + n2], &curr_data[pos + n1], curr_size - pos - n1 + 1);
+      }
+    }
+    this->set_size(curr_size + (n2 - n1));
+    return *this;
+}
+
+template<std::size_t N, typename CharT, typename Traits>
+BOOST_STATIC_STRING_CPP14_CONSTEXPR
+auto
+basic_static_string<N, CharT, Traits>::
+replace(
+    const_iterator i1,
+    const_iterator i2,
+    std::initializer_list<CharT> il) BOOST_STATIC_STRING_COND_NOEXCEPT ->
+        basic_static_string&
+{
+  const auto curr_size = size();
+  const auto curr_data = data();
+  std::size_t n1 = detail::distance(i1, i2);
+  const std::size_t pos = i1 - begin();
+  BOOST_STATIC_STRING_THROW_IF(
+    pos > curr_size, std::out_of_range{"pos > size()"});
+  BOOST_STATIC_STRING_THROW_IF(
+    curr_size - (std::min)(n1, curr_size - pos) >= max_size() - il.size(),
+    std::length_error{"replaced string exceeds max_size()"});
+  if (pos + n1 >= curr_size)
+    n1 = curr_size - pos;
+  Traits::move(&curr_data[pos + il.size()], &curr_data[pos + n1], curr_size - pos - n1 + 1);
+  detail::copy_with_traits<Traits>(il.begin(), il.end(), &curr_data[pos]);
+  this->set_size(curr_size + (il.size() - n1));
+  return *this;
+}
+
 template<std::size_t N, typename CharT, typename Traits>
 BOOST_STATIC_STRING_CPP14_CONSTEXPR
 auto
@@ -710,7 +797,7 @@ find(
     if (!n)
       return pos;
     const auto res = std::search(&data()[pos], &data()[curr_size], s, &s[n], Traits::eq);
-    return res == end() ? npos : std::distance(data(), res);
+    return res == end() ? npos : detail::distance(data(), res);
 }
 
 template<std::size_t N, typename CharT, typename Traits>
@@ -733,7 +820,7 @@ rfind(
       return pos;
     for (auto sub = &curr_data[pos]; sub >= curr_data; --sub)
       if (!Traits::compare(sub, s, n))
-        return std::distance(curr_data, sub);
+        return detail::distance(curr_data, sub);
     return npos;
 }
 
@@ -751,7 +838,7 @@ find_first_of(
     if (pos >= size() || !n)
       return npos;
     const auto res = std::find_first_of(&curr_data[pos], &curr_data[size()], s, &s[n], Traits::eq);
-    return res == end() ? npos : std::distance(curr_data, res);
+    return res == end() ? npos : detail::distance(curr_data, res);
 }
 
 template<std::size_t N, typename CharT, typename Traits>
@@ -772,7 +859,7 @@ find_last_of(
     else
       pos = curr_size - (pos + 1);
     const auto res = std::find_first_of(rbegin() + pos, rend(), s, &s[n], Traits::eq);
-    return res == rend() ? npos : curr_size - 1 - std::distance(rbegin(), res);
+    return res == rend() ? npos : curr_size - 1 - detail::distance(rbegin(), res);
 }
 
 template<std::size_t N, typename CharT, typename Traits>
@@ -790,7 +877,7 @@ find_first_not_of(
     if (!n)
       return pos;
     const auto res = detail::find_not_of<Traits>(&data()[pos], &data()[size()], s, n);
-    return res == end() ? npos : std::distance(data(), res);
+    return res == end() ? npos : detail::distance(data(), res);
 }
 
 template<std::size_t N, typename CharT, typename Traits>
@@ -810,7 +897,7 @@ find_last_not_of(
       return pos;
     pos = curr_size - (pos + 1);
     const auto res = detail::find_not_of<Traits>(rbegin() + pos, rend(), s, n);
-    return res == rend() ? npos : curr_size - 1 - std::distance(rbegin(), res);
+    return res == rend() ? npos : curr_size - 1 - detail::distance(rbegin(), res);
 }
 
 template<std::size_t N, typename CharT, typename Traits>
diff --git a/include/boost/static_string/static_string.hpp b/include/boost/static_string/static_string.hpp
index 0b9ca12..332ff71 100644
--- a/include/boost/static_string/static_string.hpp
+++ b/include/boost/static_string/static_string.hpp
@@ -425,6 +425,7 @@ public:
         @throw std::length_error if `ilist.size() > max_size()`
         @return `*this`
     */
+    BOOST_STATIC_STRING_CPP14_CONSTEXPR
     basic_static_string&
     assign(
         std::initializer_list<CharT> ilist) BOOST_STATIC_STRING_COND_NOEXCEPT
@@ -802,7 +803,13 @@ public:
     insert(
         size_type index,
         size_type count,
-        CharT ch) BOOST_STATIC_STRING_COND_NOEXCEPT;
+        CharT ch) BOOST_STATIC_STRING_COND_NOEXCEPT
+    {
+      BOOST_STATIC_STRING_THROW_IF(
+        index > size(), std::out_of_range{"index > size()"});
+      insert(begin() + index, count, ch);
+      return *this;
+    }
     
     /** Insert into the string.
     
@@ -947,13 +954,10 @@ public:
         @return An iterator to the first inserted character or pos if no insertion took place
     */
     BOOST_STATIC_STRING_CPP14_CONSTEXPR
-    iterator
-    insert(
+      iterator
+      insert(
         const_iterator pos,
-        std::initializer_list<CharT> ilist) BOOST_STATIC_STRING_COND_NOEXCEPT
-    {
-        return insert(pos, ilist.begin(), ilist.end());
-    }
+        std::initializer_list<CharT> ilist) BOOST_STATIC_STRING_COND_NOEXCEPT;
 
     /** Insert into the string.
     
@@ -981,7 +985,10 @@ public:
 #endif
     insert(
         size_type index,
-        T const& t) BOOST_STATIC_STRING_COND_NOEXCEPT;
+        T const& t) BOOST_STATIC_STRING_COND_NOEXCEPT
+    {
+      return insert(index, t, 0, npos);
+    }
 
     /** Insert into the string.
 
@@ -1010,7 +1017,11 @@ public:
         size_type index,
         T const& t,
         size_type index_str,
-        size_type count = npos) BOOST_STATIC_STRING_COND_NOEXCEPT;
+        size_type count = npos) BOOST_STATIC_STRING_COND_NOEXCEPT
+    {
+      auto const s = string_view_type(t).substr(index_str, count);
+      return insert(index, s.data(), s.size());
+    }
 
     /** Removes `min(count, size() - index)` characters starting at `index`
 
@@ -1852,10 +1863,7 @@ public:
         const_iterator i1,
         const_iterator i2,
         InputIterator j1,
-        InputIterator j2) BOOST_STATIC_STRING_COND_NOEXCEPT
-    {
-      return replace(i1, i2, basic_static_string(j1, j2));
-    }
+        InputIterator j2) BOOST_STATIC_STRING_COND_NOEXCEPT;
 
     /** Replace a subset of the string.
     
@@ -1866,14 +1874,11 @@ public:
         @return `*this`
     */
     BOOST_STATIC_STRING_CPP14_CONSTEXPR
-    basic_static_string&
-    replace(
+      basic_static_string&
+      replace(
         const_iterator i1,
         const_iterator i2,
-        std::initializer_list<CharT> il) BOOST_STATIC_STRING_COND_NOEXCEPT
-    {
-      return replace(i1, i2, il.begin(), il.size());
-    }
+        std::initializer_list<CharT> il) BOOST_STATIC_STRING_COND_NOEXCEPT;
 
     //--------------------------------------------------------------------------
     //