Suppress conversion warnings in match_results.

When passing std::size_t arguments to length(), str() and operator[] by making these templates enable_if'd on is_integral.
Fixes https://github.com/boostorg/regex/issues/197.

include/boost/regex/concepts.hpp

@@ -76,13 +76,10 @@ inline long hash_value(char_architype val)
 //
 } // namespace boost
 namespace std{
-   template<> struct char_traits<boost::char_architype>
-   {
-      // The intent is that this template is not instantiated,
-      // but this typedef gives us a chance of compilation in
-      // case it is:
-      typedef boost::char_architype char_type;
-   };
+   //
+   // We should never use this, if we do it should be an error:
+   //
+   template<> struct char_traits<boost::char_architype>;
 }
 //
 // Allocator architype:
@@ -412,6 +409,10 @@ struct BaseRegexConcept
       Regex e5(in1, in2, m_flags);
       ignore_unused_variable_warning(e5);
 
+      // equals:
+      e1 == e2;
+      e1 != e2;
+
       // assign etc:
       Regex e;
       e = m_pointer;

include/boost/regex/v5/basic_regex.hpp

@@ -488,7 +488,7 @@ public:
    }
    //
    // swap:
-   void  swap(basic_regex& that)throw()
+   void  swap(basic_regex& that)noexcept
    {
       m_pimpl.swap(that.m_pimpl);
    }
@@ -533,7 +533,21 @@ public:
          return status() - that.status();
       if(flags() != that.flags())
          return flags() - that.flags();
-      return str().compare(that.str());
+      
+      const char_type* i = m_pimpl->begin();
+      const char_type* j = that.m_pimpl->begin();
+      while ((i != m_pimpl->end()) && (j != that.m_pimpl->end()))
+      {
+         if (*i != *j)
+            return *i < *j ? -1 : 1;
+         ++i;
+         ++j;
+      }
+      if (i != m_pimpl->end())
+         return *i > static_cast<char_type>(0) ? 1 : -1;
+      if (j != that.m_pimpl->end())
+         return *j > static_cast<char_type>(0) ? -1 : 1;
+      return 0;
    }
    bool  operator==(const basic_regex& e)const
    { 

include/boost/regex/v5/match_results.hpp

@@ -97,15 +97,23 @@ public:
    bool empty() const
    { return m_subs.size() < 2; }
    // element access:
-   difference_type length(int sub = 0) const
+private:
+   difference_type do_get_length(int sub = 0) const
    {
-      if(m_is_singular)
+      if (m_is_singular)
          raise_logic_error();
       sub += 2;
-      if((sub < (int)m_subs.size()) && (sub > 0))
+      if ((sub < (int)m_subs.size()) && (sub > 0))
          return m_subs[sub].length();
       return 0;
    }
+public:
+   template <class Integer>
+   typename std::enable_if<std::is_integral<Integer>::value, difference_type>::type length(Integer sub) const
+   {
+      return do_get_length(static_cast<int>(sub));
+   }
+   difference_type length() const { return do_get_length(0); }
    difference_type length(const char_type* sub) const
    {
       if(m_is_singular)
@@ -161,7 +169,8 @@ public:
    {
       return position(sub.c_str());
    }
-   string_type str(int sub = 0) const
+private:
+   string_type do_get_string(int sub = 0) const
    {
       if(m_is_singular)
          raise_logic_error();
@@ -177,6 +186,13 @@ public:
       }
       return result;
    }
+public:
+   template <class Integer>
+   typename std::enable_if<std::is_integral<Integer>::value, string_type>::type str(Integer sub) const
+   {
+      return do_get_string(static_cast<int>(sub));
+   }
+   string_type str() const { return do_get_string(0); }
    string_type str(const char_type* sub) const
    {
       return (*this)[sub].str();
@@ -196,7 +212,8 @@ public:
    {
       return (*this)[sub].str();
    }
-   const_reference operator[](int sub) const
+   private:
+   const_reference get_at(int sub) const
    {
       if(m_is_singular && m_subs.empty())
          raise_logic_error();
@@ -207,6 +224,12 @@ public:
       }
       return m_null;
    }
+public:
+   template <class Integer>
+   typename std::enable_if<std::is_integral<Integer>::value, const_reference>::type operator[](Integer sub) const
+   {
+      return get_at(static_cast<int>(sub));
+   }
    //
    // Named sub-expressions:
    //

include/boost/regex/v5/pattern_except.hpp

@@ -52,7 +52,7 @@ public:
       , m_position(0)
    {
    }
-   ~regex_error() noexcept override {}
+   ~regex_error() noexcept override = default;
    regex_constants::error_type code()const
    { return m_error_code; }
    std::ptrdiff_t position()const