Sync from upstream.

include/boost/convert.hpp

@@ -38,10 +38,11 @@ namespace boost
 
     namespace cnv
     {
+        using char_cptr = char const*;
+
         template<typename, typename, typename> struct reference;
         struct by_default;
     }
-
     /// @brief Boost.Convert main deployment interface
     /// @param[in] value_in   Value of the TypeIn type to be converted to the TypeOut type
     /// @param[in] converter  Converter to be used for conversion
@@ -63,7 +64,6 @@ namespace boost
         boost::unwrap_ref(converter)(value_in, result);
         return result;
     }
-
     namespace cnv { namespace detail
     {
         template<typename TypeOut, typename TypeIn, typename Converter =boost::cnv::by_default>

include/boost/convert/charconv.hpp

@@ -18,56 +18,50 @@
 #endif // BOOST_NO_CXX17_IF_CONSTEXPR
 
 #include <boost/convert/base.hpp>
+#include <boost/make_default.hpp>
 #include <charconv>
 #include <type_traits>
 
-namespace boost { namespace cnv { struct charconv; }}
+namespace boost::cnv { struct charconv; }
 
-/// @brief std::to/from_chars-based extended converter
-/// @details The converter offers good overall performance and moderate formatting facilities.
-
-struct boost::cnv::charconv : boost::cnv::cnvbase<boost::cnv::charconv>
+/// @brief   std::to/from_chars-based extended converter
+/// @details Good overall performance and moderate formatting facilities.
+struct boost::cnv::charconv : public boost::cnv::cnvbase<boost::cnv::charconv>
 {
     using this_type = boost::cnv::charconv;
     using base_type = boost::cnv::cnvbase<this_type>;
 
-    private:
-
-    friend struct boost::cnv::cnvbase<this_type>;
-
     template<typename in_type>
     cnv::range<char*>
     to_str(in_type value_in, char* buf) const
     {
-        const auto [ptr, ec] = [&]{
-            if constexpr (std::is_integral_v<in_type>) {
-                return std::to_chars(buf, buf + bufsize_, value_in, static_cast<int>(base_));
-            } else {
+        auto [ptr, err] = [&]
+        {
+            if constexpr (std::is_integral_v<in_type>)
+                return std::to_chars(buf, buf + bufsize_, value_in, int(base_));
+            else
                 return std::to_chars(buf, buf + bufsize_, value_in, chars_format(), precision_);
-            }
         }();
-        bool success = ec == std::errc{};
-
-        return cnv::range<char*>(buf, success ? ptr : buf);
+        return cnv::range<char*>(buf, err == std::errc{} ? ptr : buf);
     }
-
     template<typename string_type, typename out_type>
     void
     str_to(cnv::range<string_type> range, optional<out_type>& result_out) const
     {
         out_type result = boost::make_default<out_type>();
-        const auto [ptr, ec] = [&]{
-            if constexpr (std::is_integral_v<out_type>) {
-                return std::from_chars(&*range.begin(), &*range.end(), result, static_cast<int>(base_));
-            } else {
-                return std::from_chars(&*range.begin(), &*range.end(), result, chars_format());
-            }
-        }();
+        auto [ptr, err] = [&]
+        {
+            char_cptr beg = &*range.begin();
+            char_cptr end = beg + range.size();
 
-        if (ec == std::errc{})
+            if constexpr (std::is_integral_v<out_type>)
+                return std::from_chars(beg, end, result, int(base_));
+            else
+                return std::from_chars(beg, end, result, chars_format());
+        }();
+        if (err == std::errc{})
             result_out = result;
     }
-
     std::chars_format chars_format() const
     {
         static constexpr std::chars_format format[] =
@@ -78,8 +72,6 @@ struct boost::cnv::charconv : boost::cnv::cnvbase<boost::cnv::charconv>
         };
         return format[int(notation_)];
     }
-
-    std::chars_format fmt_ = std::chars_format::fixed;
 };
 
 #endif // BOOST_CONVERT_CHARCONV_BASED_CONVERTER_HPP

makefile

@@ -18,6 +18,7 @@ target_13 = convert-test-has-member         test/has_member.cpp
 target_14 = convert-test-performance        test/performance.cpp
 target_15 = convert-test-performance-spirit test/performance_spirit.cpp
 target_16 = convert-test-charconv-converter test/charconv_converter.cpp
+target_17 = convert-test-63 				test/issue-63.cpp
 
 target_21 = convert-example-algorithms         example/algorithms.cpp
 target_22 = convert-example-default_converter  example/default_converter.cpp

test/issue-63.cpp

@@ -0,0 +1,17 @@
+#include <boost/convert.hpp>
+#include <boost/convert/charconv.hpp>
+#include <iostream>
+
+int main()
+{
+    auto cnv4 = boost::cnv::charconv();
+    std::string PuNum = "2";
+    auto optPuNum = boost::convert<size_t>(PuNum, cnv4);
+
+    if (optPuNum)
+        std::cout << *optPuNum << "\n";
+    else
+        std::cout << "nullopt\n";
+
+    return 0;
+}