Feature request: char16_t/char_32_t support for read_xml

[Dani-Hub]

Similar to this issue but less ambitious, I would like to suggest to provide minimum support for char16_t and char32_t to read XML, basically an extension of a previous BOM support issue. For example, the following code snippet does currently not compile:

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>

int main()
{
  boost::property_tree::basic_ptree<std::u16string, std::u16string> pt;
  std::string filename("utf16.xml"); 
  read_xml(filename, pt);
}

producing the following compile error:

1>c:\projects\thirdparty\boost_1_68_0\boost\property_tree\detail\rapidxml.hpp(1374): error C2664: 'void boost::property_tree::detail::rapidxml::xml_document<Ch>::parse_bom<3072>(wchar_t *&)': cannot convert argument 1 from 'Ch *' to 'char *&'
1>        with
1>        [
1>            Ch=Ch
1>        ]
1>c:\projects\thirdparty\boost_1_68_0\boost\property_tree\detail\xml_parser_read_rapidxml.hpp(116): note: see reference to function template instantiation 'void boost::property_tree::detail::rapidxml::xml_document<Ch>::parse<3072>(Ch *)' being compiled
1>        with
1>        [
1>            Ch=Ch
1>        ]
1>c:\projects\thirdparty\boost_1_68_0\boost\property_tree\detail\xml_parser_read_rapidxml.hpp(116): note: see reference to function template instantiation 'void boost::property_tree::detail::rapidxml::xml_document<Ch>::parse<3072>(Ch *)' being compiled
1>        with
1>        [
1>            Ch=Ch
1>        ]
1>c:\projects\thirdparty\boost_1_68_0\boost\property_tree\xml_parser.hpp(85): note: see reference to function template instantiation 'void boost::property_tree::xml_parser::read_xml_internal<Ptree>(std::basic_istream<char16_t,std::char_traits<char16_t>> &,Ptree &,int,const std::string &)' being compiled
1>        with
1>        [
1>            Ptree=boost::property_tree::basic_ptree<std::u16string,std::u16string,std::less<std::u16string>>
1>        ]
[...]
1>Done building project "bdal-playground.vcxproj" -- FAILED.

First inspection indicates that there are only missing specializations of the parse_bom member template for types char16_t and char_32_t.

I'm willing to make a corresponding pull request, but opened this issue first to get feedback on this idea.