format does not compile with char16_t strings and libc++

[timblechmann]

the following code does not compile with clang:

#include <boost/format.hpp>

int main(int argc, const char * argv[])
{
    std::basic_string<char16_t> abc = (boost::basic_format<char16_t>(u"Hello")).str();
    return 0;
}

the reason is that libc++ does not provide std::ctype<char16_t>, as it is not provided by the standard.

[James E. King, III]

This is not a Boost.Format concern. Implementations are not required to provide std::ctype<char16_t>. This failure also happens with libstdc++ 6.3.0, for example.

For more information, see:

​https://stackoverflow.com/questions/28360223/gcc-stdctype-specialisation-streams ​https://stackoverflow.com/questions/41315675/why-does-stdbasic-ifstreamchar16-t-not-work-in-c11