Using CRC32 algorythm to hash string at compile-time

Basically I want in my code to be able to do this:

 Engine.getById(WSID('some-id'));

Which should get transformed by

 Engine.getById('1a61bc96');

just before being compiled into asm. So at compile-time.

This is my try

constexpr int WSID(const std::string& str) {
    boost::crc_32_type result;
    result.process_bytes(str.data(), str.length());
    return result.checksum();
}

But I get this when trying to compile with MSVC 18 (CTP November 2013)

error C3757: 'const std::string &': type not allowed for 'constexpr' 
error C3249: illegal statement or sub-expression for 'constexpr' function