shift count negative or too big, undefined behavior

[mop65715]

Given:

typedef unsigned long long  ulong_long_type ;
istream& istream::operator>>(
   ulong_long_type&        dest )
{
    BGetter                    source( *this ) ;
    ulong_long_type        tmp = source.get() << 56 ;
    tmp |= source.get() << 48 ;
    tmp |= source.get() << 40 ;
    tmp |= source.get() << 32 ;
    tmp |= source.get() << 24 ;
    tmp |= source.get() << 16 ;
    tmp |= source.get() <<  8 ;
    tmp |= source.get()       ;
    return *this ;
}

During compilation I get warnings from Visual studio 2007 surrounding the left shift values 56, 48, 40 and 32, more specifically:

3>c:\sw_dev\msvc_2007-orcas\test\istream.h(158) : warning C4293: '<<' : shift count negative or too big, undefined behavior
3>c:\sw_dev\msvc_2007-orcas\test\istream.h(159) : warning C4293: '<<' : shift count negative or too big, undefined behavior
3>c:\sw_dev\msvc_2007-orcas\test\istream.h(160) : warning C4293: '<<' : shift count negative or too big, undefined behavior
3>c:\sw_dev\msvc_2007-orcas\test\istream.h(161) : warning C4293: '<<' : shift count negative or too big, undefined behavior

Of course I could ignore the warning but the claim of 'undefined behavior' is cause for concern. Thoughts.

Thanks in advance