When should a UDT activate "badbit" during I/O?

Let's say I'm reading a list of numbers:

template < typename T, typename Ch, class Tr >
std::list<T>
read_my_udt( std::basic_istream<Ch, Tr> &is )
{
    Ch const lp = is.widen('('), ca = is.widen(','), rp = is.widen(')');
    Ch c;
    std::list<T> result;

    if ( !(is >> c) )
        return std::list<T>();
    if ( !Tr::eq(c, lp) )
    {
        T sole_element;

        is.unget();
        is >> sole_element;
        result.push_back( sole_element );
    }
    else
    {
        //...
    }
    return result;
}

I didn't get into reading a comma-separated, parentheses-bounded list because the single-element case is sufficient. If there's a format error, from a read setting "eof" or "failbit", I can just return an empty list and use the stream's state to pass on
the error. But what if there's some sort of exception thrown from list operations? Should "badbit" be set, or should exceptions not having anything to do with stream operations get passed up without affecting the stream's state? After all, the stream
is still technically usable, although it has all the elements that weren't read yet at the time of the throw.

Daryle W.

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