how to force blind cast from void* to struct*

g++ says a reinterpret cast from void* is disallowed; however, all I'm
trying to do is de-capsulate a void* argument that holds the pointer
to a class (and static_cast forces cast constructor)
any solutions?
also this pops up: 43: error: expected unqualified-id before 'return'
code:
includes: stdio.h, stdlib.h, time.h, pthread.h
struct:
template <typename T> struct mergesort_thread {
    T* start;
    T* end;
    mergesort_thread(T* _start, T* _end) { start = _start; end = _end; }
    static void *invoke(void *_this) {
        pthread_t threads[2];
        mergesort_thread<T> *child1, *child2;
        T *mid, *shift, t;
        T* start = (reinterpret_cast< mergesort_thread<T> >(_this))->start;
        T* end = (reinterpret_cast< mergesort_thread<T> >(_this))->end;
        if (start >= end) return NULL; //recursed deep enough (1 element)
        //recursively sort halves of the list
        mid = (T*)(start + ((end - start) >> 1));
        child1 = new mergesort_thread<T>(start, mid);
        pthread_create(&threads[0], NULL, mergesort_thread<T>::invoke, (void
*)child1);
        child2 = new mergesort_thread<T>(mid + 1, end);
        pthread_create(&threads[1], NULL, mergesort_thread<T>::invoke, (void
*)child2);
        delete child1;
        delete child2;
        pthread_join(threads[0], NULL);
        pthread_join(threads[1], NULL);
        for (;(start <= mid) && (mid + 1 <= end); start++) {
            if (*start < *(mid + 1)) //sorted (current element belongs in 1st
half)
                continue;
            else { /* true inplace merge requires insersion-sort-like
                * methods because a value from the second half is inserted to
                * the current element */
                //copy the first element in the second half to t
                t = *(mid + 1);
                //shift first half to the right
                for (shift = mid; shift >= start; shift--)
                *(shift + 1) = *shift;
                //copy t to start
                *start = t;
                mid++;
            }
        }
    }
43: return NULL;
};