#include <valarray>
using namespace std;
 
void foo(valarray<int>& va)
{
   // You can't return by value to a non-const lvalue reference, it either
   // has to be a const lvalue reference or a (C++11) rvalue reference. In your
   // case it needs to be an r-value reference. Note that what's returned by
   // the operator is an object that has reference semantics, not a reference!
   //slice_array<int>& sa = va[slice(0,3,2)];
   slice_array<int>&& sa = va[slice(0,3,2)];
   sa = 98;
}
 
int main()
{
   valarray<int> valary(21);
   for(size_t i=0; i<valary.size(); ++i)
   {
      valary[i] = i;
   }
   foo(valary);
}

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