std::upper_bound returns const iterator in const member function

Here is a class that contains a boost::circular_buffer of some struct. I make a typedef for iterators into the contained circular_buffer.

My problem is this: when the doWork function is marked const, the returned value of std::upper_bound is not compatible with the MyIterator type due to the return value having boost::cb_details::const_traits. If I remove the const keyword from the function, all my compile errors go away.

To be clear the compiler error is this:

error: conversion from ‘boost::cb_details::iterator::Sample, std::allocator::Sample> >, boost::cb_details::const_traits::Sample> > >’ to non-scalar type ‘Wrapper::MyIterator {aka boost::cb_details::iterator::Sample, std::allocator::Sample> >, boost::cb_details::nonconst_traits::Sample> > >}’ requested [](const Sample& a, const Sample& b) { return a.foo < b.foo; });

Here is a self-contained example:

#include <algorithm>
#include <boost/circular_buffer.hpp>

template <typename T>
class Wrapper {
 public:
    struct Sample {
        T foo;
    };

    typedef typename boost::circular_buffer<Sample>::iterator MyIterator;

    Wrapper(int size) { cb.resize(size); }

    void add(T val) { cb.push_back(Sample{val}); }

    void doWork(T bound) const {
        MyIterator iter =
            std::upper_bound(cb.begin(), cb.end(), Sample{3},
                         [](const Sample& a, const Sample& b) { return a.foo < b.foo; });
    }

    boost::circular_buffer<Sample> cb;
};

int main() {
    Wrapper<int> buf(100);
    buf.add(1);
    buf.add(5);
    buf.doWork(3);
    return 0;
}

So, why can't this function be const? Why does marking it const have this side-effect? I want a non-const iterator into the container, but in my real test case I don't intend to actually modify the container at all.