dereference operators should be const

[thomas001]

Currently a lot iterators have only non-const operator* or operator-> (for example in groupby).

This is not correct according to the legacy and C++20 iterator concepts:

• LegacyInputIterator states that *i and i->m should be valid expressions given

  i and j, values of type It or const It

  Note the inclusion of const It here.

• std::indirectly_readable (which is required by std::input_iterator) has

  requires(const In in) {
    // ...
    { *in } -> std::same_as<std::iter_reference_t<In>>;
    // ....
  } &&
  

  Note that the concept requires *in vor values of const In.

Since iterators are copyable, one can always work around this by copying a const itertools iterator and dereferencing the copy. Yet, this might be inefficient for some iterators. Also, itertools iterators not implementing c++20 iterator concepts will probably make them much less useful in the future. Finally, there are other range and iterator adapter libraries out there which might assume a const iterator is dereferenceable.

For some iterators in itertools the fix seems to be straightforward by adding const to the operators. Others, like groupby, seem to require more changes.