Addresses US 303

The transform_view does not constrain the return type of the transformation function. It is invalid to pass a void-returning transformation function to the transform_view, which would cause its iterators' operator* member to return void.

Proposed change:

Change the constraints on transform_view to the following:

template<input_range V, copy_constructible F>
  requires view<V> && is_object_v<F> &&
           regular_invocable<F&, range_reference_t<V>> &&
           can-reference<invoke_result_t<F&, range_reference_t<V>>>
class transform_view;

Jonathan Wakely:

The NB comment says "The transform_view does not constrain the return type of the transformation function. It is invalid to pass a void-returning transformation function to the transform_view, which would cause its iterators' operator* member to return void."