transform_iterator end iterator with non-default-constructible unary function

[Kojoley]

Currently one have to construct/copy a second copy of transformation function for an end iterator what is unwanted and seems to be omission of the library design. For example, Boost.Range solves it via wrapping user functor with optional, unfortunately.

[Lastique]

Omitting the function object for an end iterator seems like a useful optimization, but the problem is that the end iterator can be made a non-end iterator by decrementing, and then the transform_iterator is expected to work.

I suppose, we could add a new factory function and create a specialized version of transform_iterator for an "end" iterator that will simply abort in runtime on increments, decrements and dereference. Unfortunately, this won't be caught at compile time as the respective operators need to be callable so that the iterator category is still correctly detected by the concepts.