I have not looked at it in detail. I was just looking for an update on index_type, but I don't think this PR would be OK in the future since index_type and difference_type are different types (see https://en.cppreference.com/w/cpp/container/span or https://github.com/martinmoene/span-lite#select-index-type)

@reddwarf69 no, you have connected the wrong dots.
@annagrin can we please get this PR merged, one other PR by me is also remaining to get merged.

@JordanMaples is this boiling down to gsl::span not following the standard? if yes, lets have a bigger PR making it so.

Maintainer's call: Looks good, thank you.

What have I got wrong here:

This constructor is going to be called by span::end(). span::end() is going to use span::size() as the "idx" parameter of this constructor. span::size() returns a span::index_type. After this PR "idx" is now of type span_iterator::difference_type.
As it is right now span_iterator::difference_type == span::index_type == "the type returned by span::size()" == "the type used by span::end()", no issues.

But whenever GSL's span is made to follow the standard somebody is going to create span::difference_type and will redefine span_iterator::difference_type as span::difference_type. Now if I have a span of size SIZE_MAX, and SIZE_MAX > PTRDIFF_MAX, span::end() is going to fail (and would have not before this PR).

?