<iterator>: Consider deprecating and removing stdext::checked_array_iterator/unchecked_array_iterator

[StephanTLavavej]

We've been shipping the non-Standard extensions stdext::checked_array_iterator and stdext::unchecked_array_iterator for a long time (I believe since VS 2005):

STL/stl/inc/iterator

Lines 521 to 523 in 5ef22f2

	template <class _Ptr>
	class checked_array_iterator { // wrap a pointer with checking
	static_assert(_STD is_pointer_v<_Ptr>, "checked_array_iterator requires pointers");

STL/stl/inc/iterator

Lines 718 to 720 in 5ef22f2

	template <class _Ptr>
	class unchecked_array_iterator { // wrap a pointer without checking, to silence warnings
	static_assert(_STD is_pointer_v<_Ptr>, "unchecked_array_iterator requires pointers");

As non-Standard extensions, they're rarely used. They haven't been updated to behave like modern iterators (see #943) and they supported a design that we've ripped out (where the STL algorithms used to emit warnings when raw pointers were used as output iterators; this has been superseded by static analysis). Finally, they have been essentially superseded by better types: std::span (which provides debug checks and release performance) and gsl::span (which provides unconditional security checks).

I believe that we should consider marking these stdext iterators as deprecated in VS 2019, and remove them entirely in vNext.

[StephanTLavavej]

@BillyONeal noted that checked_array_iterator is supported in C++14 mode, while std::span is C++20 - but gsl::span is the closer analogue (being always checked) and works in C++14 - on the other hand, it's a third-party dependency.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting, and we are all in favor of deprecating the stdext iterators (with an escape hatch as always). The deprecation message can suggest using either std::span or gsl::span (as the latter is supported in C++14).