[2019-02 LWG Motion 17] P1024R3 Usability Enhancements for std::span

[jensmaurer]

P1024R3

[CaseyCarter]

FYI: this paper has an interaction with P1227R2, which changes the type of span's second template parameter from ptrdiff_t to size_t. After application of P1227R2, the specializations of tuple_size and tuple_element that this paper adds will never be matched (which IIRC means they are ill-formed NDR).

LWG is hoping - @mclow can verify - that we can editorially change the ptrdiff_t's in this paper to size_t's to resolve the conflict.

[mclow]

I offered (in plenary) to consult about merge conflicts between P1024 and P1227. I have implemented both in libc++ w/o any compatibility concerns (doing what Casey suggested)