LWG 4272 For rank == 0, layout_stride is atypically convertible

[JeffGarland]

Priority 2 issue for mdspan

https://cplusplus.github.io/LWG/lwg-active.html#4272

[bstamour]

LWG polled this in Kona.

Poll: accept the proposed resolution for LWG4272 and move to immediate?
|F|A|N|
|8|0|0|