Ok, this looks like a straightforward addition. I will finalize this review once the parent review is completed though.
Rebase on ToT
Ok, so you want to allow converting implicitly to global but not to device/host?
Let's update the doc straight away!
Let's add explicit casts too.
Can you also update this comment?
Apply code review suggestions and rebase on ToT.
Added explicit cast checks to Sema tests.
I think we should extend conversion rules too.
Added documentation to cover conversion rules and rebased on ToT.