Very limited shared promotion

[Jonathan M Davis]

On Monday, June 17, 2019 5:46:44 PM MDT Manu via Digitalmars-d wrote:
> Is this valid?
>
> int x;
> void fun(scope ref shared(int) x) { ... }
> fun(x); // implicit promotion to shared in this case
>
> This appears to promote a thread-local to shared. The problem with
> such promotion is that it's not valid that a thread-local AND a shared
> reference to the same thing can exist at the same time.
>
> With scope, we can guarantee that the reference doesn't escape the callee.
> Since the argument is local to the calling thread, and since the calling
> thread can not be running other code at the same time as the call is
> executing, there is no way for any code to execute with a thread-local
> assumption while the callee makes shared assumptions.
>
> I think this might be safe?

Another thing to consider here is something that's come up before when
you've made suggestions like this - which is that unlike shared, scope is
not transitive. So, even if what you're suggesting here would work with int,
it would not work with types in general.

- Jonathan M Davis