shared - i need it to be useful

[Nicholas Wilson]

On Sunday, 21 October 2018 at 09:50:09 UTC, Walter Bright wrote:
> ---- Manu's Proposal ---
> @safe:
> int i;
> int* a = &i;
> StartNewThread(a); // Compiles! Coder has no idea!
>
> ... in the new thread ...
> void StartOfNewThread(shared(int)* b) {
>
>     ... we have two threads accessing 'i',
>     one thinks it is shared, the other unshared,
>     and StartOfNewThread() has no idea and anyone
>     writing code for StartOfNewThread() has no way
>     to know anything is wrong ...
>
>     lockedIncrement(b);  // Data Race!

No, does not compile, lockedIncrement takes an int*
Error cannot convert shared(int)* to int*

> Your proposal means that the person writing the 
> lockedIncrement(), which is a perfectly reasonable thing to do,

Indeed.

> simply cannot write it in a way that has a @safe interface, 
> because the person writing the lockedIncrement() library 
> function has no way to know that the data it receives is 
> actually unshared data.

It does, it takes an int* which is not implicitly convertible to 
given an shared(int)*

> I.e. @trusted code is obliged to proved a safe interface.

Yes.

> Your proposal makes that impossible because the compiler would 
> allow unshared data to be implicitly typed as shared.

Yes, but not the other way around.