shared - i need it to be useful

[Manu]

On Tue, Oct 16, 2018 at 3:20 AM Timon Gehr via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
>
> On 15.10.2018 20:46, Manu wrote:
> >
> > Assuming the rules above: "can't read or write to members", and the
> > understanding that `shared` methods are expected to have threadsafe
> > implementations (because that's the whole point), what are the risks
> > from allowing T* -> shared(T)* conversion?
> >
>
> Unshared becomes useless, and in turn, shared becomes useless. You can't
> have unshared/shared aliasing.

What aliasing? Please show a reasonable and likely construction of the
problem. I've been trying to think of it.

> > All the risks that I think have been identified previously assume that
> > you can arbitrarily modify the data. That's insanity... assume we fix
> > that... I think the promotion actually becomes safe now...?
>
> But useless, because there is no way to ensure thread safety of reads
> and writes if only one party to the shared state knows about the sharing.

What? I don't understand this sentence.

If a shared method is not threadsafe, then it's an implementation
error. A user should expect that a shared method is threadsafe,
otherwise it shouldn't be a shared method! Thread-local (ie, normal)
methods are for not-threadsafe functionality.