Okay, I realize there have been some discussions about this, but I have a
few questions about shared delegates because right now they are definitely
broken, but I'm not sure how.
Take this code example:

synchronized class Thing {
        void doSomeWork(void delegate() work) {
                work();
        }
        void work() {}
}

void main() {
        auto th = new Thing();
        th.doSomeWork(&th.work);
}

This doesn't compile because the type of "&th.work" is "void delegate()
shared", which cannot be cast implicitly to "void delegate()".
My first question would be whether that type is correct. It's true that the
data pointer of the delegate points to a shared object, but given that the
function locks it, does that really matter in this case? I guess I'm just
not clear on the exact meaning of "shared" in general, but it seems like
whether the data is shared or not is irrelevant when the delegate points to
a public member of a synchronized class. If it was a delegate pointing to a
private/protected member (which should be illegal in this case), that would
not be true.
If that type is correct, the problem is that "void delegate() shared"
doesn't parse as a type (there is a workaround because you can create
variables of this type through alias and typeof).

What, exactly, is wrong here?

On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com> said:

> 
> Okay, I realize there have been some discussions about this, but I have a
> few questions about shared delegates because right now they are definitely
> broken, but I'm not sure how.
> Take this code example:
> 
> synchronized class Thing {
>         void doSomeWork(void delegate() work) {
>                 work();
>         }
>         void work() {}
> }
> 
> void main() {
>         auto th = new Thing();
>         th.doSomeWork(&th.work);
> }
> 
> This doesn't compile because the type of "&th.work" is "void delegate()
> shared", which cannot be cast implicitly to "void delegate()".
> My first question would be whether that type is correct. It's true that the
> data pointer of the delegate points to a shared object, but given that the
> function locks it, does that really matter in this case? I guess I'm just
> not clear on the exact meaning of "shared" in general, but it seems like
> whether the data is shared or not is irrelevant when the delegate points to
> a public member of a synchronized class. If it was a delegate pointing to a
> private/protected member (which should be illegal in this case), that would
> not be true.
> If that type is correct, the problem is that "void delegate() shared"
> doesn't parse as a type (there is a workaround because you can create
> variables of this type through alias and typeof).
> 
> What, exactly, is wrong here?

I think what's wrong is that a shared delegate should implicitly convert to a non-shared one. The delegate is shared since it can be called safely from any thread, and making it non-shared only prevent you from propagating it to more thread so it's not harmful in any way.

-- 
Michel Fortin
michel.fortin@michelf.com
http://michelf.com/

Am 17.10.2011 22:43, schrieb Michel Fortin:
> On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com> said:
>
>>
>> Okay, I realize there have been some discussions about this, but I have a
>> few questions about shared delegates because right now they are
>> definitely
>> broken, but I'm not sure how.
>> Take this code example:
>>
>> synchronized class Thing {
>> void doSomeWork(void delegate() work) {
>> work();
>> }
>> void work() {}
>> }
>>
>> void main() {
>> auto th = new Thing();
>> th.doSomeWork(&th.work);
>> }
>>
>> This doesn't compile because the type of "&th.work" is "void delegate()
>> shared", which cannot be cast implicitly to "void delegate()".
>> My first question would be whether that type is correct. It's true
>> that the
>> data pointer of the delegate points to a shared object, but given that
>> the
>> function locks it, does that really matter in this case? I guess I'm just
>> not clear on the exact meaning of "shared" in general, but it seems like
>> whether the data is shared or not is irrelevant when the delegate
>> points to
>> a public member of a synchronized class. If it was a delegate pointing
>> to a
>> private/protected member (which should be illegal in this case), that
>> would
>> not be true.
>> If that type is correct, the problem is that "void delegate() shared"
>> doesn't parse as a type (there is a workaround because you can create
>> variables of this type through alias and typeof).
>>
>> What, exactly, is wrong here?
>
> I think what's wrong is that a shared delegate should implicitly convert
> to a non-shared one. The delegate is shared since it can be called
> safely from any thread, and making it non-shared only prevent you from
> propagating it to more thread so it's not harmful in any way.
>

I reported this exact issue already a few months ago and simply didn't get any comment on it. If you really try to use shared to are going to hit more such problems.

See the shared section of my blogpost: http://3d.benjamin-thaut.de/?p=18
-- 
Kind Regards
Benjamin Thaut

On Tue, Oct 18, 2011 at 12:53 AM, Benjamin Thaut <code@benjamin-thaut.de>wrote:

> Am 17.10.2011 22:43, schrieb Michel Fortin:
>
>  On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com>
>> said:
>>
>>
>>> Okay, I realize there have been some discussions about this, but I have a
>>> few questions about shared delegates because right now they are
>>> definitely
>>> broken, but I'm not sure how.
>>> Take this code example:
>>>
>>> synchronized class Thing {
>>> void doSomeWork(void delegate() work) {
>>> work();
>>> }
>>> void work() {}
>>> }
>>>
>>> void main() {
>>> auto th = new Thing();
>>> th.doSomeWork(&th.work);
>>> }
>>>
>>> This doesn't compile because the type of "&th.work" is "void delegate()
>>> shared", which cannot be cast implicitly to "void delegate()".
>>> My first question would be whether that type is correct. It's true
>>> that the
>>> data pointer of the delegate points to a shared object, but given that
>>> the
>>> function locks it, does that really matter in this case? I guess I'm just
>>> not clear on the exact meaning of "shared" in general, but it seems like
>>> whether the data is shared or not is irrelevant when the delegate
>>> points to
>>> a public member of a synchronized class. If it was a delegate pointing
>>> to a
>>> private/protected member (which should be illegal in this case), that
>>> would
>>> not be true.
>>> If that type is correct, the problem is that "void delegate() shared"
>>> doesn't parse as a type (there is a workaround because you can create
>>> variables of this type through alias and typeof).
>>>
>>> What, exactly, is wrong here?
>>>
>>
>> I think what's wrong is that a shared delegate should implicitly convert to a non-shared one. The delegate is shared since it can be called safely from any thread, and making it non-shared only prevent you from propagating it to more thread so it's not harmful in any way.
>>
>>
> I reported this exact issue already a few months ago and simply didn't get any comment on it. If you really try to use shared to are going to hit more such problems.
>
> See the shared section of my blogpost: http://3d.benjamin-thaut.de/?**p=18<http://3d.benjamin-thaut.de/?p=18>
>
>
Ah, I was looking through the bug reports and didn't see this exact bug. Did I just fail at searching, or should I file it?

Am 18.10.2011 08:03, schrieb Andrew Wiley:
> On Tue, Oct 18, 2011 at 12:53 AM, Benjamin Thaut <code@benjamin-thaut.de
> <mailto:code@benjamin-thaut.de>> wrote:
>
>     Am 17.10.2011 22:43, schrieb Michel Fortin:
>
>         On 2011-10-17 20:33:59 +0000, Andrew Wiley
>         <wiley.andrew.j@gmail.com <mailto:wiley.andrew.j@gmail.com>> said:
>
>
>             Okay, I realize there have been some discussions about this,
>             but I have a
>             few questions about shared delegates because right now they are
>             definitely
>             broken, but I'm not sure how.
>             Take this code example:
>
>             synchronized class Thing {
>             void doSomeWork(void delegate() work) {
>             work();
>             }
>             void work() {}
>             }
>
>             void main() {
>             auto th = new Thing();
>             th.doSomeWork(&th.work);
>             }
>
>             This doesn't compile because the type of "&th.work" is "void
>             delegate()
>             shared", which cannot be cast implicitly to "void delegate()".
>             My first question would be whether that type is correct.
>             It's true
>             that the
>             data pointer of the delegate points to a shared object, but
>             given that
>             the
>             function locks it, does that really matter in this case? I
>             guess I'm just
>             not clear on the exact meaning of "shared" in general, but
>             it seems like
>             whether the data is shared or not is irrelevant when the
>             delegate
>             points to
>             a public member of a synchronized class. If it was a
>             delegate pointing
>             to a
>             private/protected member (which should be illegal in this
>             case), that
>             would
>             not be true.
>             If that type is correct, the problem is that "void
>             delegate() shared"
>             doesn't parse as a type (there is a workaround because you
>             can create
>             variables of this type through alias and typeof).
>
>             What, exactly, is wrong here?
>
>
>         I think what's wrong is that a shared delegate should implicitly
>         convert
>         to a non-shared one. The delegate is shared since it can be called
>         safely from any thread, and making it non-shared only prevent
>         you from
>         propagating it to more thread so it's not harmful in any way.
>
>
>     I reported this exact issue already a few months ago and simply
>     didn't get any comment on it. If you really try to use shared to are
>     going to hit more such problems.
>
>     See the shared section of my blogpost:
>     http://3d.benjamin-thaut.de/?__p=18 <http://3d.benjamin-thaut.de/?p=18>
>
>
> Ah, I was looking through the bug reports and didn't see this exact bug.
> Did I just fail at searching, or should I file it?
>

I didn't file it yet, so file it. I do however think that currently there is no intention in changing the way shared works.

-- 
Kind Regards
Benjamin Thaut

On Tue, Oct 18, 2011 at 1:11 AM, Benjamin Thaut <code@benjamin-thaut.de>wrote:

> Am 18.10.2011 08:03, schrieb Andrew Wiley:
>
>> On Tue, Oct 18, 2011 at 12:53 AM, Benjamin Thaut <code@benjamin-thaut.de <mailto:code@benjamin-thaut.de**>> wrote:
>>
>>    Am 17.10.2011 22:43, schrieb Michel Fortin:
>>
>>        On 2011-10-17 20:33:59 +0000, Andrew Wiley
>>        <wiley.andrew.j@gmail.com <mailto:wiley.andrew.j@gmail.**com<wiley.andrew.j@gmail.com>>>
>> said:
>>
>>
>>            Okay, I realize there have been some discussions about this,
>>            but I have a
>>            few questions about shared delegates because right now they are
>>            definitely
>>            broken, but I'm not sure how.
>>            Take this code example:
>>
>>            synchronized class Thing {
>>            void doSomeWork(void delegate() work) {
>>            work();
>>            }
>>            void work() {}
>>            }
>>
>>            void main() {
>>            auto th = new Thing();
>>            th.doSomeWork(&th.work);
>>            }
>>
>>            This doesn't compile because the type of "&th.work" is "void
>>            delegate()
>>            shared", which cannot be cast implicitly to "void delegate()".
>>            My first question would be whether that type is correct.
>>            It's true
>>            that the
>>            data pointer of the delegate points to a shared object, but
>>            given that
>>            the
>>            function locks it, does that really matter in this case? I
>>            guess I'm just
>>            not clear on the exact meaning of "shared" in general, but
>>            it seems like
>>            whether the data is shared or not is irrelevant when the
>>            delegate
>>            points to
>>            a public member of a synchronized class. If it was a
>>            delegate pointing
>>            to a
>>            private/protected member (which should be illegal in this
>>            case), that
>>            would
>>            not be true.
>>            If that type is correct, the problem is that "void
>>            delegate() shared"
>>            doesn't parse as a type (there is a workaround because you
>>            can create
>>            variables of this type through alias and typeof).
>>
>>            What, exactly, is wrong here?
>>
>>
>>        I think what's wrong is that a shared delegate should implicitly
>>        convert
>>        to a non-shared one. The delegate is shared since it can be called
>>        safely from any thread, and making it non-shared only prevent
>>        you from
>>        propagating it to more thread so it's not harmful in any way.
>>
>>
>>    I reported this exact issue already a few months ago and simply
>>    didn't get any comment on it. If you really try to use shared to are
>>    going to hit more such problems.
>>
>>    See the shared section of my blogpost:
>>    http://3d.benjamin-thaut.de/?_**_p=18<http://3d.benjamin-thaut.de/?__p=18><
>> http://3d.benjamin-thaut.de/?**p=18 <http://3d.benjamin-thaut.de/?p=18>>
>>
>>
>>
>> Ah, I was looking through the bug reports and didn't see this exact bug. Did I just fail at searching, or should I file it?
>>
>>
> I didn't file it yet, so file it. I do however think that currently there is no intention in changing the way shared works.
>
>
This much is clearly a bug, and Michel's explanation of how shared delegates
should work makes a lot of sense.
As for the synchronized classes that synchronize access to not-really-shared
members, well, I don't think that's changing. The problem is that the
compiler can't *guarantee* that the reference you hold is the only
reference. TDPL has a discussion of why an "A owns B, so A's lock should be
good enough for B" wasn't implemented.
As for the overloading on shared, the idea there was that you should really
either be multithreaded or not - trying to implement both shared and
non-shared versions of code is generally a bad idea and quite bug prone. If
it *might* be shared, just synchronize it. Premature optimization is the
root of quite a bit of evil, and trying to make some things thread safe and
some things not is walking a tightrope without any sense of balance - you
won't know when you fall, you'll just see the corruption when you hit the
floor.

On Tue, Oct 18, 2011 at 1:48 AM, Andrew Wiley <wiley.andrew.j@gmail.com>wrote:

> On Tue, Oct 18, 2011 at 1:11 AM, Benjamin Thaut <code@benjamin-thaut.de>wrote:
>
>> Am 18.10.2011 08:03, schrieb Andrew Wiley:
>>
>>> On Tue, Oct 18, 2011 at 12:53 AM, Benjamin Thaut <code@benjamin-thaut.de <mailto:code@benjamin-thaut.de**>> wrote:
>>>
>>>    Am 17.10.2011 22:43, schrieb Michel Fortin:
>>>
>>>        On 2011-10-17 20:33:59 +0000, Andrew Wiley
>>>        <wiley.andrew.j@gmail.com <mailto:wiley.andrew.j@gmail.**com<wiley.andrew.j@gmail.com>>>
>>> said:
>>>
>>>
>>>            Okay, I realize there have been some discussions about this,
>>>            but I have a
>>>            few questions about shared delegates because right now they
>>> are
>>>            definitely
>>>            broken, but I'm not sure how.
>>>            Take this code example:
>>>
>>>            synchronized class Thing {
>>>            void doSomeWork(void delegate() work) {
>>>            work();
>>>            }
>>>            void work() {}
>>>            }
>>>
>>>            void main() {
>>>            auto th = new Thing();
>>>            th.doSomeWork(&th.work);
>>>            }
>>>
>>>            This doesn't compile because the type of "&th.work" is "void
>>>            delegate()
>>>            shared", which cannot be cast implicitly to "void delegate()".
>>>            My first question would be whether that type is correct.
>>>            It's true
>>>            that the
>>>            data pointer of the delegate points to a shared object, but
>>>            given that
>>>            the
>>>            function locks it, does that really matter in this case? I
>>>            guess I'm just
>>>            not clear on the exact meaning of "shared" in general, but
>>>            it seems like
>>>            whether the data is shared or not is irrelevant when the
>>>            delegate
>>>            points to
>>>            a public member of a synchronized class. If it was a
>>>            delegate pointing
>>>            to a
>>>            private/protected member (which should be illegal in this
>>>            case), that
>>>            would
>>>            not be true.
>>>            If that type is correct, the problem is that "void
>>>            delegate() shared"
>>>            doesn't parse as a type (there is a workaround because you
>>>            can create
>>>            variables of this type through alias and typeof).
>>>
>>>            What, exactly, is wrong here?
>>>
>>>
>>>        I think what's wrong is that a shared delegate should implicitly
>>>        convert
>>>        to a non-shared one. The delegate is shared since it can be called
>>>        safely from any thread, and making it non-shared only prevent
>>>        you from
>>>        propagating it to more thread so it's not harmful in any way.
>>>
>>>
>>>    I reported this exact issue already a few months ago and simply
>>>    didn't get any comment on it. If you really try to use shared to are
>>>    going to hit more such problems.
>>>
>>>    See the shared section of my blogpost:
>>>    http://3d.benjamin-thaut.de/?_**_p=18<http://3d.benjamin-thaut.de/?__p=18><
>>> http://3d.benjamin-thaut.de/?**p=18 <http://3d.benjamin-thaut.de/?p=18>>
>>>
>>>
>>>
>>> Ah, I was looking through the bug reports and didn't see this exact bug. Did I just fail at searching, or should I file it?
>>>
>>>
>> I didn't file it yet, so file it. I do however think that currently there is no intention in changing the way shared works.
>>
>>
> This much is clearly a bug, and Michel's explanation of how shared delegates should work makes a lot of sense.
>

Filed as http://d.puremagic.com/issues/show_bug.cgi?id=6823 For now, I can just cast things at the call site.

On Mon, Oct 17, 2011 at 3:43 PM, Michel Fortin <michel.fortin@michelf.com>wrote:

> On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com> said:
>
>
>> Okay, I realize there have been some discussions about this, but I have a
>> few questions about shared delegates because right now they are definitely
>> broken, but I'm not sure how.
>> Take this code example:
>>
>> synchronized class Thing {
>>        void doSomeWork(void delegate() work) {
>>                work();
>>        }
>>        void work() {}
>> }
>>
>> void main() {
>>        auto th = new Thing();
>>        th.doSomeWork(&th.work);
>> }
>>
>> This doesn't compile because the type of "&th.work" is "void delegate()
>> shared", which cannot be cast implicitly to "void delegate()".
>> My first question would be whether that type is correct. It's true that
>> the
>> data pointer of the delegate points to a shared object, but given that the
>> function locks it, does that really matter in this case? I guess I'm just
>> not clear on the exact meaning of "shared" in general, but it seems like
>> whether the data is shared or not is irrelevant when the delegate points
>> to
>> a public member of a synchronized class. If it was a delegate pointing to
>> a
>> private/protected member (which should be illegal in this case), that
>> would
>> not be true.
>> If that type is correct, the problem is that "void delegate() shared"
>> doesn't parse as a type (there is a workaround because you can create
>> variables of this type through alias and typeof).
>>
>> What, exactly, is wrong here?
>>
>
> I think what's wrong is that a shared delegate should implicitly convert to a non-shared one. The delegate is shared since it can be called safely from any thread, and making it non-shared only prevent you from propagating it to more thread so it's not harmful in any way.
>
>
Actually, I've been thinking about this some more, and I think that the
delegate should only implicitly convert if the argument types are safe to
share across threads as well.
If I had a class that looked like this:
synchronized class Thing2 {
       void doSomeWork(int i) {}
       void doSomeOtherWork(Thing2 t) {}
       void work() {}
}

The actual argument type in doSomeOtherWork is required to be shared(Thing2)
(which isn't a problem here because Thing2 is a synchronized class, but you
see the point).
These same rules should apply to shared delegates. I'm not sure exactly how
this works for value types because they're always safe to pass as arguments
yet shared(int) clearly isn't the same as int, but the rules will be the
same for delegates as for member functions.

With that in mind, I'm not sure exactly how the implicit conversions should work, but the argument types should always be properly share-able.

On 2011-10-19 20:36:37 +0000, Andrew Wiley <wiley.andrew.j@gmail.com> said:

> 
> On Mon, Oct 17, 2011 at 3:43 PM, Michel Fortin <michel.fortin@michelf.com>wrote:
> 
>> On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com>
>> said:
>> 
>> 
>>> Okay, I realize there have been some discussions about this, but I have a
>>> few questions about shared delegates because right now they are definitely
>>> broken, but I'm not sure how.
>>> Take this code example:
>>> 
>>> synchronized class Thing {
>>> void doSomeWork(void delegate() work) {
>>> work();
>>> }
>>> void work() {}
>>> }
>>> 
>>> void main() {
>>> auto th = new Thing();
>>> th.doSomeWork(&th.work);
>>> }
>>> 
>>> This doesn't compile because the type of "&th.work" is "void delegate()
>>> shared", which cannot be cast implicitly to "void delegate()".
>>> My first question would be whether that type is correct. It's true that
>>> the
>>> data pointer of the delegate points to a shared object, but given that the
>>> function locks it, does that really matter in this case? I guess I'm just
>>> not clear on the exact meaning of "shared" in general, but it seems like
>>> whether the data is shared or not is irrelevant when the delegate points
>>> to
>>> a public member of a synchronized class. If it was a delegate pointing to
>>> a
>>> private/protected member (which should be illegal in this case), that
>>> would
>>> not be true.
>>> If that type is correct, the problem is that "void delegate() shared"
>>> doesn't parse as a type (there is a workaround because you can create
>>> variables of this type through alias and typeof).
>>> 
>>> What, exactly, is wrong here?
>> 
>> I think what's wrong is that a shared delegate should implicitly convert to
>> a non-shared one. The delegate is shared since it can be called safely from
>> any thread, and making it non-shared only prevent you from propagating it to
>> more thread so it's not harmful in any way.
> 
> Actually, I've been thinking about this some more, and I think that the
> delegate should only implicitly convert if the argument types are safe to
> share across threads as well.

I disagree.


> If I had a class that looked like this:
> synchronized class Thing2 {
>        void doSomeWork(int i) {}
>        void doSomeOtherWork(Thing2 t) {}
>        void work() {}
> }
> 
> The actual argument type in doSomeOtherWork is required to be shared(Thing2)
> (which isn't a problem here because Thing2 is a synchronized class, but you
> see the point).

Is it? Whether the argument was shared or not, the thread in which the function code runs can only access thread-local data from that thread, including the arguments and global variables. It won't be able to send  references to non-shared data to other threads just because its context pointer is shared (or synchronized).


-- 
Michel Fortin
michel.fortin@michelf.com
http://michelf.com/

On Wed, Oct 19, 2011 at 4:11 PM, Michel Fortin <michel.fortin@michelf.com>wrote:

> On 2011-10-19 20:36:37 +0000, Andrew Wiley <wiley.andrew.j@gmail.com> said:
>
>
>> On Mon, Oct 17, 2011 at 3:43 PM, Michel Fortin <michel.fortin@michelf.com
>> >**wrote:
>>
>>  On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com>
>>> said:
>>>
>>>
>>>  Okay, I realize there have been some discussions about this, but I have
>>>> a
>>>> few questions about shared delegates because right now they are
>>>> definitely
>>>> broken, but I'm not sure how.
>>>> Take this code example:
>>>>
>>>> synchronized class Thing {
>>>> void doSomeWork(void delegate() work) {
>>>> work();
>>>> }
>>>> void work() {}
>>>> }
>>>>
>>>> void main() {
>>>> auto th = new Thing();
>>>> th.doSomeWork(&th.work);
>>>> }
>>>>
>>>> This doesn't compile because the type of "&th.work" is "void delegate()
>>>> shared", which cannot be cast implicitly to "void delegate()".
>>>> My first question would be whether that type is correct. It's true that
>>>> the
>>>> data pointer of the delegate points to a shared object, but given that
>>>> the
>>>> function locks it, does that really matter in this case? I guess I'm
>>>> just
>>>> not clear on the exact meaning of "shared" in general, but it seems like
>>>> whether the data is shared or not is irrelevant when the delegate points
>>>> to
>>>> a public member of a synchronized class. If it was a delegate pointing
>>>> to
>>>> a
>>>> private/protected member (which should be illegal in this case), that
>>>> would
>>>> not be true.
>>>> If that type is correct, the problem is that "void delegate() shared"
>>>> doesn't parse as a type (there is a workaround because you can create
>>>> variables of this type through alias and typeof).
>>>>
>>>> What, exactly, is wrong here?
>>>>
>>>
>>> I think what's wrong is that a shared delegate should implicitly convert
>>> to
>>> a non-shared one. The delegate is shared since it can be called safely
>>> from
>>> any thread, and making it non-shared only prevent you from propagating it
>>> to
>>> more thread so it's not harmful in any way.
>>>
>>
>> Actually, I've been thinking about this some more, and I think that the delegate should only implicitly convert if the argument types are safe to share across threads as well.
>>
>
> I disagree.
>
>
>
>  If I had a class that looked like this:
>> synchronized class Thing2 {
>>       void doSomeWork(int i) {}
>>       void doSomeOtherWork(Thing2 t) {}
>>       void work() {}
>> }
>>
>> The actual argument type in doSomeOtherWork is required to be
>> shared(Thing2)
>> (which isn't a problem here because Thing2 is a synchronized class, but
>> you
>> see the point).
>>
>
> Is it? Whether the argument was shared or not, the thread in which the
> function code runs can only access thread-local data from that thread,
> including the arguments and global variables. It won't be able to send
>  references to non-shared data to other threads just because its context
> pointer is shared (or synchronized).
>
>
>
The problem is that what's behind the context pointer is also shared. If
this delegate is just a closure, that doesn't matter, since the context is
basically immutable.
The problem I see is when the delegate is actually a member function that
stores data in an object. If it was passed a reference to non-shared data,
it could store that reference in a shared object, breaking transitive
shared.