Debugging silent exit of threads in Phobos calls

[Russel Winder]

On Fri, 2018-06-01 at 16:19 -0400, Steven Schveighoffer via
Digitalmars-d-learn wrote:
> On 6/1/18 1:41 PM, Russel Winder wrote:
> > struct Datum {
> > 	public const int a;
> > 	public const int b;
> > }
> > 
> > struct Message {
> > 	Datum datum;
> > }
> 
> I found the bug. Basically, the Variant static if is failing because
> the 
> assignment it is checking (assigning a Message to a Message) would 
> overwrite const data. But it's not really overwriting existing data, 
> it's emplacing into new data (always). So this check is not correct.

I think then my bug report 18934 is slightly wrong and it isn't the
struct within struct that is the problem, it is the const fields in a
struct that is?

> Much simpler test case:
> 
> struct S
> {
>     const int x;
> }
> 
> void main()
> {
>     import std.variant;
>     import std.stdio;
>     Variant v = S(1);
>     writeln(v.get!S); // same failure
> }
> 

Aha, much nicer since it is far more localised to the issue. I was
still focused on the receive and the struct within struct, which seems
to be the wrong issue: it is the const fields that are the problem.
Feel free to ignore my example and stack trace on

https://issues.dlang.org/show_bug.cgi?id=18934

and replace it with the above!

> If I replace the check in std.variant:
> 
>                  static if (is(typeof(*cast(T*) target = *src)) ||
> 
> with:
> 
>                  static if (is(typeof(delegate T() {return *src;}))
> ||
> 
> Then it works (both your code, and the simple example).
> 
> Note that in all cases, variant is returning a COPY of the data, not
> a 
> reference, so it shouldn't be possible to violate const.

Excellently done. Thanks for attacking this problem and finding a
solution. I will not now even contemplate hacking up my code. Actually
I would have put const in and still got a problem it seems, as I had
the wrong reason for the problem.

> Please, file a bug. I will see if I can submit a PR to fix.
> 

Bug report is 18934.

If there can be a fast release of this via 2.080.1 with subsequence
fast releases via d-apt and Debian and Fedora packaging, I would be a
very happy bunny.

In the meantime onward with ACCU 2019 organisation.

ACCU needs more D content. Yes it is 50% C++, but that is exactly why
it needs D content. It also needs Go content. in 2018 we had Rust
content and it went down well.

-- 
Russel.
===========================================
Dr Russel Winder      t: +44 20 7585 2200
41 Buckmaster Road    m: +44 7770 465 077
London SW11 1EN, UK   w: www.russel.org.uk
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