=void in struct definition

[Johan Engelen]

On Monday, 9 April 2018 at 11:06:50 UTC, Shachar Shemesh wrote:
> struct S {
>   int a;
>   int[5000] arr = void;
> }
>
> void func() {
>   S s;
> }
>
> During the s initialization, the entire "S" area is 
> initialized, including the member arr which we asked to be = 
> void.
>
> Is this a bug?

Could be optimized, yes, provided that the spec is updated. We 
discussed this live at the end of my DConf talk last year, and 
Walter (in audience) agreed upon the needed spec change. I 
haven't had/taken the time to work on it yet :(

The optimization of simplifying the initialization isn't too 
hard. But it is a bit tricky, Johannes wrote down some good 
points here: https://issues.dlang.org/show_bug.cgi?id=15951
(note the padding bytes issue).  The good news is that there 
doesn't appear to be any spec about it, so technically there is 
no language breakage and currently it is an "accepts invalid" 
bug...

Over dinner me, deadalnix and some others discussed further 
optimization where emission of the large S.init could be 
eliminated. We worked out some details, but it's a little harder 
thing to do.

cheers,
   Johan