In what order static if conditions are evaluated?

[Don]

On Thursday, 28 March 2013 at 11:29:49 UTC, Artur Zawłocki wrote:
> Hi,
>
> DMD (I'm using v2.060) seems to evaluate conditions in static 
> if's from top to bottom (at least in a given scope). For 
> example, consider the following program:
>
>   module test;
>
>   const bool x = true;
>   const bool y = true;
>
>   struct S {
>
>     static assert(y);
>
>     static if(x)
>        static const bool y = false;
>
>     static if(y)
>        static const bool x = false;
>   }
>
> 'x' in the first static if condition refers to .x and the 
> static if introduces S.y to which 'y' in the second static if 
> refers. 'y' in the static assert also refers to S.y and thus 
> the assertion fails:
>
>   test.d(8): Error: static assert  (y) is false
>
> Now, I guess if the second static if were evaluated first then 
> 'y' in the static assert condition would refer to .y and the 
> program would compile. Note that dmd evaluates the static 
> assert *after* both static if's.
>
> So clearly D program semantics depends on the order static if's 
> and static assert's are evaluated. Are there any standard rules 
> here (I cannot find anything in Language Reference) or is this 
> behaviour implementation dependent?
>
> Artur

Yes, it currently evaluates it top to bottom. An algorithm has 
been discussed which would remove that restriction; your example 
would then compile. It's difficult to implement though. It has to 
deal with paradoxes:

static if (!is(typeof(x)) int y = 1;
static if (!is(typeof(y)) int x = 1;

Who wins?