Structures and CTFE

[bauss]

On Monday, 3 September 2018 at 13:39:25 UTC, agorkvmh wrote:
> Hi all,
>
> Running this:
> ---
> static assert(Foo(1).pos == 2);
>
>
> struct Foo
> {
>     this(int i){ advance(); }
>
>     size_t pos =1;
>
>     void advance()
>     {
>         pragma(msg, pos);
>         pos = pos+1;
>         pragma(msg, pos);
>     }
> }
>
> dmd -o- -unittest source/pgs/parser.d
> 1LU
> 1LU
> ---
>
> The static assert passes, but why the second pragma print '1'?
>
> Thanks

It prints 1, because pragma(msg) is called when the compiler is 
analyzing the code through the semantic process and not when the 
body of the function is executed during CTFE.

And since "pos = pos+1" is a runtime construct (technically) then 
the expression is ignored, unless the function is called during 
CTFE, but by the time the function is called during CTFE then the 
pragma(msg) has already been executed.