Functions cannot be interpreted while being compiled

[Timon Gehr via Digitalmars-d]

On 25.08.2016 05:11, ketmar wrote:
> On Thursday, 25 August 2016 at 02:20:31 UTC, Andrei Alexandrescu wrote:
>> Just got the error above. Is this fundamental or an implementation
>> artifact? -- Andrei
>
> fundamental.

(The remainder of the post is describing implementation details; this 
doesn't explain why the limitation is fundamental.)

> interpreter operates only on fully semanticed functions,
> where all the analysis, type inferencing and lowering is already
> complete (otherwise interpreter have to reimplement *all* the frontend
> again, to do exactly same work).
> ...

Usually, to do exactly the same work, one would just use exactly the 
same code.

> the error mesage you got means that CTFE tried to execute something that
> is still in semantic analysis stage.

No, it means CTFE tried to execute a function of which some parts are 
still in semantic analysis stage. E.g., it is clear what the following 
function should mean, if it was assigned a meaning:

int foo(int x){
     if(x==0) return 1;
     enum r=foo(0);
     return r;
}

Usually CTFE only fails if the dynamic branch actually taken runs into a 
problem, so this is somewhat inconsistent.