CTFE with C functions not possible?

[Daniel Kozak via Digitalmars-d-learn]

On Thursday, 31 December 2015 at 11:52:01 UTC, Shriramana Sharma 
wrote:
> Using DMD 2.0.69.2, the following code:
>
> extern (C) double sqrt(double x);
> enum q = sqrt(4.0);
>
> gives the error:
>
> Error: sqrt cannot be interpreted at compile time, because it 
> has no available source code
>
> But if I do:
>
> import std.math;
> enum q = sqrt(4.0);
>
> There is no problem. So two questions:
>
> 1) Why exactly can't the compiler call a C function at compile 
> time whereas it can call a D function?
>
> 2) <same as above> ... which itself only in the end calls that 
> very same C function IIANM?
>
> I see druntime/import/core/math.d l 91:
>
> double sqrt(double x);  /* intrinsic */

No you can't call your own C function with CTFE. I do not say it 
is not possible to implement this in dmd compiler, but IMHO it is 
not something too much usefull