CTFE with C functions not possible?

[Rikki Cattermole via Digitalmars-d-learn]

On 01/01/16 12:52 AM, 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 */

 From what I've been able to tell, std.math have some special yucky 
rules about it and intrinsics.
It will make it very hard to split std.math up.