I'm getting an unhelpful linker error, what've I got wrong?

[pineapple via Digitalmars-d-learn]

On Wednesday, 28 October 2015 at 11:40:14 UTC, tcak wrote:
> The "writebuffer" is defined to take an array as parameter. 
> Yet, you are passing a pointer and a length to it. Instead, 
> pass the parameter "str" to it directly. Also, you do not have 
> to put "!char" to there. Compiler will solve it out by itself.

There's also a writebuffer method in the interface with this 
signature, though:

     streamint writebuffer(T)(in T* buffer, in streamint count);

And regardless, changing the problematic code to this doesn't 
address the linker error:

     final streamint writestring(in char[] str){
         return this.writebuffer(str);
     }
     final streamint writestring(in string str){
         return this.writebuffer(str);
     }