Missing Symbol Accessing Templated Function Through Interface

[Peter via Digitalmars-d-learn]

Hi,

Can anyone explain to me what's causing the following code to 
generate a missing symbol error...

import std.stdio;

interface IProblem {
    void writeln(T...)(T arguments);
}

class Problem : IProblem {
    void writeln(T...)(T arguments) {
       // This is just here to have code in an implementation.
       stdout.writeln("The implementation function was called.");
    }
}

IProblem getProblem() {
    return(new Problem());
}

void main() {
    auto problem = getProblem();

    problem.writeln("Some text and a number - ", 1234);
}

Now obviously this codes a bit contrived but its my attempt to 
reduce an issue I'm seeing to its more concise form. Note that if 
I cast the return value from the call to getProblem() to a 
Problem instance this rectifies the issue but I'm unclear as to 
why this might be. The error I get when I compile this is...

    Undefined symbols for architecture x86_64:
      "_D3app8IProblem18__T7writelnTAyaTiZ7writelnMFAyaiZv", 
referenced from:
          __Dmain in template_issue.o
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v 
to see invocation)
    --- errorlevel 1
    dmd failed with exit code 1.

Apologies is the cause is blatantly obvious to more experienced D 
coders.

Regards,

Peter