Missing Symbol Accessing Templated Function Through Interface

[Nicholas Wilson via Digitalmars-d-learn]

On Tuesday, 8 March 2016 at 11:50:32 UTC, Peter wrote:
> 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

Probably because the idea of a template function in an interface 
is
wrong, and I'm surprised that dmd accepts that. The reason is that
declaring a (non final)template function in an interface is like 
declaring a
function pointer to an uninstansiated(sp?) template function. Also
how many copies should appear in the vtbl for different template
argument combinations?
  Calling a templates on a class is fine, which explains why 
casting
solves the issue.

Solutions: if the function in the interface must take template 
args
parameterise the interface not the function (not sure if this 
works)
OR
if the function must take a variable number of args just use 
regular
runtime varadic arguments.

Nic