Linker Error with Template Function

[Kyle Ingraham]

I am writing a library where I would like to be able to store 
instances of a type of class to an associative array for later 
usage. Each class stored has to implement a function as part of 
the required interface. The argument given is always the same 
type but the return value should be flexible. I solved this with 
an interface:

```d
interface PathConverter
{
     T toD(T)(const string value) @safe;
}
```

That interface lets me type the associative array and any other 
part of the library that needs to use implementers of that 
interface e.g.

```d
PathConverter[string] converters;
```

The problem I'm running into is that when compile the library I 
receive the following error during linking:

```
error LNK2019: unresolved external symbol 
_D3app13PathConverter__T3toDTAyaZQjMFNfxAyaZQp referenced in 
function 
_D3app14TypedURLRouter__T10setHandlerTPFNfC4vibe4http6server17HTTPServerRequestCQBlQBjQBh18HTTPServerResponseAyaZvZQDmMFEQDaQCy6common10HTTPMethodQBlQEhZ__T9__lambda4TQEvTQDoTASQGt16PathCaptureGroupZQBrMFNfQGiQFaQBlZv

fatal error LNK1120: 1 unresolved externals

Error: linker exited with status 1120
```

The call is within a delegate to a function that returns a class 
instance from the associative array. At runtime I fill the 
associative array. The call looks like this:

```d
tailArgs[i] = getPathConverter("id 
string").toD!(Parameters!(handler)[i])("string to convert");
```

Am I running into this error because the linker can't find the 
instantiation of the template method? How would I give the linker 
the information it needs? Is there a better way to have an 
interface with flexible return values?