How to use template Object in interface?

[zoujiaqing]

hunt-cache current version use template implemention adapter 
changes.

I want use Interface to define Adapter, this master code unable 
to comple.

How to do it? D programming language design flaws?

```bash
git clone https://github.com/huntlabs/hunt-cache
cd hunt-cache/example
dub run -v

...

source/app.d(29,19): Error: no property name for type 
Nullable!(User)

```