sort!("...") with template function?

[Vladimirs Nordholm via Digitalmars-d-learn]

Hello.

I wish to sort an array by calling a template function on a 
struct. In essence I want to do

    foos.sort!("a.get!Dummy < b.get!Dummy");

but I get the error message

    
/dlang/dmd/linux/bin64/../../src/phobos/std/functional.d-mixin-215(215): Error: undefined identifier Dummy

Is there anyway I can get `sort` to recognise my Dummy type?

Example: https://run.dlang.io/is/9zDfdd