Restriction on interface function types

[Chris Williams]

On Wednesday, 12 March 2014 at 11:56:43 UTC, Steven Schveighoffer 
wrote:
> What you want simply isn't possible. An interface binds at 
> runtime, and you need to declare types at compile-time. You 
> can't use an interface method to define the type of y.

Here's the method that is used in Phobos:

http://forum.dlang.org/thread/yagdbyegdkibhaqlbyid@forum.dlang.org#post-zkdhdiszjklaepgkbxpj:40forum.dlang.org