How to make a global immutable associative array?

[Dan Killebrew]

> You just have to construct it first and then claim that it is 
> unique and so safely cast it to immutable:
>
>
> import std.exception : assumeUnique;
>
> immutable int[int] aa;
>
> static this(){
>    auto temp = [1:2, 3:4];
>    aa = assumeUnique(temp);
> }

Seems unintuitive and roundabout. Is this a bug or a feature?