Associative array literal. Why doesn't it compile?

[Adel Mamin via Digitalmars-d-learn]

Compiler dmd_2.068.0-0_amd64.deb on Ubuntu 12.04 Linux:

auto lookup = [ "one":1, "two":2 ];

The dmd error:
Error: non-constant expression ["one":1, "two":2]

Why doesn't it compile?
As a workaround I could do the assignment one element at a time 
in a loop. It would be uglier though.