.length as lvalue in ctfe?

[Lutger]

As in the subject, is it (meant to be) possible to set the length of an 
array in a function that is executed at compile time? The compiler says 
no. All that the documentation says is that length can be executed at 
compile time, it makes no distinction for setting or getting the length.

Given that opConcat and opConcatAssign are allowed, and thus the 
workarounds are trivial, would this be a bug?