Implicit cast to immutable

[bearophile]

Daniel Murphy:

> 2)
> immutable(int[]) fun() { return new int[]; } // conversion happens here
> immutable x  = fun();
> 
> Bearophile's example is of the second, where it definately matters what the 
> purity of the function is.

This is the enhancement request I have written days ago:
http://d.puremagic.com/issues/show_bug.cgi?id=6783

Bye,
bearophile