implicit conversion to mutable via alias this

[via Digitalmars-d]

I can only guess Kenji's reasons, but I believe it's related to 
DIP49, which he created:

http://wiki.dlang.org/DIP49

The DIP describes how copying between differently qualified types 
should work. I guess his stance is that this should always go via 
the postblit, probably to avoid uncertainties about postblit or 
alias this has priority over the other. IMO this makes sense.

(And you're right, this is currently not implemented.)