Implicit conversion from const to mutable

[Balagopal Komarath via Digitalmars-d-learn]

Is it possible to make structs containing slices support implicit 
conversion from const to mutable? I tried adding a postblit that 
dupes the member 'a'. That didn't work.

struct A
{
     int[] a;
}

void main()
{
     static assert (is(const(A) : A)); // fails
}