My guess is that:
struct S {
int x;
int[] a;
}
S foo(const S b) {
foo makes the promise that b's members won't change: so 'b.x' is
implicitely const(int) and 'b.a' is implicitely const(int[]).
S other = b;
Here you assign 'b.x', which is const(int), to 'other.x', which is int.
You can assign an const(int) to an int: after the value is copied,
changing 'other.x' won't change 'b.x'.
But then you assign 'b.a', which is const(int[]), to 'other.a' which is
int[]. This is forbidden: this would copy the reference to the array
from 'b' to 'other', and after the copy, you would be able to change the
content of 'b.a' by changing the content of 'other.a', thus going
against const.
I'm not sure if this is completely right nor if I'm completely clear,
though... :)
Nicolas
