why won't byPair work with a const AA?

[Steven Schveighoffer via Digitalmars-d-learn]

On 8/1/17 7:15 PM, H. S. Teoh via Digitalmars-d-learn wrote:
> On Tue, Aug 01, 2017 at 07:09:45PM -0400, Steven Schveighoffer via Digitalmars-d-learn wrote:
>> If this were a true implementation without the opaqueness, it would
>> not work properly.
> [...]
> 
> Actually, a proper implementation would still work, provided you declare
> your pointer types carefully.  Sketch of idea:
> 
> 	auto byKeyValue(AA)(AA aa) {
> 		struct Result {
> 			const(Slot)* current; // N.B.: proper type
> 			bool empty() { ... }
> 			auto front() { return Pair(*current); }
> 			void popFront() {
> 				current = current.next;
> 				...
> 			}
> 		}
> 		return Result(aa);
> 	}
> 
> Basically, the type of `current` must be const(Slot)* rather than
> const(Slot*), which would be the default inferred type. But since it's
> legal to assign a const pointer to a pointer to const (you can't modify
> the original pointer, nor what it points to, but it's valid to copy the
> pointer to a mutable pointer variable, as long as what is pointed to is
> still const), this actually will work without breaking / bypassing the
> type system.

No, you can't const the Slot, because if the value type is a pointer, 
you can't then cast away the const-ness of it legally.

There are ways to get it right, it involves templating for mutability.

The current code is pretty horrific though, any way you slice it.

-Steve