string <-> null/bool implicit conversion

[Steven Schveighoffer via Digitalmars-d]

On 8/21/15 7:34 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm at gmx.net>" 
wrote:
> On Thursday, 20 August 2015 at 19:41:44 UTC, Jonathan M Davis wrote:
>> On Thursday, 20 August 2015 at 17:50:11 UTC, Steven Schveighoffer wrote:
>>> if(arr != null)
>>
>> Definitely don't do that. IMHO, "== null and "!= null" should be
>> illegal. If you really want to check for null, then you need to use
>> "is null" or "!is null", whereas if you want to check that an array is
>> empty, check its length or call empty. By using "== null" or "!=
>> null", you tend to give the false impression that you're checking
>> whether the object or array is null - which is not what you're
>> actually doing.
>
> I disagree. `is null` is the one that should be illegal. `is` is
> supposed to do a bitwise comparison, but `null` is usually just a
> pointer/reference, while a slice consists of both a reference and a
> length. Which of those are compared?

Both. null in this context is actually an array (with null pointer and 
zero length).

null is technically a no-type placeholder (defaulting to void * without 
context). In different contexts it means different things.

arr is null -> both pointer and length are 0
arr == null -> arr contains the same elements that null contains.
arr.ptr == null -> arr contains a null pointer (length could technically 
be non-zero).

-Steve