More operators inside `is(...)` expressions

[Steven Schveighoffer]

On 8/23/20 5:12 PM, Stefan Koch wrote:
> On Sunday, 23 August 2020 at 21:08:30 UTC, Per Nordlöw wrote:
>> Why aren't more operators allowed inside `is(...)`-expressions?
>>
>> For instance
>>
>>     if (!is(CommonType!(typeof(min), typeof(max)) == void))
>>
>> could be written as
>>
>>     if (is(CommonType!(typeof(min), typeof(max)) != void))

I agree. There's a reason we have != and don't require people to write 
!(x == y)

> 
> Because you don't want to make them even more complicated than they are 
> already.

This one though is really annoying. I've bristled at that a few times 
when I had to write it.

What is the cost here? Just use the same AST node for the is expression, 
and wrap it in a `not` AST node.

-Steve