Fixing spurious "statement is not reachable" in template code

[Steven Schveighoffer via Digitalmars-d]

On 10/27/15 12:04 PM, tsbockman wrote:
> On Tuesday, 27 October 2015 at 12:28:38 UTC, Steven Schveighoffer wrote:
>> On 10/26/15 1:08 PM, tsbockman wrote:
>>> My solution #2 is the same as the one you proposed - the dummy parameter
>>> stuff is my vague proposal for a way to actual implement this behavior
>>> in the compiler.
>>>
>>> The halting problem is no more of an issue than it is for the compiler
>>> today. (That is to say, the halting problem leads to false negatives,
>>> but not false positives.)
>>
>> OK, as long as the default behavior isn't to reject the code, I
>> suppose this is simply an optimization.
>
> Right.
>
>> But the problem I see is that only static ifs that reduce to static
>> if(true) would be considered to cause a short-circuit (e.g. someint <=
>> int.max). Unless the compiler can VRP according to the template
>> constraint, which may be possible in simple circumstances, but not in
>> all circumstances.
>
> I think the next piece of Lionello Lunesu's PR which I am updating might
> actually do that; I'll have to check later. Regardless, you are correct
> solution #1 will substantially reduce the opportunities for constant
> folding.

I'm not a compiler dev, so I'm not sure how it works or should work. I'm 
looking at this from a user standpoint.

I didn't consider that the compiler is likely already using VRP to 
determine whether a line of code is unnecessary. If it's a matter of 
just keeping track of 2 VRP ranges (the actual VRP and the VRP just for 
checking reachability) for each line of code, then perhaps it's just 
easier to extend VRP. I don't know the answer.

All I was saying is that it's not necessary to add on an additional 
layer if it's not present.

I defer to the actual devs as to what is easier.

-Steve