Docs: Section on local variables

[Steven Schveighoffer]

On Thu, 26 Apr 2012 14:46:38 -0400, Stewart Gordon <smjg_1998 at yahoo.com>  
wrote:

> I can begin to see why it makes errors for unused variables a bad idea.   
> But why no warnings?  Obviously the user wouldn't like to see warnings  
> thrown at them when they try using templates with such constraints.  But:
>
> - The average programmer is, the vast majority of the time, not writing  
> template constraints, but trying to write bug-free application code.
> - A quality compiler would swallow warnings generated by the content of  
> IsExpressions, just as it already swallows errors generated by them -  
> the only difference being that warnings don't cause the IsExpression to  
> return false.

I think the mechanism is highly desired, but gets in the way in a select  
few situations.  The best answer IMO is to disable that mechanism when  
it's not desired.  It's then an opt-out mechanism that doesn't require  
instrumenting most code.

Some ideas:

pragma(used) int x;
@used int x;

-Steve