write a function template specialisation that tests if an argument is known at compile time

[Cecil Ward]

On Sunday, 12 August 2018 at 00:55:50 UTC, Paul Backus wrote:
> On Sunday, 12 August 2018 at 00:15:37 UTC, Cecil Ward wrote:
>> Paul, what would the calls look like?
>>
>> I am about to misunderstand things completely so here goes :-)
>>
>> It would be a bit kludgy having to switch from one calling 
>> syntax to another, putting the mask argument in the template 
>> parameters or in the normal position. Or have I misunderstood? 
>> And if the caller did not use the right call syntax variant 
>> then the optimisation would not happen. Thing is, as it is the 
>> details are nicely hidden and the caller does not even need to 
>> thing about the fact that an (eponymous) template is being 
>> used.
>
> As far as I know, there's no way to *guarantee* the 
> optimization and keep the normal function call syntax. Probably 
> the best you can do is write the mask check as a regular if 
> statement, put `pragma(inline, true)` in the function, and hope 
> the optimizer is smart enough to get rid of the branch.

I was thinking about reflection and powerful things like traits. 
Would a test to see if a static if compile do the trick ? You ask 
the question using traits : "does the following compile? : { 
static if ( mask == 3 ) { }; }"  - any use?