test if the alias of a template is a literal

[TheFlyingFiddle via Digitalmars-d-learn]

On Thursday, 27 October 2016 at 14:45:22 UTC, Gianni Pisetta 
wrote:
> On Thursday, 27 October 2016 at 14:34:38 UTC, TheFlyingFiddle 
> wrote:
>> On Thursday, 27 October 2016 at 14:04:23 UTC, Gianni Pisetta 
>> wrote:
>>> Hi all,
>>> but at the moment isStringLiteral will return true even with 
>>> variables of type string. So i searched for a metod to check 
>>> if an alias is a literal value, but found nothing. Anyone 
>>> have any clue on how can be done?
>>>
>>> Thanks,
>>> Gianni Pisetta
>>
>> Not really understanding your problem. Could you include an 
>> example use that is problematic?
>
> Yea, sorry I missed that.
> A really stupid example would be
>
> string var;
>
> alias Sequence = Optimize!( "The", " ", "value", " ", "of", " 
> ", "var is ", var );
>
> static assert( is( Sequence == AliasSeq!( "The value of var is 
> ", var ) ) );
>
> writeln( Sequence );
>
> given that you include the code snippet in the first post.
>
> Thanks, Gianni

I think this fixes the problem:

template isStringLiteral(T...) if (T.length == 1) {
     static if(is( typeof(T[0]) == string ))
     {
         enum bool isStringLiteral = !__traits(compiles, &T[0]);
     }
     else
     {
         enum bool isStringLiteral = false;
     }
}

Literals do not have an address but variables do.

However note that:
string var;
static assert(!is(AliasSeq!(var) == AliasSeq!(var)));

It still works at run-time though:
string var = " hello ";
alias Seq = Optimize!("This", " is", " a", " variable! ", var);
//pragma(msg, Seq) //Fails to compile at var
//static assert(is(Seq == AliasSeq!("This is a variable!", 
var))); //Also fails
writeln(Seq); //Still works