Compare TypeTuple element with another type

[John Colvin via Digitalmars-d]

On Friday, 1 August 2014 at 11:55:02 UTC, Tudor Berariu wrote:
> On Friday, 1 August 2014 at 10:34:02 UTC, Rene Zwanenburg wrote:
>> On Friday, 1 August 2014 at 10:25:39 UTC, Tudor Berariu wrote:
>>> Is it possible to compare at compile time an element from a 
>>> TypeTuple with another type?
>>>
>>> This code fails:
>>>
>>>   alias T = Tuple!(int, bool);
>>>   static assert(is(T[0] == int));
>>>
>>>
>>>   Error: static assert  (is(Tuple!(int, bool)[0] == int)) is 
>>> false
>>>
>>>
>>> Tudor
>>
>> Tuple is not a type tuple, it's a runtime tuple. TypeTuple is 
>> located in std.typetuple. Use that and your code will work.
>
> Thank you! That works indeed, but my problem is actually a bit 
> different.
> I called that a "TypeTuple" because I read this line in the 
> documentation "If a tuple's elements are solely types, it is 
> called a TypeTuple" (http://dlang.org/tuple.html).
>
> What I want to achieve is something like this:
>
>     template isNeededType(T) {
>       enum bool isNeededType = is(T[0] == int);
>     }
>
>     ...
>
>     enum auto t = Tuple!(int, bool)(3, false);
>     alias T = typeof(t);
>     static assert(isNeededType!T);
>
> I know that "is(typeof(t[0]) == int)" works for the above 
> example, but I need to get, if possible, the types of the 
> elements from the type of that tuple only.

template isNeededType(T)
{
     enum isNeededType = is(T[0] == int);
}

enum t = tuple(3, false);
alias T = typeof(t.expand);
static assert(isNeededType!T);