Why function template 'among' is of complexity O(1) and not O(n)?

[Gopan]

Recently, I came across the following thread
http://forum.dlang.org/thread/l8nocr$1dv5$1@digitalmars.com?page=2

On Monday, 16 December 2013 at 22:10:52 UTC, Andrei Alexandrescu 
wrote:
> On 12/16/13 1:45 PM, Walter Bright wrote:
>> On 12/16/2013 12:38 PM, Andrei Alexandrescu wrote:
>>> uint among(T, Us...)(T v, Us vals)
>>> {
>>>     foreach (i, U; Us)
>>>     {
>>>         if (v == vals[i]) return i + 1;
>>>     }
>>>     return 0;
>>> }
>>
>> This has O(n) behavior, which might be unexpected for the user.
>
> It's O(1) because the data size is in the source text. There's 
> no variable n.

To me, it looks like a Linear Search and hence of complexity O(n).
Could somebody please elaborate why it is O(1) and not O(n)?
If there is an article/document that I am to read to understand 
this, please cite it.

(I am a beginner level programmer and new to D Language.)

Thanks,
Gopan.