Indexing with an arbitrary type

[Alex via Digitalmars-d-learn]

Hi everybody,
I have a question and a half on templates and ranges, this time.
Say, I have two functions:

auto f1(T, S)(T t, S s) if(isIntegral!T && isRandomAccessRange!S)
{
     return s[t];
}

auto f2(T, S)(T t, RandomAccessFinite!S raf) if(isIntegral!T)
{
     return raf[t];
}

and a
class myClass : RandomAccessFinite!ubyte {...}
which implements all the methods needed by the RandomAccessFinite 
interface.

then, I could use this in my main by just

void main()
{
     myClass mC = new myClass();
     writeln(f2(1, mC));
     writeln(f1(1, mC));

     ubyte[] arr = [0, 42, 2, 3, 4];
     writeln(f1(1, arr));
     //writeln(f2(1, arr)); //this fails and is the first part of 
the question.
}

so, the first question is, why the method using the 
RandomAccessFinite interface fails on using an array? Did I miss 
a method, which is not supported by an array?

But the main question is about the other part, about the 
constraint to the first parameter to my functions.
It seems strange to me to use "isIntegral" here, as this is some 
kind of unrelated for something used as an index.
Is there anything more appropriate to check? What kind of 
interface/trait has an index to fulfill?