Template-Parameterized Variadic isInstaceOf

["Nordlöw" via Digitalmars-d-learn]

On Friday, 7 August 2015 at 11:45:22 UTC, Nordlöw wrote:
> Can somebody please explain and help out with variadic version 
> of `isInstanceOf`?

Here's a try at isSortedRange:

enum bool isSortedRange(T, alias pred = "a < b") = is(T == 
SortedRange!(Args[0], pred), Args...);

unittest
{
     alias R = int[];
     enum pred = "a < b";
     static assert(isSortedRange!(SortedRange!(R, pred),
                                  pred));
}

but it fails. This simplified case

enum bool isSortedRange(T, alias pred = "a < b") = is(T == 
SortedRange!Args, Args...);

works.

How do check that the second template argument to the instance of 
SortedRange matches `pred`?