constraint on variadic template

[Alex via Digitalmars-d-learn]

On Wednesday, 7 December 2016 at 17:08:09 UTC, Gary Willoughby 
wrote:
>
> Will some like this work?
>
>     import std.range.primitives;
>
>     mixin template S(T...) if(__traits(hasMember, 
> ElementType!(T), "run"))
>     {
>         ...
>     }
>
> https://dlang.org/phobos/std_range_primitives.html#ElementType

Nope:
template instance ElementType!(rFS, rAS) does not match template 
declaration ElementType(R)