constraint on variadic template

[Alex via Digitalmars-d-learn]

Hi people,
have searched the history, but didn't find something similar:

say I have a template like

mixin template S(T...)
{
     void run()
     {
         foreach(s; T)
         {
             static assert(__traits(hasMember, s, "run"));
         }
     }
}

How to formulate the check inside the foreach as a template 
constraint with
mixin template S(T...) if(???)

have tried so far:
mixin template S(T...) if(__traits(hasMember, T, "run"))
but the compiler complies about
Error: expected 2 arguments for hasMember but had 3
which says to me, that the input is not taken value by value, but 
all values as entirety.

tried:
if(T.each!(s => __traits(hasMember, s, "run")))
but it complies about
cannot deduce function from argument types !()(void, void)
which is ok, as the the provided types are templates by 
themselves.

The formulation at the beginning works, but the intention is a 
little bit different.

Thanks in advance
Alex