opEquals for structs

[Mafi]

Just tried to implemnt Perl6-like junctions. Despite template functions 
overloading against varadic and non-variadic (ie T[] and T[]...) does 
not work, why has a struct opEquals to be S.opEquals(ref const(S))? Why 
can't I compare a class against a struct or in  my case a struct against 
an int? I can't even compare a struct against a struct of another type.
I'm curious for the reason of this.
Mafi