class B {}
class C: B {}

var f1: ((B, B)) -> () = {p in }
var f2: ((C, C)) -> () = f1 //note: compiler allows this assignment

var v: (C, C) = (C(), C())

//at this point f1 & f2 are effectively the same instance
//but compiler doesn't allow to invoke f1 with v arg

f1(v) //Cannot express tuple conversion '(C, C)' to '(B, B)'
f2(v)