func foo(_ closure:(Int,Int,Int)->Void) {
    closure(1,2,3)
}

foo { } // ERROR: "expects 3 arguments, which cannot be implicitly ignored"
foo { print($0) } // ERROR: "expects 3 arguments, but 1 was used"
foo { print($1) } // ERROR: "expects 3 arguments, but 2 were used" Huh? There was only 1 used.
foo { print($0,$1) } // ERROR: "expects 3 arguments, but 2 were used"
foo { print($0,$1,$2) } // OK
foo { print($2) } // OK
// ^^^ Though it might as well say "expects 3 arguments, but 1 was used"

func each(_ closure: (T, Int) -> Void) { .. }
// should be able to call using first parameter only:
foo.each { $0.doSomething } // ERROR: "expects 2 arguments, but 1 was used"