Why Swift throws error when using optional param in closure func?

Question

Can someone explain why the swift compiler complains about the \"offset:\" parameter which is optional anyway?

If I pass zero as param incrementByFive(0)

Answer 1

Only functions can have default parameters in Swift, but not closures.

The closure returned from makeIncrementer() has the type (Int?) -> Int, i.e. it takes one argument of type Int?, and that must be provided when calling the closure.

Compare also SR-531 Allow default parameters in closure parameters, in particular the comment

Doesn't really make sense, if you need this behavior you should be using a function. Closures are usually called anonymously with their arguments all provided, so there is no need for default parameter values.