Unexpected error with integer generic parameters

Using Swift
, , ,
1

While playing with integer generic parameters in form of implementing a compile time optional type:

struct Optional<let isNil: Int, Wrapped> {
  private let wrapped: Wrapped!

  init(wrapped: Wrapped) where isNil == 1 {
    self.wrapped = wrapped
  }

  init() where isNil == 0 {
    self.wrapped = nil
  }

  func wrappedValue() -> Wrapped where isNil == 1 {
    return wrapped
  }
}

func nilCheck<let isNil: Int, T>(_ value: Optional<isNil, T>) {
  if isNil == 1 {
    print(value.wrappedValue()) // - error: instance method 'wrappedValue()' requires the types 'isNil' and '1' be equivalent
  } else {
    print("nil")
  }
}

nilCheck(Optional<0, Int>())
nilCheck(Optional(wrapped: 1))

I am getting error:

:21:17: error: instance method 'wrappedValue()' requires the types 'isNil' and '1' be equivalent
12 | }
13 |
14 | func wrappedValue() -> Wrapped where isNil == 1 {
| - note: where 'isNil' = 'isNil' 15 | return wrapped 16 | } : 19 | func nilCheck<let isNil: Int, T>(_ value: Optional<isNil, T>) { 20 | if isNil == 1 { 21 | print(value.wrappedValue()) | - error: instance method 'wrappedValue()' requires the types 'isNil' and '1' be equivalent
22 | } else {
23 | print("nil")

But shouldn’t this kind of usage work since the wrappedValue method is only accessed in the context isNil is 1?

2

No. An if statement doesn’t add any static information, without binding.

What you have is the same kind of thing as this, with Swift.Optional:

if value != nil {

This is the kind of binding you need:

if case let value as Optional<1, T> = value {
3

@Danny shouldn’t compiler be able to infer I am in a context where the isNil is 1 is already satisfied. I see other languages are able to do that. Is this a limitation on Swift compiler?

4

Is there somewhere else in Swift that you think you’ve seen anything like this?

Constraints and overloads are how Swift deals with this.

func nilCheck(_ value: Optional<1, some Any>) {
  print(value.wrappedValue())
}

func nilCheck(_: Optional<0, some Any>) {
  print("nil")
}
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