Function Types as Return Types in TSPL

Using Swift
1

I’m confused about Function Types as Return Types in TSPL.

Xcode 26.0
maxOS Tahoe 26.0

Why?
chooseStepFunction1(:) --> error
chooseStepFunction2(:) --> It works
chooseStepFunction3(:) --> It works

The TSPL doesn't explain ...

encountered chooseStepFunction1 error:

type of expression is ambiguous without a type annotation
return backward ? stepBackward : stepForward
       ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

func stepForward(_ input: Int) -> Int {
    return input + 1
}

func stepBackward(_ input: Int) -> Int {
    return input - 1
}

// type of expression is ambiguous without a type annotation
func chooseStepFunction1(backward: Bool) -> (Int) -> Int {
    return backward ? stepBackward : stepForward
}

// works
func chooseStepFunction2(backward: Bool) -> @Sendable (Int) -> Int {
    return backward ? stepBackward : stepForward
}

// works
func chooseStepFunction3(backward: Bool) -> (Int) -> Int {
    if backward {
        return stepForward
    } else {
        return stepBackward
    }
}
2

I’m no expert, but that sounds like a bug to me.

FYI, your code compiles on Linux using Swift 5.8

ternary.swift:16:21: warning: converting non-sendable function value to '@Sendable (Int) -> Int' may introduce data races
    return backward ? stepBackward : stepForward
3

When I try this, chooseStepFunction1 compiles fine, it's chooseStepFunction2 that gives a diagnostic (for converting non-sendable (Int) -> Int to @Sendable (Int) -> Int). What are your build settings (e.g. do you have approachable concurrency and/or default MainActor isolation turned on? Is it Swift 5 or Swift 6? etc)

4

chooseStepFunction1 produces an error for me in Swift 6 mode, but not Swift 5 mode.

5

The errors that occur are different depending on what scope everything is defined within. For example, wrap it all in a function, and:

func f() {
  func stepForward(_ input: Int) -> Int { input + 1 }
  func stepBackward(_ input: Int) -> Int { input - 1 }

  // Compiles.
  func chooseStepFunction1(backward: Bool) -> (Int) -> Int {
    backward ? stepBackward : stepForward
  }

  // Converting non-Sendable function value to '@Sendable (Int) -> Int' may introduce data races
  func chooseStepFunction2(backward: Bool) -> @Sendable (Int) -> Int {
    backward ? stepBackward : stepForward
  }
}

I don't think the casting that makes everything compile should be necessary, but that's how things have been for a while.

@Sendable func stepForward(_ input: Int) -> Int { input + 1 }
@Sendable func stepBackward(_ input: Int) -> Int { input - 1 }

func chooseStepFunction1(backward: Bool) -> (Int) -> Int {
  backward ? stepBackward : stepForward as (_) -> _
}

func chooseStepFunction2(backward: Bool) -> @Sendable (Int) -> Int {
  backward ? stepBackward : stepForward
}
6

(post deleted by author)