Crash on the boundary between Swift 5 and Swift 6 code

Using Swift
,
1

I'm getting a crash in the runtime using Xcode 16.4 (Apple Swift version 6.1.2). Also reproducible with a nightly toolchain (2025-06-22).

// MyLib: Swift 6

@preconcurrency public typealias Action = @MainActor () -> Void

@preconcurrency @MainActor
public func useIt(_ actions: [String: Action]) {
    actions["xyz"]?()
}

// MyApp: Swift 5, minimal checking
import MyLib

func getActions() -> [String: Action] {
    return [
        "abc": { print("abc") },
        "xyz": { print("xyz") }
    ]
}

// CRASH: Could not cast value of type '() -> ()' (0x104818518) to '@Swift.MainActor @Sendable () -> ()' (0x10487ebf8).
//
// #0 __pthread_kill ()
// #1 pthread_kill ()
// #2 abort ()
// #3 swift::fatalErrorv ()
// #4 swift::fatalError ()
// #5 swift::swift_dynamicCastFailure ()
// #6 swift::swift_dynamicCastFailure ()
// #7 swift_dynamicCast ()
// #8 Swift._dictionaryUpCast<τ_0_0, τ_0_1, τ_0_2, τ_0_3 where τ_0_0: Swift.Hashable, τ_0_2: Swift.Hashable>(Swift.Dictionary<// τ_0_0, τ_0_1>) -> Swift.Dictionary<τ_0_2, τ_0_3> ()
// 
useIt(getActions())

Issue does not reproduce when app is compiled with complete isolation checking.
There are no compile time diagnostics emitted.

Is this a compiler bug or am I using @preconcurrency incorrectly?

1 Like
2

Does it still crash if you try updating getActions() like so:

func getActions() -> [String: Action] {
    return [
        "abc": { @MainActor in print("abc") },
        "xyz": { @MainActor in print("xyz") }
    ]
}
3

Still crashes:

func getActions() -> [String: Action] {
    return [
        "abc": { @MainActor in print("abc") }, // ⚠️ Converting function value of type '@MainActor () -> ()' to '() -> Void' loses global actor 'MainActor'; this is an error in the Swift 6 language mode
        "xyz": { @MainActor in print("xyz") }  // ⚠️ Converting function value of type '@MainActor () -> ()' to '() -> Void' loses global actor 'MainActor'; this is an error in the Swift 6 language mode
    ]
}
4

Reported as App crashes on the boundary between Swift 5 and Swift 6 code · Issue #82618 · swiftlang/swift · GitHub

5

It seems that adding -warn-concurrency to the Swift 5 target fixes the issue, aligning dictionary types as Dictionary<String, @MainActor @Sendable () -> ()>.

@Douglas_Gregor, @beccadax, is it expected that @preconcurrency works only in tandem with -warn-concurrency?

EDIT: But with -warn-concurrency calls to Action in MyApp also trigger the error, while calls to useIt() do not. Is this another bug?

6

Since MyLib is compiled with -swift-version 6, why bother putting @preconcurrency on those 2 nominal decls? There shouldn't be any crash after removing those 2 occurrences of @preconcurrency in MyLib.

7

Because MyApp is compiled in Swift 5 and is still missing concurrency annotations. Without @preconcurrency usages of those APIs in MyApp trigger an error even with -strict-concurrency=minimal.

8

I understand your motivations.

The consequence of using @preconcurrency on MyLib.Action is, at all use sites with minimal concurrency checking, Action will be treated as () -> Void instead of the preferred @MainActor @Sendable () -> Void) (reference).

I agree a relevant runtime conversion crash is surprising (although not common), but is understandable from my perspective.

9

Playing with this further, with generic types that don't have special treat (i.e. other than Array, Set, Dictionary), compiler produces a compilation error:

// MyLib: Swift 6

public struct Box<T> {
    public var value: T
    public init(value: T) {
        self.value = value
    }
}

@preconcurrency @MainActor
public func useBox(_ box: Box<Action>) {
    box.value()
}

// MyApp: Swift 5, minimal checking

func getBox() -> Box<Action> {
    return Box { print("box") }
}

useBox(getBox()) // error: Cannot convert value of type 'Box<() -> Void>' to expected argument type 'Box<Action>' (aka 'Box<@MainActor @Sendable () -> ()>')

Should compiler produce the same error for the Array, Set and Dictionary? @Douglas_Gregor, @beccadax, WDYT?