CasePaths subscript not working on Swift 6

[gianfilippocisternino]

Compiler is giving unexpected errors when using CasePath subscript on Swift 6. This error happens only in some cases (see snippet below).

While looking for a potential bug i found this commented code: e39069d

May that fix the issue?

This snippet compiles fine:

let index = state.navigationPath[topMainDestination]?.lastIndex(
  where: { destination in
    guard let modalDestination = destination[case: \.modal] as? MD else {
      return false
    }

    return modalDestination[case: keyPath] != nil
  }
)

guard
  let index,
  let destination = state.navigationPath[topMainDestination]?[index],
  let modalDestination = destination[case: \.modal] as? MD
else {
  return nil
}

return modalDestination[case: keyPath]

This one triggers an error somewhere else in the function:

return state.navigationPath[topMainDestination]?.compactMap { destination in
  guard let modalDestination = destination[case: \.modal] as? MD else {
    return nil
  }
  
  return modalDestination[case: keyPath]
}.last

Example
CasePathsDemo.zip

Environment

• swift-case-paths 1.5.5
• Xcode 16.0 (16A242)
• Swift 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
• OS: iOS 18.0 macOS 15.0 (24A335)

[mbrandonw]

Hi @gianfilippocisternino, can you please provide a minimal reproduction of the problem?

[gianfilippocisternino]

Hi @mbrandonw, i uploaded a sample app that shows the problem. Thanks

[mbrandonw]

Hi @gianfilippocisternino, this just seems like a regression in the Swift compiler in Swift 6. If you add some explicit type annotations it works just fine:

get: { [unowned self] () -> Value? in
…
return navigationPath[topMainDestination]?.compactMap { destination -> Value? in

Since this isn't an issue with the library I am going to convert it to a discussion. Please feel free to continue the conversation over there!