SendableKeyPath?

Evolution Discussion
3

As the bug report notes, KeyPaths should be Sendable, and captures should already be restricted to Sendable values.

SE-0302 - Sendable and @Sendable closures:

Key path literals

Key paths themselves conform to the Sendable protocol. However, to ensure that it is safe to share key paths, key path literals can only capture values of types that conform to the Sendable protocol. This affects uses of subscripts in key paths:

class SomeClass: Hashable {
  var value: Int
}

class SomeContainer {
  var dict: [SomeClass : String]
}

let sc = SomeClass(...)

// error: capture of 'sc' in key path requires 'SomeClass' to conform
// to 'Sendable'
let keyPath = \SomeContainer.dict[sc]

But apparently that hasn't been implemented. That snippet doesn't produce the expected error.

There was some discussion about loosening this, and having sendable keypath literals produce KeyPath & Sendable, but AFAICT that never made it in to a proposal.

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