Key path formed to setter with unavailable key path is writable

[stephencelis]

Description

(First posted to the forums.)

Currently, a key path formed to a property that has an unavailable setter are WritableKeyPaths. This seems like a bug, since it allows code like dynamic member lookup to quietly compile what should crash at runtime.

While tracking the availability itself in the key path seems like a good idea and would allow library users to surface their own error messaging, I imagine that is a complex project to take on. Maybe as a baby step, key paths should at the very least be simply non-writable KeyPaths when the setter is unavailable.

Reproduction

struct S {
  var p: String {
    get { "" }
    @available(*, unavailable)
    set { fatalError() }
  }
}
let kp: WritableKeyPath = \S.p

Expected behavior

The key path returned should be KeyPath, not WritableKeyPath, so the above should not compile.

Environment

swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

Additional information

No response

[xedin]

Sounds reasonable to me, we do this for declarations and conformance in the solver already but not in key path context. Shouldn't be hard to fix either.

[stephencelis]

@xedin Do you think this should also propagate availability (warnings/errors) when forming such key paths in contexts that require writability?

[stephencelis]

/cc @tshortli