AnyObject key path diagnostic is broken

[stephencelis]

Description

The following code has a compile error:

struct S {
  let object: AnyObject
  let keyPath: AnyKeyPath
  func f1() {
    object[keyPath: keyPath]  // 🛑
  }
}

🛑 The root type of a Swift key path cannot be 'AnyObject'

According to this very old comment AnyObject key paths are forbidden for Objective-C messaging reasons. I'm not sure that still applies, but I would expect an AnyObject to be able to apply an AnyKeyPath regardless.

Stranger still, the diagnostic goes away with the mere act of copying object over to a local, mutable variable:

func f2() {
  var object = object
  object[keyPath: keyPath]  // ✅
}

Regardless of the correct behavior, I believe the above should at least behave consistently.

See also: https://forums.swift.org/t/anyobject-and-key-paths/71671

Reproduction

struct S {
  let object: AnyObject
  let keyPath: AnyKeyPath
  func f1() {
    object[keyPath: keyPath]  // 🛑
  }
  func f2() {
    var object = object
    object[keyPath: keyPath]  // ✅
  }
}

Expected behavior

struct S {
  let object: AnyObject
  let keyPath: AnyKeyPath
  func f1() {
    object[keyPath: keyPath]  // ✅
  }
  func f2() {
    var object = object
    object[keyPath: keyPath]  // ✅
  }
}

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