Add support for classes in _createOffsetBasedKeyPath

Author: Azoy

stdlib/public/core/KeyPath.swift

@@ -3912,7 +3912,7 @@ public func _createOffsetBasedKeyPath(
   // The buffer header is 32 bits, but components must start on a word
   // boundary.
   let kpBufferSize = MemoryLayout<Int>.size + MemoryLayout<Int32>.size
-  return kpTy._create(capacityInBytes: kpBufferSize) {
+  let kp = kpTy._create(capacityInBytes: kpBufferSize) {
     var builder = KeyPathBuffer.Builder($0)
     let header = KeyPathBuffer.Header(
       size: kpBufferSize - MemoryLayout<Int>.size,
@@ -3923,7 +3923,7 @@ public func _createOffsetBasedKeyPath(
     builder.pushHeader(header)
 
     let componentHeader = RawKeyPathComponent.Header(
-      stored: .struct,
+      stored: _MetadataKind(root) == .struct ? .struct : .class,
       mutable: false,
       inlineOffset: UInt32(offset)
     )
@@ -3935,6 +3935,12 @@ public func _createOffsetBasedKeyPath(
 
     component.clone(into: &builder.buffer, endOfReferencePrefix: false)
   }
+
+  if _MetadataKind(root) == .struct {
+    kp.assignOffsetToStorage(offset: offset)
+  }
+
+  return kp
 }
 
 @_spi(ObservableRerootKeyPath)

test/stdlib/KeyPath.swift

@@ -1073,6 +1073,16 @@ struct Dog {
   var age: Int
 }
 
+class Cat {
+  var name: String
+  var age: Int
+
+  init(name: String, age: Int) {
+    self.name = name
+    self.age = age
+  }
+}
+
 if #available(SwiftStdlib 5.9, *) {
   keyPath.test("_createOffsetBasedKeyPath") {
     let dogAgeKp = _createOffsetBasedKeyPath(
@@ -1086,6 +1096,18 @@ if #available(SwiftStdlib 5.9, *) {
     let sparky = Dog(name: "Sparky", age: 7)
 
     expectEqual(sparky[keyPath: dogAgeKp!], 7)
+
+    let catNameKp = _createOffsetBasedKeyPath(
+      root: Cat.self,
+      value: String.self,
+      offset: 16
+    ) as? KeyPath<Cat, String>
+
+    expectNotNil(catNameKp)
+
+    let chloe = Cat(name: "Chloe", age: 4)
+
+    expectEqual(chloe[keyPath: catNameKp!], "Chloe")
   }
 }