Merge pull request #78252 from swiftlang/egorzhdan/frt-conditional-cast-test

[cxx-interop] Add tests for conditional cast to foreign reference type

Author: egorzhdan

test/Interop/Cxx/foreign-reference/witness-table-typechecker.swift

@@ -0,0 +1,14 @@
+// RUN: %target-typecheck-verify-swift -cxx-interoperability-mode=default -disable-availability-checking -I %S/Inputs
+
+import WitnessTable
+
+public protocol ListNode {
+  associatedtype Element
+  func next() -> Element?
+}
+
+@available(SwiftStdlib 5.8, *)
+extension CxxLinkedList : ListNode { }
+
+let existential: any ListNode = makeLinkedList()
+let cast: CxxLinkedList? = existential as? CxxLinkedList

test/Interop/Cxx/foreign-reference/witness-table.swift

@@ -36,6 +36,11 @@ extension CxxLinkedList : ListNode { }
 @available(SwiftStdlib 5.8, *)
 extension MyCxxSequence : Sequence, IteratorProtocol { }
 
+class SwiftLinkedList: ListNode {
+  typealias Element = Int
+  func next() -> Int? { return nil }
+}
+
 if #available(SwiftStdlib 5.8, *) {
 
 var WitnessTableTestSuite = TestSuite("Use foreign reference in a generic context")
@@ -68,6 +73,10 @@ WitnessTableTestSuite.test("As an existential") {
   expectEqual(cast?.value, 0)
   expectEqual(cast?.next()?.value, 1)
   expectEqual(cast?.next()?.next()?.value, 2)
+
+  let notACxxValue: any ListNode = SwiftLinkedList()
+  let invalidCast = notACxxValue as? CxxLinkedList
+  expectNil(invalidCast)
 }
 
 }