[Concurrency] Fix asSerialExecutor() for complex equality.

stdlib/public/Concurrency/Executor.swift

@@ -808,7 +808,13 @@ public struct UnownedSerialExecutor: Sendable {
 
   @available(StdlibDeploymentTarget 6.2, *)
   public func asSerialExecutor() -> (any SerialExecutor)? {
-    return unsafe unsafeBitCast(executor, to: (any SerialExecutor)?.self)
+    // The low bits of the witness table are used to encode the executor kind.
+    // any SerialExecutor needs a raw witness table pointer, so mask off the low
+    // bits, knowing the witness table pointer is aligned to the pointer size.
+    let rawExecutorData = unsafe unsafeBitCast(executor, to: (UInt, UInt).self)
+    let mask = ~(UInt(MemoryLayout<UnsafeRawPointer>.alignment) - 1)
+    let alignedExecutor = unsafe (rawExecutorData.0, rawExecutorData.1 & mask)
+    return unsafe unsafeBitCast(alignedExecutor, to: (any SerialExecutor)?.self)
   }
 }
 

test/Concurrency/Runtime/custom_executors_complex_equality.swift

@@ -66,6 +66,11 @@ actor MyActor {
 
   func test(expectedExecutor: NaiveQueueExecutor, expectedQueue: DispatchQueue) {
     expectedExecutor.preconditionIsolated("Expected deep equality to trigger for \(expectedExecutor) and our \(self.executor)")
+
+    // Ensure we get a usable value from asSerialExecutor() when the executor
+    // has complex equality.
+    _ = expectedExecutor.asUnownedSerialExecutor().asSerialExecutor()!.asUnownedSerialExecutor()
+
     print("\(Self.self): [\(self.executor.name)] on same context as [\(expectedExecutor.name)]")
   }
 }