[Concurrency] Clarify/fix error object handling in futures.

Author: DougGregor

include/swift/ABI/Task.h

@@ -31,6 +31,7 @@ class AsyncContext;
 class Executor;
 class Job;
 struct OpaqueValue;
+struct SwiftError;
 class TaskStatusRecord;
 
 /// An ExecutorRef isn't necessarily just a pointer to an executor
@@ -323,6 +324,12 @@ class AsyncTask : public HeapObject, public Job {
           reinterpret_cast<char *>(this) + storageOffset(resultType));
     }
 
+    /// Retrieve the error.
+    SwiftError *&getError() {
+      return *reinterpret_cast<SwiftError **>(
+           reinterpret_cast<char *>(this) + storageOffset(resultType));
+    }
+
     /// Compute the offset of the storage from the base of the future
     /// fragment.
     static size_t storageOffset(const Metadata *resultType)  {

stdlib/public/Concurrency/Task.cpp

@@ -34,7 +34,7 @@ void FutureFragment::destroy() {
     break;
 
   case Status::Error:
-    swift_unknownObjectRelease(getStoragePtr());
+    swift_unknownObjectRelease(reinterpret_cast<OpaqueValue *>(getError()));
     break;
   }
 }
@@ -300,8 +300,7 @@ swift::swift_task_future_wait(AsyncTask *task, AsyncTask *waitingTask) {
    case FutureFragment::Status::Error:
       return TaskFutureWaitResult{
           TaskFutureWaitResult::Error,
-          *reinterpret_cast<OpaqueValue **>(
-            task->futureFragment()->getStoragePtr())};
+          reinterpret_cast<OpaqueValue *>(task->futureFragment()->getError())};
   }
 }