racket/promise: fix promise/force on the result of delay/sync

Closes #1933

pkgs/racket-test-core/tests/racket/syntax.rktl

@@ -907,6 +907,14 @@
   (test (void) sync p)
   (test (list (void)) sync (wrap-evt p list)))
 
+(let ()
+  (define pr (delay/sync 'done))
+  (test #f promise-forced? pr)
+  (test #f sync/timeout 0 pr)
+  (test 'done force pr)
+  (test #t promise-forced? pr)
+  (test (void) sync/timeout 0 pr))
+
 (test '(list 3 4) 'quasiquote `(list ,(+ 1 2) 4))
 (test '(list a (quote a)) 'quasiquote (let ((name 'a)) `(list ,name ',name)))
 (test '(a 3 4 5 6 b) 'quasiquote `(a ,(+ 1 2) ,@(map abs '(4 -5 6)) b))

racket/collects/racket/promise.rkt

@@ -26,7 +26,11 @@
 (define-struct (running-thread running) (thread))
 
 ;; used in promise/sync until it's forced
-(define-struct syncinfo ([thunk #:mutable] done-evt done-sema access-sema))
+(define-struct syncinfo ([thunk #:mutable] done-evt done-sema access-sema)
+  ;; We don't want to apply a `syncinfo`, but declaring the `syncinfo`
+  ;; as a procedure tells `promise-forced?` when the promise is not
+  ;; yet forced
+  #:property prop:procedure (case-lambda))
 
 (define-struct (promise/sync promise) ()
   #:property prop:custom-write