allow promises to be impersonated and thus allow impersonator contracts

related to PR 12861

collects/racket/contract/private/misc.rkt

@@ -827,6 +827,8 @@
     (let* ([ctc (coerce-contract 'promise/c ctc-in)]
            [ctc-proc (contract-projection ctc)])
       (define chap? (chaperone-contract? ctc))
+      (define c/i-struct (if chap? chaperone-struct impersonate-struct))
+      (define c/i-procedure (if chap? chaperone-procedure impersonate-procedure))
       ((if chap? make-chaperone-contract make-contract)
        #:name (build-compound-type-name 'promise/c ctc)
        #:projection
@@ -839,15 +841,13 @@
                 val
                 '(expected "<promise>," given: "~e")
                 val))
-                   (if chap?
-                       (chaperone-struct
-                        val
-                        promise-forcer (λ (_ proc) 
-                                         (chaperone-procedure
-                                          proc
-                                          (λ (promise)
-                                            (values p-app promise)))))
-                       (delay (p-app (force val)))))))
+             (c/i-struct
+              val
+              promise-forcer (λ (_ proc) 
+                               (c/i-procedure
+                                proc
+                                (λ (promise)
+                                  (values p-app promise))))))))
        #:first-order promise?))))
 
 (define/subexpression-pos-prop (parameter/c x)

collects/racket/private/promise.rkt

@@ -147,12 +147,7 @@
 ;; property value for the right forcer to use
 (define-values [prop:force promise-forcer]
   (let-values ([(prop pred? get) ; no need for the predicate
-                (make-struct-type-property 'forcer
-                  (lambda (v info)
-                    (unless (and (procedure? v)
-                                 (procedure-arity-includes? v 1))
-                      (raise-argument-error 'prop:force "(any/c . -> . any)" v))
-                    v))])
+                (make-struct-type-property 'forcer 'can-impersonate)])
     (values prop get)))
 
 ;; A promise value can hold

collects/tests/racket/contract-test.rktl

@@ -9255,6 +9255,16 @@
                         'pos
                         'neg)))
    #t)
+  
+  (test/spec-passed/result
+   'promise/c6
+   '(let ([a (delay 7)])
+      (equal? a
+              (contract (promise/c (new-∃/c 'α))
+                        a
+                        'pos
+                        'neg)))
+   #t)
 
   
 ;