syntax/parse: fix bug in specialized code for ellipsis patterns

The result of 'stx->list' is not always either a list or #f.

Merge to 5.3.
(cherry picked from commit e10951024f)

collects/syntax/parse/experimental/private/substitute.rkt

@@ -222,7 +222,7 @@ A HeadGuide (HG) is one of:
        (lambda (env lenv)
          (let* ([v (f1 env lenv)]
                 [v* (stx->list v)])
-           (unless v*
+           (unless (list? v*)
              (raise-syntax-error 'template
                                  "splicing template did not produce a syntax list"
                                  stx))

collects/syntax/parse/private/residual.rkt

@@ -205,7 +205,7 @@
 
 (define (predicate-ellipsis-parser x cx pr es pred? desc rl)
   (let ([elems (stx->list x)])
-    (if (and elems (andmap pred? elems))
+    (if (and elems (list? elems) (andmap pred? elems))
         (values 'ok elems)
         (let loop ([x x] [cx cx] [i 0])
           (cond [(syntax? x)

collects/tests/stxparse/test.rkt

@@ -459,3 +459,14 @@
     (not #rx"expected foo") ;; y:nat was incorrectly considered part of opaque region
     #rx"expected exact-nonnegative-integer")
   )
+
+;; from Neil Van Dyke (7/28/2012)
+(test-case "specialized predicate-ellipsis-parser"
+  ;; test that it works on improper lists
+  ;; ... when input is syntax
+  (check-eq? (syntax-parse #'(a b c . d) [(x:id ...) #t] [_ #f]) #f)
+  ;; ... and when input is stx pair (but not syntax)
+  (check-eq? (syntax-parse #'(a b c . d) [(_ x:id ...) #t] [_ #f]) #f)
+  ;; test that it works on proper lists w/ embedded stxpairs
+  (check-eq? (syntax-parse #'(a b . (c  d)) [(x:id ...) #t] [_ #f]) #t)
+  (check-eq? (syntax-parse #'(a b . (c  d)) [(_ x:id ...) #t] [_ #f]) #t))