Avoid wrapping 5 when contracted as (Sequenceof Integer)

Closes racket/math#13

typed-racket-lib/typed-racket/private/type-contract.rkt

@@ -10,7 +10,7 @@
  (env type-name-env row-constraint-env)
  (rep core-rep rep-utils free-ids type-mask values-rep
       base-types numeric-base-types)
- (types resolve utils printer match-expanders union)
+ (types resolve utils printer match-expanders union subtype)
  (prefix-in t: (types abbrev numeric-tower subtype))
  (private parse-type syntax-properties)
  racket/match racket/syntax racket/list
@@ -525,6 +525,11 @@
        [(? Fun? t) (t->sc/fun t)]
        [(? DepFun? t) (t->sc/fun t)]
        [(Set: t) (set/sc (t->sc t))]
+       [(Sequence: (list t))
+        #:when (subtype t:-Nat t)
+        ;; sequence/c is always a wrapper, so avoid it when we just have a number
+        (or/sc (flat/sc #'exact-nonnegative-integer?)
+               (sequence/sc (t->sc t)))]
        [(Sequence: ts) (apply sequence/sc (map t->sc ts))]
        [(Vector: t) (vectorof/sc (t->sc/both t))]
        [(HeterogeneousVector: ts) (apply vector/sc (map t->sc/both ts))]

typed-racket-test/succeed/sequenceof-integer.rkt

@@ -0,0 +1,19 @@
+#lang racket/base
+
+(module typed typed/racket/base
+  (provide foo)
+  (: foo (-> (U Integer (Sequenceof Integer)) String))
+  (define (foo x)
+    (if (integer? x)
+        (format "I got an integer: ~a" x)
+        (error "I did not get an integer: ~a" x))))
+
+(module other-typed typed/racket/base
+  (provide bar)
+  (require (submod ".." typed))
+  (define (bar) (foo 0)))
+
+(require 'typed
+         'other-typed)
+(foo 0)
+(bar)