repair the type for vector->immutable-vector (#1228)

closes #1131

Author: capfredf

typed-racket-lib/typed-racket/base-env/base-env.rkt

@@ -893,7 +893,9 @@
                       (list -vec-len))]
 [vector (-poly (a) (->* (list) a (-mvec a)))]
 [vector-immutable (-poly (a) (->* (list) a (-ivec a)))]
-[vector->immutable-vector (-poly (a) (-> (-vec a) (-ivec a)))]
+[vector->immutable-vector (-poly (a)
+                                 (cl-> [((-vec a)) (-ivec a)]
+                                       [(-VectorTop) (-ivec Univ)]))]
 [vector-fill! (-poly (a) (-> (-vec a) a -Void))]
 [vector-argmax (-poly (a) (-> (-> a -Real) (-vec a) a))]
 [vector-argmin (-poly (a) (-> (-> a -Real) (-vec a) a))]

typed-racket-test/unit-tests/typecheck-tests.rkt

@@ -1388,7 +1388,10 @@
                                (if (symbol? x) 'x #f)) ((U Symbol String) -> (U Symbol #f) : Symbol))
                       (list "three" 'four 'five "six"))
               (-lst -Symbol)]
-
+        [tc-e (let ([x : Any '#()])
+                (and (vector? x)
+                     (vector->immutable-vector x)))
+              (t:Un (-ivec Univ) (-val #f))]
 
         [tc-e (vector-filter path-string? (ann (vector "a" 4 5 "b") (Mutable-Vectorof Any)))
               (-mvec (t:Un -Path -String))]