Fix check-unreachable-code? mode in TR

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-if.rkt

@@ -25,7 +25,7 @@
       ;; otherwise, this code is unreachable
       ;; and the resulting type should be the empty type
       [(check-unreachable-code?)
-       (tc-expr/check expr Univ)
+       (tc-expr expr)
        (ret (Un))]
       [else (ret (Un))]))
   (match (single-value tst)

pkgs/typed-racket-pkgs/typed-racket-test/tests/typed-racket/fail/check-unreachable.rkt

@@ -0,0 +1,17 @@
+#;
+(exn-pred #rx"expected: Symbol.*given: String")
+#lang racket/load
+
+;; This test makes sure that the check-unreachable-code mode
+;; actually works.
+
+(require (for-syntax typed-racket/utils/tc-utils))
+
+(begin-for-syntax
+  (check-unreachable-code? #t))
+
+(require typed/racket)
+
+(if #t
+    "foo"
+    (symbol->string "foo"))