Refine type of cadr on (Pairof A (Listof B)).

Closes #1026.

Author: samth

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

@@ -525,9 +525,12 @@
                     [-> (-lst (-lst a)) (-lst a)]))]
 [cadr (-poly (a b c)
              (cl->* [->acc (list (-pair a (-pair b c))) b (list -car -cdr)]
+                    [-> (-pair a (-lst b)) b]
                     [-> (-lst a) a]))]
 [cddr  (-poly (a b c)
               (cl->* [->acc (list (-pair a (-pair b c))) c (list -cdr -cdr)]
+                     [-> (-pair a (-lst b)) (-lst b)]
+                     [-> (-pair a (-pair b (-lst c))) (-lst c)]
                      [-> (-lst a) (-lst a)]))]
 
 [caaar (-poly (a b c d)

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

@@ -5319,6 +5319,10 @@
          ;; https://github.com/racket/typed-racket/issues/605
          (hash 0 1 2 3 4 5 6 7 8 0)
          (-Immutable-HT -Byte -Byte))
+
+
+       (tc-e (cadr (ann (cons "a" (list 1 2 3)) (Pairof String (Listof Byte))))
+             -Byte)
        )
       (if (extflonum-available?)