Handle subtyping with all-optional kw args better.

Problem reported by @schackbrian2012. Closes #608.

typed-racket-lib/typed-racket/infer/infer-unit.rkt

@@ -381,11 +381,15 @@
 
      (define s-seq (seq ss (rest->end s-rest)))
      (define t-seq (seq ts (rest->end t-rest)))
-     (and (null? s-kws)
-          (null? t-kws)
-          (% cset-meet
-           (cgen context s t)
-           (cgen/seq context t-seq s-seq)))]))
+     (and
+      ;; if all keywords are optional, then we can just treat
+      ;; them like they aren't there (or if there are none)
+      (for/and ([s-kw (in-list s-kws)])
+        (not (Keyword-required? s-kw)))
+      (null? t-kws)
+      (% cset-meet
+         (cgen context s t)
+         (cgen/seq context t-seq s-seq)))]))
 
 (define/cond-contract (cgen/flds context flds-s flds-t)
   (context? (listof fld?) (listof fld?)  . -> . (or/c #f cset?))

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

@@ -4204,6 +4204,11 @@
                    [else 42]))
                (void))
              -Void]
+       [tc-e (map string-trim (ann '("a" "b") (Listof String)))
+             (-lst -String)]
+       [tc-err (let ()
+                 (define (f #:x x y) 1)
+                 (map f (list 1 2 3)))]
        )
 
   (test-suite