fix explanation of :missing in README

Author: seancorfield

README.md

@@ -202,12 +202,14 @@ and returns them to the caller. Unknown options, missing required arguments,
 validation errors, and exceptions thrown during `:parse-fn` are all added to
 the errors vector.
 
-By default, the error message when a required argument is omitted is:
+Any option can be flagged as required by providing a `:missing` key in the
+option spec with a string that should be used for the error message if the
+option is omitted.
 
-`Missing required argument for ...`
+The error message when a required argument is omitted (either a short opt with
+`:require` or a long opt describing an argument) is:
 
-This can be overridden by providing a `:missing` key in the option spec with a
-string that should be used for the error message instead of the default.
+`Missing required argument for ...`
 
 Correspondingly, `parse-opts` returns the following map of values:
 

src/main/clojure/clojure/tools/cli.cljc

@@ -413,9 +413,12 @@
           :else
           (validate value spec opt optarg))))
 
-(defn- neg-flag? [spec opt]
+(defn- allow-no? [spec]
   (and (:long-opt spec)
-       (re-find #"^--\[no-\]" (:long-opt spec))
+       (re-find #"^--\[no-\]" (:long-opt spec))))
+
+(defn- neg-flag? [spec opt]
+  (and (allow-no? spec)
        (re-find #"^--no-" opt)))
 
 (defn- parse-optarg [spec opt optarg]