[doc_comment plugin] Allow documenting return type on return-less functions

Closes #566

Author: marijnh

plugin/doc_comment.js

@@ -419,7 +419,10 @@
           fn.argNames[i] += "?";
         if (known) propagateWithWeight(known, fn.args[i]);
       }
-      if (ret) propagateWithWeight(ret, fn.retval);
+      if (ret) {
+        if (fn.retval == infer.ANull) fn.retval = new infer.AVal;
+        propagateWithWeight(ret, fn.retval);
+      }
       if (self) propagateWithWeight(self, fn.self);
     } else if (type) {
       propagateWithWeight(type, aval);

test/cases/jsdoc.js

@@ -105,3 +105,10 @@ stringOrNumber; //: string|number
 function unionFunction(a) { return argh(); }
 
 unionFunction; //: fn(a: string) -> [Foo|number]
+
+/**
+ * @returns {string}
+ */
+function ui() {}
+
+ui(); //: string