MATLAB: R2018a functionSignatures.json (user function tab complete) not working as documented outside of live-script

MATLABtab completionuser defined functions

In a recent thread, I came across a mention of a new feature in R2018a: tab completion suggestions for user-defined functions. (release notes link to this feature)
I applaud this development. I know I am not the only one complaining that it didn't already exist. So I immediately tried to check it out. I used the guide linked in the release notes, which states: "To experiment with code suggestions, start to call the function from a live script and observe the suggestions."
That suggests that it's also possible to use this outside of live scripts, but they just suggest you use that. I don't appear to be the only one thinking that, as suggested by Sean de Wolski: "While I can't talk about the future, if you look at 18a, there is new infrastructure for adding function signatures for code complete/suggest especially in regards to the live editor."
But I failed to get it to work. In the normal editor I only got the normal header-derived hint, and in the live script I could only get a single input type, instead of the two included in my JSON file. What am I doing wrong? Or is the feature not yet at the level the documentation has us believe? Is this a bug that I should report (there aren't any about this yet)?
(Function header, result in live script, and JSON file below and in zip)
function outfilename=WBM(filename,url_part,varargin)
% syntax:
% outfilename=WBM(filename,url_part)
% outfilename=WBM(___,options)
% outfilename=WBM(___,Name,Value)
.
{
  "WBM":
  {
    "inputs":
    [
      {"name":"filename", "kind":"required", "type":["char"],
      "purpose":"Target file name"},
      {"name":"url_part", "kind":"required", "type":["char"],
      "purpose":"URL of the file/website"},
      {"name":"options",  "kind":"ordered",  "type":["struct"],
      "purpose":"Struct containing the options"}
    ]
  },
  "WBM":
  {
    "inputs":
    [
      {"name":"filename",  "kind":"required",  "type":["char"],
      "purpose":"Target file name"},
      {"name":"url_part",  "kind":"required",  "type":["char"],
      "purpose":"URL of the file/website"},
      {"name":"date_part", "kind":"namevalue", "type":["char"],
      "purpose":"Partial or complete date of the capture"},
      {"name":"tries",   "kind":"namevalue", "type":["numeric","numel=3"],
      "purpose":"Number of times a [load,save,timeout] is allowed"},
      {"name":"verbose",  "kind":"namevalue", "type":["numeric","scalar","integer"],
      "purpose":"The amount of text displayed"},
      {"name":"m_date_r",  "kind":"namevalue", "type":["char", "choices={'ignore','warning','error'}"],
      "purpose":"Missing date response"},
      {"name":"response",  "kind":"namevalue", "type":["char"],
      "purpose":"Cell with responses"},
      {"name":"ignore",  "kind":"namevalue", "type":["char"],
      "purpose":"HTML error codes to be ignored"}
    ]
  }
}

Best Answer

See the "comments" section for more discussion. In this particular case, the use of the argument name "options" is making the display a little ambiguous, since "options" is also generically used as the name for trailing name/value pairs (and, more rare, trailing flags).
In R2018a specifically, JSON definitions are use to display:
  • Function hints and suggested completions in the Live Editor
  • Tab-completion only in the command window and plain-text editor. Function hints still use older facilities.
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