How do I reproduce the problem this causes?

I am also awaiting further instructions. https://discourse.phabricator-community.org/t/cannot-display-placeholder-in-src-view-form-control-aphrontformtokenizercontrol-php/89/11

The code does seem wrong though?

I'll admit I missed reproduction steps on first pass (and didn't ask until after I wrote the diff)

I don't see any callsites, so I believe this doesn't cause any reproducible bad behavior -- i.e., this is support for third-party development, not a bug fix. If there are no callsites, we should just remove setPlaceholder() and the placeholder property if you want to clean this up.

(Without blaming it, my guess is that this method predates the modularization of TypeaheadDatasource into a separate subclass and has been obsolete since then.)

¯\_(ツ)_/¯

Not the worst thing I've wasted my time on today.

it's a nice diff though. I used a getter.