Display a warning when composing unlisted toots with something looking like a hashtag (#6132)

app/javascript/mastodon/features/compose/containers/warning_container.js

@@ -5,20 +5,27 @@ import PropTypes from 'prop-types';
 import { FormattedMessage } from 'react-intl';
 import { me } from '../../../initial_state';
 
+const APPROX_HASHTAG_RE = /(?:^|[^\/\)\w])#(\S+)/i;
+
 const mapStateToProps = state => ({
   needsLockWarning: state.getIn(['compose', 'privacy']) === 'private' && !state.getIn(['accounts', me, 'locked']),
+  hashtagWarning: state.getIn(['compose', 'privacy']) !== 'public' && APPROX_HASHTAG_RE.test(state.getIn(['compose', 'text'])),
 });
 
-const WarningWrapper = ({ needsLockWarning }) => {
+const WarningWrapper = ({ needsLockWarning, hashtagWarning }) => {
   if (needsLockWarning) {
     return <Warning message={<FormattedMessage id='compose_form.lock_disclaimer' defaultMessage='Your account is not {locked}. Anyone can follow you to view your follower-only posts.' values={{ locked: <a href='/settings/profile'><FormattedMessage id='compose_form.lock_disclaimer.lock' defaultMessage='locked' /></a> }} />} />;
   }
+  if (hashtagWarning) {
+    return <Warning message={<FormattedMessage id='compose_form.hashtag_warning' defaultMessage="This toot won't be listed under any hashtag as it is unlisted. Only public toots can be searched by hashtag." />} />;
+  }
 
   return null;
 };
 
 WarningWrapper.propTypes = {
   needsLockWarning: PropTypes.bool,
+  hashtagWarning: PropTypes.bool,
 };
 
 export default connect(mapStateToProps)(WarningWrapper);