Rails 6.1 — add `importmap-rails`, gem bundles but methods unavailable

Ruby: 3.1.2, Rails 6.1

Trying to add gem importmap-rails to an existing Rails 6.1 app.

The gem bundles, but neither the rake importmap:install task nor the gem helper method <%= javascript_import_tags %> are available.

In rails console, the class Importmap is undefined.

It seems the gem does not install properly or completely. No useful error messages raised.

How can I debug a gem that bundles but is not available? The situation doesn't make sense to me.

(Related question from earlier attempt)

The issue was that the importmap-rails gem was inside group :rails in the Gemfile, and so the class did not get required.

This does not work (linked question above has the explanation):

# gem("rails", "~> 7.0")

group :rails do
  gem("actioncable")
  # gem("actionmailbox")
  gem("actionmailer")
  gem("actionpack")
  # gem("actiontext")
  gem("actionview")
  gem("activejob")
  gem("activemodel")
  gem("activerecord")
  # gem("activestorage")
  gem("activesupport")
  gem("bundler")
  gem("importmap-rails")
  gem("railties")
  gem("sprockets-rails")
  gem("turbo-rails")
end

Needs to be:

group :rails do
  ...
  gem("bundler")
  gem("railties")
end

gem("importmap-rails")
gem("sprockets-rails")
gem("turbo-rails")