Test gem github: in a Gemfile

.github/workflows/test.yml

@@ -100,6 +100,11 @@ jobs:
     - run: bundle exec rake --version
     - run: bundle exec rake
 
+    - name: Test `gem github:` in a Gemfile
+      run: bundle install
+      env:
+        BUNDLE_GEMFILE: ${{ github.workspace }}/gemfiles/gem_from_github.gemfile
+
     - name: which ruby, rake
       if: "!startsWith(matrix.os, 'windows')"
       run: which -a ruby rake

gemfiles/gem_from_github.gemfile

@@ -0,0 +1,8 @@
+source "https://rubygems.org"
+
+# Ruby < 2.3 only support Bundler 1, which no longer works with gem github:
+if RUBY_VERSION >= '2.3'
+  # From https://github.com/ruby/setup-ruby/issues/358#issuecomment-1195899304
+  # Tests using github: and the repository uses a non-master default branch.
+  gem 'rack-test', github: 'rack/rack-test'
+end