Fix NoMethodError during `bundle update --group`

The `bundler` gem does not participate in the lockfile, but it can still
be included in the list of dependencies that are being updated by
`bundle update` if `--group` is specified.

For example, if a Gemfile contains `bundler-audit` (which depends on
`bundler`) in the `:development` group, then updating with the option
`--group=development` will naturally include `bundler` in the list of
gems to evaluate for updating.

The trouble is that since `bundler` is excluded from the lockfile,
searching the locked gems for a gemspec for bundler will return `nil`.
This caused the following error during `bundle update`:

    NoMethodError: undefined method `version' for nil:NilClass

This commit solves this bug by skipping over gems (i.e `bundler`) that
are not in the lockfile when comparing gem versions at the conclusion of
the upgrade command.

1 files changed, 17 insertions, 0 deletions

diff --git a/spec/commands/update_spec.rb b/spec/commands/update_spec.rb
index a8283cf593..a2842f0998 100644
--- a/spec/commands/update_spec.rb
+++ b/spec/commands/update_spec.rb
@@ -195,6 +195,23 @@ RSpec.describe "bundle update" do
         expect(the_bundle).not_to include_gems "foo 2.0"
       end
     end
+
+    context "when bundler itself is a transitive dependency" do
+      it "executes without error" do
+        install_gemfile <<-G
+          source "file://#{gem_repo1}"
+          gem "activesupport", :group => :development
+          gem "rack"
+        G
+        update_repo2 do
+          build_gem "activesupport", "3.0"
+        end
+        bundle "update --group development"
+        expect(the_bundle).to include_gems "activesupport 2.3.5"
+        expect(the_bundle).to include_gems "bundler #{Bundler::VERSION}"
+        expect(the_bundle).not_to include_gems "rack 1.2"
+      end
+    end
   end
 
   describe "in a frozen bundle" do