[rubygems/rubygems] Fix bundle update --bundler when restarts disabled

There's no reason why we should not update bundler as requested, even if
restarts are disabled.

https://github.com/rubygems/rubygems/commit/e59acd2a0d

lib/bundler/self_manager.rb

@@ -31,8 +31,6 @@ module Bundler
     end
 
     def update_bundler_and_restart_with_it_if_needed(target)
-      return unless autoswitching_applies?
-
       spec = resolve_update_version_from(target)
       return unless spec
 
@@ -40,7 +38,7 @@ module Bundler
 
       Bundler.ui.info "Updating bundler to #{version}."
 
-      install(spec)
+      install(spec) unless installed?(version)
 
       restart_with(version)
     end
@@ -102,8 +100,7 @@ module Bundler
     def needs_switching?(restart_version)
       autoswitching_applies? &&
         released?(restart_version) &&
-        !running?(restart_version) &&
-        !updating?
+        !running?(restart_version)
     end
 
     def autoswitching_applies?
@@ -172,10 +169,6 @@ module Bundler
       $PROGRAM_NAME != "-e"
     end
 
-    def updating?
-      "update".start_with?(ARGV.first || " ") && ARGV[1..-1].any? {|a| a.start_with?("--bundler") }
-    end
-
     def installed?(restart_version)
       Bundler.configure
 

spec/bundler/commands/update_spec.rb

@@ -1684,6 +1684,24 @@ RSpec.describe "bundle update --bundler" do
     expect(err).to eq("The `bundle update --bundler` target version (999.999.999) does not exist")
   end
 
+  it "errors if the explicit target version does not exist, even if auto switching is disabled" do
+    pristine_system_gems "bundler-9.9.9"
+
+    build_repo4 do
+      build_gem "myrack", "1.0"
+    end
+
+    install_gemfile <<-G
+      source "https://gem.repo4"
+      gem "myrack"
+    G
+
+    bundle :update, bundler: "999.999.999", raise_on_error: false, env: { "BUNDLER_VERSION" => "9.9.9" }
+
+    expect(last_command).to be_failure
+    expect(err).to eq("The `bundle update --bundler` target version (999.999.999) does not exist")
+  end
+
   it "allows updating to development versions if already installed locally" do
     system_gems "bundler-9.9.9"