diff --git a/compile.c b/compile.c
index cbf00c9da113b8..4a0a5e83f3a3c0 100644
--- a/compile.c
+++ b/compile.c
@@ -3149,41 +3149,56 @@ find_destination(INSN *i)
 static int
 remove_unreachable_chunk(rb_iseq_t *iseq, LINK_ELEMENT *i)
 {
-    LINK_ELEMENT *first = i, *end;
+    LINK_ELEMENT *first = i, *end, *scan;
     int *unref_counts = 0, nlabels = ISEQ_COMPILE_DATA(iseq)->label_no;
 
     if (!i) return 0;
     unref_counts = ALLOCA_N(int, nlabels);
     MEMZERO(unref_counts, int, nlabels);
-    end = i;
+
+    scan = i;
     do {
         LABEL *lab;
-        if (IS_INSN(i)) {
-            if (IS_INSN_ID(i, leave)) {
-                end = i;
+        if (IS_INSN(scan)) {
+            if (IS_INSN_ID(scan, leave)) {
                 break;
             }
-            else if ((lab = find_destination((INSN *)i)) != 0) {
+            else if ((lab = find_destination((INSN *)scan)) != 0) {
                 unref_counts[lab->label_no]++;
             }
         }
-        else if (IS_LABEL(i)) {
-            lab = (LABEL *)i;
+        else if (IS_LABEL(scan)) {
+            lab = (LABEL *)scan;
             if (lab->unremovable) return 0;
+        }
+        else if (IS_ADJUST(scan)) {
+            return 0;
+        }
+    } while ((scan = scan->next) != 0);
+
+    end = i;
+    scan = i;
+    do {
+        LABEL *lab;
+        if (IS_INSN(scan)) {
+            if (IS_INSN_ID(scan, leave)) {
+                end = scan;
+                break;
+            }
+        }
+        else if (IS_LABEL(scan)) {
+            lab = (LABEL *)scan;
             if (lab->refcnt > unref_counts[lab->label_no]) {
-                if (i == first) return 0;
+                if (scan == first) return 0;
                 break;
             }
             continue;
         }
-        else if (IS_TRACE(i)) {
-            /* do nothing */
-        }
-        else if (IS_ADJUST(i)) {
+        else if (IS_ADJUST(scan)) {
             return 0;
         }
-        end = i;
-    } while ((i = i->next) != 0);
+        end = scan;
+    } while ((scan = scan->next) != 0);
     i = first;
     do {
         if (IS_INSN(i)) {
diff --git a/doc/syntax/refinements.rdoc b/doc/syntax/refinements.rdoc
index 80595eb4455ab6..836dac2c99ad15 100644
--- a/doc/syntax/refinements.rdoc
+++ b/doc/syntax/refinements.rdoc
@@ -246,7 +246,10 @@ invokes that method since +foo+ does not exist on Integer.
 
 When +super+ is invoked, method lookup starts:
 
-* If the method is in a refinement, at the refined class or module
+* If the method is in a refinement:
+  * At the next active refinement in scope at the +super+ call site,
+    excluding the current refinement, if such a refinement exists
+  * Otherwise, at the refined class or module
 * Otherwise, at the next ancestor
 
 Method lookup then proceeds as described in the Method Lookup section
diff --git a/lib/bundler/bundler.gemspec b/lib/bundler/bundler.gemspec
index 49319e81b4f08c..77e957108ccf6c 100644
--- a/lib/bundler/bundler.gemspec
+++ b/lib/bundler/bundler.gemspec
@@ -24,9 +24,9 @@ Gem::Specification.new do |s|
 
   s.metadata = {
     "bug_tracker_uri" => "https://github.com/ruby/rubygems/issues?q=is%3Aopen+is%3Aissue+label%3ABundler",
-    "changelog_uri" => "https://github.com/ruby/rubygems/blob/master/bundler/CHANGELOG.md",
+    "changelog_uri" => "https://github.com/ruby/rubygems/blob/master/CHANGELOG-bundler.md",
     "homepage_uri" => "https://bundler.io/",
-    "source_code_uri" => "https://github.com/ruby/rubygems/tree/master/bundler",
+    "source_code_uri" => "https://github.com/ruby/rubygems",
   }
 
   s.required_ruby_version     = ">= 3.2.0"
@@ -39,6 +39,9 @@ Gem::Specification.new do |s|
   # include the gemspec itself because warbler breaks w/o it
   s.files += %w[lib/bundler/bundler.gemspec]
 
+  # These live next to the gemspec when Bundler ships as a gem, but not when
+  # it is synced into Ruby core, where the gemspec moves under lib/bundler.
+  s.files += %w[CHANGELOG-bundler.md LICENSE-bundler.md README-bundler.md].select {|f| File.file?(f) }
   s.bindir        = "exe"
   s.executables   = %w[bundle bundler]
   s.require_paths = ["lib"]
diff --git a/lib/bundler/ci_detector.rb b/lib/bundler/ci_detector.rb
index e5fedbdea8d383..227e127c71c229 100644
--- a/lib/bundler/ci_detector.rb
+++ b/lib/bundler/ci_detector.rb
@@ -3,7 +3,7 @@
 module Bundler
   module CIDetector
     # NOTE: Any changes made here will need to be made to both lib/rubygems/ci_detector.rb and
-    # bundler/lib/bundler/ci_detector.rb (which are enforced duplicates).
+    # lib/bundler/ci_detector.rb (which are enforced duplicates).
     # TODO: Drop that duplication once bundler drops support for RubyGems 3.4
     #
     # ## Recognized CI providers, their signifiers, and the relevant docs ##
diff --git a/lib/rubygems/ci_detector.rb b/lib/rubygems/ci_detector.rb
index 7a2d4ee29a9360..22e9552079cae1 100644
--- a/lib/rubygems/ci_detector.rb
+++ b/lib/rubygems/ci_detector.rb
@@ -3,7 +3,7 @@
 module Gem
   module CIDetector
     # NOTE: Any changes made here will need to be made to both lib/rubygems/ci_detector.rb and
-    # bundler/lib/bundler/ci_detector.rb (which are enforced duplicates).
+    # lib/bundler/ci_detector.rb (which are enforced duplicates).
     # TODO: Drop that duplication once bundler drops support for RubyGems 3.4
     #
     # ## Recognized CI providers, their signifiers, and the relevant docs ##
diff --git a/lib/rubygems/commands/setup_command.rb b/lib/rubygems/commands/setup_command.rb
index 175599967cf62d..ad4aaab384a0c6 100644
--- a/lib/rubygems/commands/setup_command.rb
+++ b/lib/rubygems/commands/setup_command.rb
@@ -292,7 +292,6 @@ def shebang
 
   def install_lib(lib_dir)
     libs = { "RubyGems" => "lib" }
-    libs["Bundler"] = "bundler/lib"
     libs.each do |tool, path|
       say "Installing #{tool}" if @verbose
 
@@ -366,7 +365,7 @@ def install_default_bundler_gem(bin_dir)
       target_specs_dir
     end
 
-    new_bundler_spec = Dir.chdir("bundler") { Gem::Specification.load("bundler.gemspec") }
+    new_bundler_spec = Gem::Specification.load("bundler.gemspec")
     full_name = new_bundler_spec.full_name
     gemspec_path = "#{full_name}.gemspec"
 
@@ -390,26 +389,24 @@ def install_default_bundler_gem(bin_dir)
 
     require_relative "../installer"
 
-    Dir.chdir("bundler") do
-      built_gem = Gem::Package.build(new_bundler_spec)
-      begin
-        installer = Gem::Installer.at(
-          built_gem,
-          env_shebang: options[:env_shebang],
-          format_executable: options[:format_executable],
-          force: options[:force],
-          bin_dir: bin_dir,
-          install_dir: default_dir,
-          wrappers: true
-        )
-        # We need to install only executable and default spec files.
-        # lib/bundler.rb and lib/bundler/* are available under the site_ruby directory.
-        installer.extract_bin
-        installer.generate_bin
-        installer.write_default_spec
-      ensure
-        FileUtils.rm_f built_gem
-      end
+    built_gem = Gem::Package.build(new_bundler_spec)
+    begin
+      installer = Gem::Installer.at(
+        built_gem,
+        env_shebang: options[:env_shebang],
+        format_executable: options[:format_executable],
+        force: options[:force],
+        bin_dir: bin_dir,
+        install_dir: default_dir,
+        wrappers: true
+      )
+      # We need to install only executable and default spec files.
+      # lib/bundler.rb and lib/bundler/* are available under the site_ruby directory.
+      installer.extract_bin
+      installer.generate_bin
+      installer.write_default_spec
+    ensure
+      FileUtils.rm_f built_gem
     end
 
     new_bundler_spec.executables.each {|executable| bin_file_names << target_bin_path(bin_dir, executable) }
@@ -499,7 +496,7 @@ def remove_old_bin_files(bin_dir)
 
   def remove_old_lib_files(lib_dir)
     lib_dirs = { File.join(lib_dir, "rubygems") => "lib/rubygems" }
-    lib_dirs[File.join(lib_dir, "bundler")] = "bundler/lib/bundler"
+    lib_dirs[File.join(lib_dir, "bundler")] = "lib/bundler"
     lib_dirs.each do |old_lib_dir, new_lib_dir|
       lib_files = files_in(new_lib_dir)
 
diff --git a/lib/rubygems/platform.rb b/lib/rubygems/platform.rb
index 367b00e7e1a286..dac1ce885829fe 100644
--- a/lib/rubygems/platform.rb
+++ b/lib/rubygems/platform.rb
@@ -196,7 +196,7 @@ def hash # :nodoc:
   # the runtime platform.
   #
   #--
-  # NOTE: Until it can be removed, changes to this method must also be reflected in `bundler/lib/bundler/rubygems_ext.rb`
+  # NOTE: Until it can be removed, changes to this method must also be reflected in `lib/bundler/rubygems_ext.rb`
 
   def ===(other)
     return nil unless Gem::Platform === other
@@ -221,7 +221,7 @@ def ===(other)
   end
 
   #--
-  # NOTE: Until it can be removed, changes to this method must also be reflected in `bundler/lib/bundler/rubygems_ext.rb`
+  # NOTE: Until it can be removed, changes to this method must also be reflected in `lib/bundler/rubygems_ext.rb`
 
   def normalized_linux_version
     return nil unless @version
diff --git a/ractor.rb b/ractor.rb
index 2dc60f5ff64926..366a1192c818d2 100644
--- a/ractor.rb
+++ b/ractor.rb
@@ -615,7 +615,7 @@ def value
 
   #
   # call-seq:
-  #    ractor.monitor(port) -> self
+  #    ractor.monitor(port) -> true or false
   #
   # Registers the port as a monitoring port for this ractor. When the ractor terminates,
   # the port receives a Symbol object.
@@ -623,6 +623,10 @@ def value
   # * +:exited+ is sent if the ractor terminates without an unhandled exception.
   # * +:aborted+ is sent if the ractor terminates by an unhandled exception.
   #
+  # Returns +true+ if the monitor was registered (the ractor is still running).
+  # Returns +false+ if the ractor had already terminated; in that case the
+  # termination message (+:exited+ or +:aborted+) is sent to the port immediately.
+  #
   #     r = Ractor.new{ some_task() }
   #     r.monitor(port = Ractor::Port.new)
   #     port.receive #=> :exited and r is terminated
diff --git a/spec/bin/bundle b/spec/bin/bundle
index 8f8b5352951537..23e386505d6e7a 100755
--- a/spec/bin/bundle
+++ b/spec/bin/bundle
@@ -1,6 +1,7 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
+require_relative "../bundler/support/switch_rubygems"
 require_relative "../bundler/support/activate"
 
 load File.expand_path("bundle", Spec::Path.exedir)
diff --git a/spec/bundler/commands/update_spec.rb b/spec/bundler/commands/update_spec.rb
index f100bdfb9edb55..907f81d756ff7e 100644
--- a/spec/bundler/commands/update_spec.rb
+++ b/spec/bundler/commands/update_spec.rb
@@ -1582,8 +1582,11 @@
         999.0.0
     L
 
-    expect(the_bundle).to include_gems "bundler 999.0.0"
-    expect(the_bundle).to include_gems "myrack 1.0"
+    bundle "--version"
+    expect(out).to include("999.0.0")
+
+    bundle "list"
+    expect(out).to include("myrack (1.0)")
   end
 
   it "does not claim to update to Bundler version to a wrong version when cached gems are present" do
@@ -1665,8 +1668,11 @@
         9.9.9
     L
 
-    expect(the_bundle).to include_gems "bundler 9.9.9"
-    expect(the_bundle).to include_gems "myrack 1.0"
+    bundle "--version"
+    expect(out).to include("9.9.9")
+
+    bundle "list"
+    expect(out).to include("myrack (1.0)")
   end
 
   it "errors if the explicit target version does not exist" do
diff --git a/spec/bundler/install/gemfile_spec.rb b/spec/bundler/install/gemfile_spec.rb
index 83875a3d0e300f..b8cd66ca7580ce 100644
--- a/spec/bundler/install/gemfile_spec.rb
+++ b/spec/bundler/install/gemfile_spec.rb
@@ -135,7 +135,7 @@ def source(source, *args, &blk)
     it "shows culprit file and line" do
       skip "ruby-core test setup has always \"lib\" in $LOAD_PATH so `require \"bundler\"` always activates the local version rather than using RubyGems gem activation stuff, causing conflicts" if ruby_core?
 
-      install_gemfile "source 'https://gem.repo1'", requires: [bundler_bug], artifice: nil, raise_on_error: false
+      install_gemfile "source 'https://gem.repo1'", requires: [bundler_bug], artifice: nil, raise_on_error: false, bundle_bin: dev_binstub
       expect(err).to include("bundler_bug.rb:6")
     end
   end
diff --git a/spec/bundler/runtime/setup_spec.rb b/spec/bundler/runtime/setup_spec.rb
index ceb6fcf66acf39..2b4ac3085c318b 100644
--- a/spec/bundler/runtime/setup_spec.rb
+++ b/spec/bundler/runtime/setup_spec.rb
@@ -144,7 +144,7 @@ def clean_load_path(lp)
 
       ruby <<-RUBY
         require 'bundler'
-        gem "bundler", "#{Bundler::VERSION}" if #{ruby_core?}
+        gem "bundler", "#{Bundler::VERSION}"
         Bundler.setup
         puts $LOAD_PATH
       RUBY
@@ -987,7 +987,8 @@ def clean_load_path(lp)
       puts Bundler.rubygems.installed_specs.map(&:name)
     R
 
-    expect(out).to eq("activesupport\nbundler\nactivesupport\nbundler")
+    expect(out).to include("activesupport")
+    expect(out).not_to include("myrack")
   end
 
   describe "when a vendored gem specification uses the :path option" do
@@ -1309,12 +1310,16 @@ def lock_with(ruby_version = nil)
     end
 
     context "is not present" do
-      # Skipped on ruby-core because `ruby "require 'bundler/setup'"` does not
-      # activate bundler as a gem there, so Source::Metadata falls back to a
-      # synthetic spec whose cache_file does not exist on disk and
-      # LockfileGenerator#bundler_checksum drops the bundler checksum, while
-      # the on-disk lockfile still has it.
+      # Skipped on ruby-core, and on the release-version CI variant, because
+      # `ruby "require 'bundler/setup'"` does not activate bundler as a gem
+      # there, so Source::Metadata falls back to a synthetic spec whose
+      # cache_file does not exist on disk and LockfileGenerator#bundler_checksum
+      # drops the bundler checksum, while the on-disk lockfile still has it.
+      # In-development (.dev) versions never write a bundler checksum, so the
+      # regular suite stays unaffected.
       it "does not change the lock", :ruby_repo do
+        skip "bundler is loaded from the source tree, not installed as a gem" unless Bundler::VERSION.end_with?(".dev")
+
         expect { ruby "require 'bundler/setup'" }.not_to change { lockfile }
       end
     end
diff --git a/spec/bundler/support/build_metadata.rb b/spec/bundler/support/build_metadata.rb
index 2eade4137bd68f..8469d96a760112 100644
--- a/spec/bundler/support/build_metadata.rb
+++ b/spec/bundler/support/build_metadata.rb
@@ -44,7 +44,7 @@ def git_commit_sha
     end
 
     def release_date_for(version, dir:)
-      changelog = File.expand_path("CHANGELOG.md", dir)
+      changelog = File.expand_path("CHANGELOG-bundler.md", dir)
       File.readlines(changelog)[2].scan(/^## #{Regexp.escape(version)} \((.*)\)/).first&.first if File.exist?(changelog)
     end
 
diff --git a/spec/bundler/support/path.rb b/spec/bundler/support/path.rb
index 4b80c5b4a444d9..6b81cb19963dcf 100644
--- a/spec/bundler/support/path.rb
+++ b/spec/bundler/support/path.rb
@@ -10,7 +10,7 @@ module Path
     include Spec::Env
 
     def source_root
-      @source_root ||= Pathname.new(ruby_core? ? "../../.." : "../../bundler").expand_path(__dir__)
+      @source_root ||= Pathname.new(ruby_core? ? "../../.." : "../..").expand_path(__dir__)
     end
 
     def root
@@ -50,7 +50,7 @@ def dev_binstub
     end
 
     def bindir
-      @bindir ||= source_root.join(ruby_core? ? "spec/bin" : "../bin")
+      @bindir ||= source_root.join(ruby_core? ? "spec/bin" : "bin")
     end
 
     def exedir
@@ -76,7 +76,7 @@ def path
     end
 
     def spec_dir
-      @spec_dir ||= source_root.join(ruby_core? ? "spec/bundler" : "../spec")
+      @spec_dir ||= source_root.join(ruby_core? ? "spec/bundler" : "spec")
     end
 
     def man_dir
@@ -123,7 +123,7 @@ def tmp_root
         end
         Pathname(real)
       else
-        (ruby_core? ? source_root : source_root.parent).join("tmp")
+        source_root.join("tmp")
       end
     end
 
@@ -291,14 +291,14 @@ def replace_required_ruby_version(version, dir:)
     end
 
     def replace_changelog(version, dir:)
-      changelog = File.expand_path("CHANGELOG.md", dir)
+      changelog = File.expand_path("CHANGELOG-bundler.md", dir)
       contents = File.readlines(changelog)
       contents = [contents[0], contents[1], "## #{version} (2100-01-01)\n", *contents[3..-1]].join
       File.open(changelog, "w") {|f| f << contents }
     end
 
     def git_root
-      ruby_core? ? source_root : source_root.parent
+      source_root
     end
 
     def rake_path
@@ -345,11 +345,11 @@ def git_ls_files(glob)
     end
 
     def tracked_files_glob
-      ruby_core? ? "libexec/bundle* lib/bundler lib/bundler.rb spec/bundler man/bundle*" : "lib exe CHANGELOG.md LICENSE.md README.md bundler.gemspec"
+      ruby_core? ? "libexec/bundle* lib/bundler lib/bundler.rb spec/bundler man/bundle*" : "exe/bundle exe/bundler lib/bundler lib/bundler.rb bundler.gemspec CHANGELOG-bundler.md LICENSE-bundler.md README-bundler.md"
     end
 
     def lib_tracked_files_glob
-      ruby_core? ? "lib/bundler lib/bundler.rb" : "lib"
+      "lib/bundler lib/bundler.rb"
     end
 
     def man_tracked_files_glob
@@ -371,7 +371,7 @@ def standard_gemfile_basename
     end
 
     def tool_dir
-      ruby_core? ? source_root.join("tool/bundler") : source_root.join("../tool/bundler")
+      source_root.join("tool/bundler")
     end
 
     def templates_dir
diff --git a/test/ruby/test_iseq.rb b/test/ruby/test_iseq.rb
index b4760dc412779c..30931587f8b6e4 100644
--- a/test/ruby/test_iseq.rb
+++ b/test/ruby/test_iseq.rb
@@ -953,6 +953,10 @@ def test_unreachable_next_in_block
     end
   end
 
+  def test_unreachable_find_pattern_in_else_branch
+    assert_in_out_err([], "if []; else; a => [*, 42, *]; end")
+  end
+
   def test_serialize_anonymous_outer_variables
     iseq = RubyVM::InstructionSequence.compile(<<~'RUBY')
       obj = Object.new
diff --git a/test/rubygems/bundler_test_gem.rb b/test/rubygems/bundler_test_gem.rb
index ca2980e04b519a..1f356bb42c3059 100644
--- a/test/rubygems/bundler_test_gem.rb
+++ b/test/rubygems/bundler_test_gem.rb
@@ -408,7 +408,7 @@ def with_local_bundler_at(path)
     require "bundler"
 
     # If bundler gemspec exists, pretend it's installed
-    bundler_gemspec = File.expand_path("../../bundler/bundler.gemspec", __dir__)
+    bundler_gemspec = File.expand_path("../../bundler.gemspec", __dir__)
     if File.exist?(bundler_gemspec)
       target_gemspec_location = "#{path}/specifications/bundler-#{Bundler::VERSION}.gemspec"
 
diff --git a/test/rubygems/test_gem_commands_setup_command.rb b/test/rubygems/test_gem_commands_setup_command.rb
index b33e05ab28dc55..7df59ecfc6dbbb 100644
--- a/test/rubygems/test_gem_commands_setup_command.rb
+++ b/test/rubygems/test_gem_commands_setup_command.rb
@@ -15,15 +15,15 @@ def setup
       lib/rubygems.rb
       lib/rubygems/requirement.rb
       lib/rubygems/ssl_certs/rubygems.org/foo.pem
-      bundler/exe/bundle
-      bundler/exe/bundler
-      bundler/lib/bundler.rb
-      bundler/lib/bundler/b.rb
-      bundler/bin/bundler/man/bundle-b.1
-      bundler/lib/bundler/man/bundle-b.1.ronn
-      bundler/lib/bundler/man/gemfile.5
-      bundler/lib/bundler/man/gemfile.5.ronn
-      bundler/lib/bundler/templates/.circleci/config.yml
+      exe/bundle
+      exe/bundler
+      lib/bundler.rb
+      lib/bundler/b.rb
+      lib/bundler/man/bundle-b.1
+      lib/bundler/man/bundle-b.1.ronn
+      lib/bundler/man/gemfile.5
+      lib/bundler/man/gemfile.5.ronn
+      lib/bundler/templates/.circleci/config.yml
     ]
 
     create_dummy_files(filelist)
@@ -34,7 +34,7 @@ def setup
       s.files = ["lib/bundler.rb"]
     end
 
-    File.open "bundler/bundler.gemspec", "w" do |io|
+    File.open "bundler.gemspec", "w" do |io|
       io.puts gemspec.to_ruby
     end
 
@@ -171,8 +171,18 @@ def test_destdir_flag_regenerates_binstubs
   end
 
   def test_files_in
-    assert_equal %w[rubygems.rb rubygems/requirement.rb rubygems/ssl_certs/rubygems.org/foo.pem],
-                 @cmd.files_in("lib").sort
+    assert_equal %w[
+      bundler.rb
+      bundler/b.rb
+      bundler/man/bundle-b.1
+      bundler/man/bundle-b.1.ronn
+      bundler/man/gemfile.5
+      bundler/man/gemfile.5.ronn
+      bundler/templates/.circleci/config.yml
+      rubygems.rb
+      rubygems/requirement.rb
+      rubygems/ssl_certs/rubygems.org/foo.pem
+    ], @cmd.files_in("lib").sort
   end
 
   def test_install_lib
@@ -475,7 +485,7 @@ def new_bundler_specification_path
   end
 
   def bundler_spec
-    Gem::Specification.load("bundler/bundler.gemspec")
+    Gem::Specification.load("bundler.gemspec")
   end
 
   def bundler_version
diff --git a/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb b/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb
index 129c216259e63a..780846d0156b1a 100644
--- a/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb
+++ b/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb
@@ -231,5 +231,54 @@ def omit_unless_support_pqc
     # mode :pqc requires Ruby OpenSSL >= 4.0.
     omit "PQC test requires Ruby OpenSSL >= 4.0" unless
       Gem::Version.new(OpenSSL::VERSION) >= Gem::Version.new("4.0")
+    # Even with a new enough OpenSSL, the runtime may keep PQC groups and
+    # signature algorithms out of its default negotiation lists (for example
+    # RHEL's system-wide crypto policies). The PQC server forces both, while
+    # the gem fetcher connects with the default client configuration, so a
+    # real loopback handshake is the only reliable way to tell whether this
+    # environment can negotiate PQC at all.
+    omit "PQC handshake is not available in this OpenSSL configuration" unless
+      self.class.support_pqc_handshake?
+  end
+
+  # Probe an actual PQC handshake between a forced-PQC server and a
+  # default-configured client, mirroring what the integration tests exercise.
+  # Memoized so the probe runs at most once per process.
+  def self.support_pqc_handshake?
+    return @support_pqc_handshake unless @support_pqc_handshake.nil?
+
+    @support_pqc_handshake = probe_pqc_handshake
+  end
+
+  def self.probe_pqc_handshake
+    server = TCPServer.new("127.0.0.1", 0)
+    ctx = OpenSSL::SSL::SSLContext.new
+    ctx.cert = OpenSSL::X509::Certificate.new(File.read(File.join(__dir__, "mldsa65_ssl_cert.pem")))
+    ctx.key = OpenSSL::PKey.read(File.read(File.join(__dir__, "mldsa65_ssl_key.pem")))
+    ctx.groups = "X25519MLKEM768"
+    ssl_server = OpenSSL::SSL::SSLServer.new(server, ctx)
+
+    port = server.addr[1]
+    server_thread = Thread.new do
+      client = ssl_server.accept
+      client.close
+    rescue OpenSSL::OpenSSLError
+      nil
+    end
+
+    client_ctx = OpenSSL::SSL::SSLContext.new
+    client_ctx.verify_mode = OpenSSL::SSL::VERIFY_NONE
+    socket = TCPSocket.new("127.0.0.1", port)
+    ssl = OpenSSL::SSL::SSLSocket.new(socket, client_ctx)
+    ssl.connect
+    ssl.close
+    true
+  rescue OpenSSL::OpenSSLError, SystemCallError
+    false
+  ensure
+    server_thread&.join(5)
+    server_thread&.kill if server_thread&.alive?
+    ssl_server&.close
+    server&.close
   end
 end if Gem::HAVE_OPENSSL
diff --git a/tool/sync_default_gems.rb b/tool/sync_default_gems.rb
index db64e20274b4e0..3242dee5d2b541 100755
--- a/tool/sync_default_gems.rb
+++ b/tool/sync_default_gems.rb
@@ -241,11 +241,11 @@ def lib((upstream, branch), gemspec_in_subdir: false)
       ["lib/rubygems.rb", "lib/rubygems.rb"],
       ["lib/rubygems", "lib/rubygems"],
       ["test/rubygems", "test/rubygems"],
-      ["bundler/lib/bundler.rb", "lib/bundler.rb"],
-      ["bundler/lib/bundler", "lib/bundler"],
-      ["bundler/exe/bundle", "libexec/bundle"],
-      ["bundler/exe/bundler", "libexec/bundler"],
-      ["bundler/bundler.gemspec", "lib/bundler/bundler.gemspec"],
+      ["lib/bundler.rb", "lib/bundler.rb"],
+      ["lib/bundler", "lib/bundler"],
+      ["exe/bundle", "libexec/bundle"],
+      ["exe/bundler", "libexec/bundler"],
+      ["bundler.gemspec", "lib/bundler/bundler.gemspec"],
       ["spec", "spec/bundler"],
       *["bundle", "parallel_rspec", "rspec"].map {|binstub|
         ["bin/#{binstub}", "spec/bin/#{binstub}"]
@@ -369,12 +369,15 @@ def replace_rdoc_ref_all_full
   end
 
   def rubygems_do_fixup
-    gemspec_content = File.readlines("lib/bundler/bundler.gemspec").map do |line|
-      next if line =~ /LICENSE\.md/
+    gemspec = "lib/bundler/bundler.gemspec"
+    if File.exist?(gemspec)
+      gemspec_content = File.readlines(gemspec).map do |line|
+        next if line =~ /LICENSE\.md/
 
-      line.gsub("bundler.gemspec", "lib/bundler/bundler.gemspec")
-    end.compact.join
-    File.write("lib/bundler/bundler.gemspec", gemspec_content)
+        line.gsub("bundler.gemspec", "lib/bundler/bundler.gemspec")
+      end.compact.join
+      File.write(gemspec, gemspec_content)
+    end
 
     ["bundle", "parallel_rspec", "rspec"].each do |binstub|
       path = "spec/bin/#{binstub}"