[rubygems/rubygems] Warn non flattened require paths in old RubyGems versions too

https://github.com/rubygems/rubygems/commit/b3cdccc6fb

lib/bundler/rubygems_ext.rb

@@ -56,6 +56,9 @@ module Gem
   # Can be removed once RubyGems 3.5.14 support is dropped
   VALIDATES_FOR_RESOLUTION = Specification.new.respond_to?(:validate_for_resolution).freeze
 
+  # Can be removed once RubyGems 3.3.15 support is dropped
+  FLATTENS_REQUIRED_PATHS = Specification.new.respond_to?(:flatten_require_paths).freeze
+
   class Specification
     require_relative "match_metadata"
     require_relative "match_platform"
@@ -161,6 +164,27 @@ module Gem
       end
     end
 
+    unless FLATTENS_REQUIRED_PATHS
+      def flatten_require_paths
+        return unless raw_require_paths.first.is_a?(Array)
+
+        warn "#{name} #{version} includes a gemspec with `require_paths` set to an array of arrays. Newer versions of this gem might've already fixed this"
+        raw_require_paths.flatten!
+      end
+
+      class << self
+        module RequirePathFlattener
+          def from_yaml(input)
+            spec = super(input)
+            spec.flatten_require_paths
+            spec
+          end
+        end
+
+        prepend RequirePathFlattener
+      end
+    end
+
     private
 
     def dependencies_to_gemfile(dependencies, group = nil)

spec/bundler/realworld/edgecases_spec.rb

@@ -198,18 +198,7 @@ RSpec.describe "real world edgecases", realworld: true do
     expect(lockfile).to include(rubygems_version("paperclip", "~> 5.1.0"))
   end
 
-  it "outputs a helpful error message when gems have invalid gemspecs", rubygems: "< 3.3.16" do
+  it "outputs a helpful warning when gems have a gemspec with invalid `require_paths`" do
-    install_gemfile <<-G, standalone: true, raise_on_error: false, env: { "BUNDLE_FORCE_RUBY_PLATFORM" => "1" }
-      source 'https://rubygems.org'
-      gem "resque-scheduler", "2.2.0"
-      gem "redis-namespace", "1.6.0" # for a consistent resolution including ruby 2.3.0
-      gem "ruby2_keywords", "0.0.5"
-    G
-    expect(err).to include("You have one or more invalid gemspecs that need to be fixed.")
-    expect(err).to include("resque-scheduler 2.2.0 has an invalid gemspec")
-  end
-
-  it "outputs a helpful warning when gems have a gemspec with invalid `require_paths`", rubygems: ">= 3.3.16" do
     install_gemfile <<-G, standalone: true, env: { "BUNDLE_FORCE_RUBY_PLATFORM" => "1" }
       source 'https://rubygems.org'
       gem "resque-scheduler", "2.2.0"