Bug #21708: Ruby 3.4 Forwardable: setter delegations trigger SyntaxError from forwardable/impl.rb

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: 3.4.7
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[jfi (James Inman)]

• forwardable: 1.3.3 (bundled stdlib)
• OS: Ubuntu 24.04 (also reproducible on macOS)
• RUBYOPT / $DEBUG: default (but with $DEBUG=true or RUBYOPT=-d the errors are emitted)

Reproduction Script (minimal)

  require "forwardable"

  class Host
    attr_accessor :scheme
  end

  class Delegator
    extend Forwardable
    def initialize
      @host = Host.new
    end
    def_delegator :@host, :scheme=, :scheme=
    def_delegator :@host, :scheme,  :scheme
  end

  $DEBUG = true # force forwardable to log the SyntaxError
  d = Delegator.new
  d.scheme = "https"
  puts d.scheme

Output running with ruby -d:

  Exception 'SyntaxError' at .../forwardable/impl.rb:6 - syntax errors found
  > 6 | BEGIN{throw tag}; ().scheme=
      |                    ^ expected an expression after `=`

Despite the SyntaxError, the program continues, but stderr is flooded whenever a setter is delegated. Similar errors appear for any delegated setter (host=, port=, etc.), causing noise in test runs and CI logs.

Expected behavior

Delegating setter methods should not raise or log SyntaxError; forwardable should generate the delegator quietly, as in Ruby 3.3 and earlier.

Other reports

• ruby/forwardable issue #35 shows the same SyntaxError output pattern on Ruby 3.4: https://github.com/ruby/forwardable/issues/35