Gem::SpecificationPolicy#validate_licenses

# File tmp/rubies/ruby-3.4.1/lib/rubygems/specification_policy.rb, line 392
  def validate_licenses
    licenses = @specification.licenses

    licenses.each do |license|
      next if Gem::Licenses.match?(license) || license.nil?
      license_id_deprecated = Gem::Licenses.deprecated_license_id?(license)
      exception_id_deprecated = Gem::Licenses.deprecated_exception_id?(license)
      suggestions = Gem::Licenses.suggestions(license)

      if license_id_deprecated
        main_message = "License identifier '#{license}' is deprecated"
      elsif exception_id_deprecated
        main_message = "Exception identifier at '#{license}' is deprecated"
      else
        main_message = "License identifier '#{license}' is invalid"
      end

      message = <<-WARNING
#{main_message}. Use an identifier from
https://spdx.org/licenses or '#{Gem::Licenses::NONSTANDARD}' for a nonstandard license,
or set it to nil if you don't want to specify a license.
      WARNING
      message += "Did you mean #{suggestions.map {|s| "'#{s}'" }.join(", ")}?\n" unless suggestions.nil?
      warning(message)
    end

    warning <<-WARNING if licenses.empty?
licenses is empty, but is recommended. Use an license identifier from
https://spdx.org/licenses or '#{Gem::Licenses::NONSTANDARD}' for a nonstandard license,
or set it to nil if you don't want to specify a license.
    WARNING
  end