Gem::SpecificationPolicy#validate_licenses

# File tmp/rubies/ruby-3.2.0/lib/rubygems/specification_policy.rb, line 362
  def validate_licenses
    licenses = @specification.licenses

    licenses.each do |license|
      if !Gem::Licenses.match?(license)
        suggestions = Gem::Licenses.suggestions(license)
        message = <<-WARNING
license value '#{license}' is invalid.  Use a license identifier from
http://spdx.org/licenses or '#{Gem::Licenses::NONSTANDARD}' for a nonstandard license.
        WARNING
        message += "Did you mean #{suggestions.map {|s| "'#{s}'" }.join(', ')}?\n" unless suggestions.nil?
        warning(message)
      end
    end

    warning <<-WARNING if licenses.empty?
licenses is empty, but is recommended.  Use a license identifier from
http://spdx.org/licenses or '#{Gem::Licenses::NONSTANDARD}' for a nonstandard license.
    WARNING
  end