Gem::Resolver::ActivationRequest#installed?

# File tmp/rubies/ruby-3.3.0/lib/rubygems/resolver/activation_request.rb, line 101
def installed?
  case @spec
  when Gem::Resolver::VendorSpecification then
    true
  else
    this_spec = full_spec

    Gem::Specification.any? do |s|
      s == this_spec
    end
  end
end