Gem::Resolver#resolve

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/resolver.rb, line 143
def resolve
  # Pre-check: raise UnsatisfiableDependencyError for root deps with no
  # platform match. We filter by platform ONLY here (not required_ruby_version
  # / required_rubygems_version): a foreign-platform gem is genuinely "not
  # found", but a gem that exists yet is incompatible with the running Ruby
  # should flow through the solver to a DependencyResolutionError that names
  # the Ruby requirement. That matches Bundler (which models Ruby as a
  # synthetic dependency, so this surfaces as a solve failure) and gives a
  # clearer message than the platform-oriented UnsatisfiableDependencyError.
  @needed.each do |dep|
    next if @soft_missing
    dep_request = DependencyRequest.new(dep, nil)
    all = @set.find_all(dep_request)
    matching = select_local_platforms(all)

    next unless matching.empty?

    exc = Gem::UnsatisfiableDependencyError.new(dep_request, all)
    exc.errors = @set.errors
    raise exc
  end

  solver = Gem::PubGrub::VersionSolver.new(
    source: self,
    root: @root_package,
    strategy: Gem::Resolver::Strategy.new(self),
    logger: make_logger
  )
  result = solver.solve

  # Convert to Array<ActivationRequest>
  needed_by_name = @needed.group_by(&:name)
  requests = result.filter_map do |package, version|
    next if Gem::PubGrub::Package.root?(package)
    spec = spec_for(package.to_s, version)
    dep = needed_by_name[package.to_s]&.first || Gem::Dependency.new(package.to_s)
    dep_request = DependencyRequest.new(dep, nil)
    ActivationRequest.new(spec, dep_request)
  end

  @cooldown_skipped = cooldown_skipped_summary(requests) if @cooldown&.active?

  requests
rescue Gem::PubGrub::SolveFailure => e
  extended = extract_extended_explanation(e.incompatibility)
  if extended
    message = "#{e.explanation}\n\n#{extended}"
    raise Gem::DependencyResolutionError, Struct.new(:explanation).new(message)
  else
    raise Gem::DependencyResolutionError, e
  end
end