class Sass::Tree::Visitors::Perform

def visit_extend(node)

and then parses the result into a {Sass::Selector::CommaSequence}.
Runs SassScript interpolation in the selector,

def visit_extend(node)
  parser = Sass::SCSS::StaticParser.new(run_interp(node.selector),
    node.filename, node.options[:importer], node.line)
  node.resolved_selector = parser.parse_selector
  node
end