def foo(x)
  x
end

alias bar foo

class Test
  def baz(x)
    x
  end

  alias qux baz
end

foo(1)
bar("str")
Test.new.baz(1)
Test.new.qux("str")

__END__
# Classes
class Object
  private
  def foo: (Integer | String x) -> (Integer | String)
  alias bar foo
end

class Test
  def baz: (Integer | String x) -> (Integer | String)
  alias qux baz
end