$builtin: get-function(rgb);
$same-builtin: get-function(rgb);
$different-builtin: get-function(lighten);
@function lighten() {
  @return wut;
}
$redefined-builtin: get-function(lighten);
.should-be-true {
  identical: $builtin == $builtin;
  same: $builtin == $same-builtin;
}

.should-be-false {
  different: $different-builtin == $builtin;
  redefined: $different-builtin == $redefined-builtin;
}