SCSS ampersand operator

I'm migrating from LESS to SCSS.

In LESS I can do this:

.btn {
  a&.disabled,
  fieldset[disabled] a& {
    pointer-events: none;
  }
}

CSS result:

a.btn.disabled,
fieldset[disabled] a.btn {
  pointer-events: none;
}

What is the SCSS equivalent of that?