less.js/test/less/extend-nest.less

.sidebar {
  width: 300px;
  background: red;

  .box {
    background: #FFF;
    border: 1px solid #000;
    margin: 10px 0;
  }
}

.sidebar2 {
  &:extend(.sidebar all);
  background: blue;
}

.type1 {
    .sidebar3 {
      &:extend(.sidebar all);
      background: green;
    }
}

.type2 {
  &.sidebar4 {
    &:extend(.sidebar all);
    background: red;
  }
}