Set width of div to auto inside another div

Question

I have a div. I want to have another div2 with width to be auto(self-adjustable). But width: auto doesn’t work. div2 needs to be aligned to the left.

.parent {
  background: lightblue;
  display: block;
  padding: 10px;
}
.child {
  border: 2px solid green;
  display: flex;
  padding: 10px;
  ///////////
  width: auto;
}

<div class='parent'>
  <div class='child'>
    <span>
      Some text
    </span>
  </div>
</div>

Just like this, but without width: fit-content.

.parent {
  background: lightblue;
  padding: 10px;
  display: block;
}
.child {
  border: 2px solid green;
  border-radius: 4px;
  padding: 10px;
  width: fit-content;
}

<div class='parent'>
  <div class='child'>
    <span>
      Some text
    </span>
  </div>
</div>

P.S. Cannot use display: inline-block for child div. Cannot touch/edit parent div, all css need to remain the same. Cannot use width: max-content, width: min-content and width: fit-content