What is the proper way to interpolate boolean attributes like checked in an input in the new HEEx engine?

For example, in LEEx if I wanted to include it based on some condition, I would do this:

<input type="checkbox" name="web_pages[]" value="<%= w.id %>" <%= if w.id in @selected, do: "checked" %> />

Which would generate HTML like this:

<input type="checkbox" name="web_pages[]" value="1" />

<input type="checkbox" name="web_pages[]" value="2" checked />

Now in HEEx I do this

<input type="checkbox" name="web_pages[]" value={w.id} checked={if w.id in @selected, do: "true", else: nil} />

Which also works, if a nil value is passed then the checked attribute won’t be passed at all, but if I pass true then it will generate this:

<input type="checkbox" name="web_pages[]" value="2" checked="true" />

Which is correct, but longer and a bit uglier. I would have expected that this would work:

<input type="checkbox" name="web_pages[]" value={w.id} {if w.id in @selected, do: "checked"} />

But it doesn’t, it fails with protocol Enumerable not implemented for nil of type Atom. It looks like interpolation must always be bound to an attribute name in the HTML tag, right?