My code:

{% assign selected_products = [section.settings.featured_product1, section.settings.featured_product2, section.settings.featured_product3] %}

<ul class="featured-products">
  {% for product in selected_products %}
    {% render 'product', product: product %}
  {% endfor %}
</ul>

{% schema %}
{
  "name": "Featured products",
  "settings": [
    {
      "type": "product",
      "id": "featured_product1",
      "label": "Featured product 1"
    },
    {
      "type": "product",
      "id": "featured_product2",
      "label": "Featured product 2"
    },
    {
      "type": "product",
      "id": "featured_product3",
      "label": "Featured product 3"
    }
  ]
}
{% endschema %}

vscode highlights the use of selected_products in a loop as a warning with the text: Unknown object 'selected_products' used.theme-checkUndefinedObject.

Error:

• 16:49:23 ERROR » update sections/featured_products.liquid:
Liquid syntax error (line 1): Expected close_square but found comma in “{{[section.settings.featured_product1, section.settings.featured_product2, section.settings.featured_product3] }}”