Best method to use helper in any template

Question

It seems to be impossible to use a helper in the template Magento_Catalog\templates\product\list\items.phtml.

I have tried it via argument injection:

app\design\frontend\company\base\Magento_Catalog\layout\override\base\catalog_product_view.xml

<!-- Related Products Slider -->
    <block class="Magento\Catalog\Block\Product\ProductList\Related"
           name="catalog.product.related"
           template="Magento_Catalog::product/list/items.phtml"
    >
        <arguments>
            <argument name="type" xsi:type="string">related</argument>
            <argument name="company_helper" xsi:type="object">Company\Base\Helper\Product</argument>
        </arguments>
        <block class="Magento\Catalog\Block\Product\ProductList\Item\Container"
               name="related.product.addto" as="addto"
        >
            <block class="Magento\Catalog\Block\Product\ProductList\Item\AddTo\Compare"
                   name="related.product.addto.compare" as="compare"
                   template="Magento_Catalog::product/list/addto/compare.phtml"
            />
        </block>
    </block>

<!-- Upsell Products Slider -->
    <block class="Magento\Catalog\Block\Product\ProductList\Upsell"
           name="product.info.upsell"
           template="Magento_Catalog::product/list/items.phtml"
    >
        <arguments>
            <argument name="type" xsi:type="string">upsell</argument>
            <argument name="company_helper" xsi:type="object">Company\Base\Helper\Product</argument>
        </arguments>
        <block class="Magento\Catalog\Block\Product\ProductList\Item\Container"
               name="upsell.product.addto" as="addto"
        >
            <block class="Magento\Catalog\Block\Product\ProductList\Item\AddTo\Compare"
                   name="upsell.product.addto.compare" as="compare"
                   template="Magento_Catalog::product/list/addto/compare.phtml"
            />
        </block>
    </block>

and by extending the block classes.

I even extended the block class \Magento\Catalog\Block\Product\AbstractProduct

<?php

namespace Company\Base\Block\Product;

class AbstractProduct extends \Magento\Catalog\Block\Product\AbstractProduct
{
    protected $productHelper;

    /**
     * @param Context $context
     * @param array $data
     */
    public function __construct(
        \Magento\Catalog\Block\Product\Context $context
        , array $data = []
        , \Company\Base\Helper\Product $productHelper
    ) {
        $this->productHelper = $productHelper;
        parent::__construct($context, $data);
    }

    public function getProductHelper()
    {
        return $this->productHelper;
    }

}

But if I try to use the helper in the template:

$productHelper = $block->getProductHelper();  // also tried $this instead of $block
echo $productHelper->test();  // should output "success"

Then I get an error saying that $productHelper is null

Answer 1

try with

$this->helper(\Company\Base\Helper\Product::class);

inside the template

and why it didn't work for you when you overridden the AbstractProduct class...

You extended the class, but your new class is never instantiated. The blocks used in by the template still use the original AbstractProduct class.
overriding works only for classes that get instantiated. The AbstractProduct class is only extended by other classes, but is never instantiated.