Restricts which items can be added to cart based on whether a specific item is already in the cart or being added to the cart.

woocommerce-cart-restrictions.php

<?php
/**
 * WooCommerce code snippet that restricts which items can be added to cart:
 *
 * 1) The $trial_product should not be added to cart if existing items are in the cart.
 * 2) Other products should not be added to cart if $trial_product is already in cart.
 */
function example_cart_filtering( $passed_validation, $product_id ) {

	// ID of the trial product
	$trial_product_id = 9344;

	// If cart is empty, no need for additional validation.
	if ( WC()->cart->is_empty() ) {
		return $passed_validation;
	}

	// If trial product is being added to cart, existing products should be removed.
	if ( $trial_product_id === $product_id ) {
		$notice = sprintf(
			// Translators: Placeholder is for cart url.
			__( 'Trial Product can not be purchased with other products. Please empty your <a href="%s">existing cart</a>.' ),
			esc_url( wc_get_cart_url() )
		);
		wc_add_notice( $notice, 'notice' );
		return false;
	}

	// If trial product is in cart, additional products should not be added.
	$products = WC()->cart->get_cart_contents();
	foreach ( $products as $product => $values ) {
		$product_id = $values['data']->get_id();
		if ( $trial_product_id === $product_id ) {
			$notice = sprintf(
				// Translators: Placeholder is for cart url.
				__( 'The Trial Product can not be ordered with other items.<br> Please remove the Trial Product from <a href="%s">your cart</a> if you wish to make other purchases.' ),
				esc_url( wc_get_cart_url() )
			);
			wc_add_notice( $notice, 'notice' );
			return false;
		}
	}

	// Trial Product is not in cart or being added.
	return $passed_validation;

}
add_filter( 'woocommerce_add_to_cart_validation', 'example_cart_filtering', 100, 2 );