The loop can throw warning if `WP_Query::have_posts()` not called.

[peterwilsoncc]

Starting or overrunning the loop can throw a warning if WP_Query::have_posts() is not called correctly via the developer. The issue can be seen in these tests:

<?php
function test_no_posts() {
        $query = new WP_Query( array( 'post_type' => 'unregistered_cpt' ) );

        $query->the_post();
        $this->assertEmpty( $query->posts );
}

function test_too_many_calls() {
        $post_id = $this->factory()->post->create();
        $query   = new WP_Query( array( 'p' => $post_id ) );

        // Call the post twice.
        $query->the_post();
        $query->the_post();
        $this->assertNotEmpty( $query->posts );
}

In each case the tests will throw an error before reaching the assertion, as they are attempting to access an array key that does not exist.

A part of me is tempted to close this as wontfix, as it's very clearly documented that one should use while ( have_posts() ) for the primary loop or while $query->have_posts() for a secondary loop.