hybrid_locate_template() as discussed in https://github.com/justintadlock/hybrid-core/issues/111

hybrid-locate-template.php

<?php
/*
Plugin Name: Hybrid Core hybrid_locate_template()
Plugin URI: https://github.com/justintadlock/hybrid-core/issues/111
Description: Free yourself from TEMPLATEPATH and STYLESHEETPATH constants
Author: Leho Kraav
Author URI: http://conversionready.com
Version: 2015.06.10
License: MIT
*/

if ( defined( "DOING_AJAX" ) && DOING_AJAX ) {
    return;
}

if ( is_admin() ) {
    return;
}

# TODO what's the hybrid standard priority for "always last"?
# TODO has to be module-level opt-in for now, overrides existing filters too aggressively
#      add_filter( "hybrid_content_template", "hybrid_content_template", 15, 2 );

/**
 * Because plugins need to host their own templates, but locate_template()
 * forces theme-only hierarchy.
 *
 * wp_templating_constants() runs very early, after "setup_theme" action.
 * Plugins may not able to determine request responsibility until much later.
 *
 * By that time templating constants are set and you're screwed.
 *
 * http://chat.stackexchange.com/transcript/message/22088394#22088394
 * https://developer.wordpress.org/reference/functions/locate_template/
 * https://github.com/justintadlock/hybrid-core/issues/111
 * https://core.trac.wordpress.org/ticket/13239
 * https://core.trac.wordpress.org/ticket/14310
 * https://core.trac.wordpress.org/ticket/22355
 * https://github.com/GaryJones/Gamajo-Template-Loader/blob/develop/class-gamajo-template-loader.php#L141
 */
function hybrid_locate_template( $template_names, $load = false, $require_once = true ) {

    $located = '';
    $stylesheetpath = get_stylesheet_directory();
    $templatepath = get_template_directory();

    foreach ( (array) $template_names as $template_name ) {

        if ( ! $template_name ) {
            continue;
        }

        # TODO such non-DRY core code, optimize
        if ( file_exists( $stylesheetpath . '/' . $template_name ) ) {

            $located = $stylesheetpath . '/' . $template_name;

            break;

        } elseif ( file_exists( $templatepath . '/' . $template_name ) ) {

            $located = $templatepath . '/' . $template_name;

            break;

        }

    }

    if ( $load && "" !== $located ) {
        load_template( $located, $require_once );
    }

    return $located;

}

/**
 * Overrides locate_template() result with hybrid_locate_template()
 */
function hybrid_content_template( $template_name, $templates ) {

    $template_name = hybrid_locate_template( $templates, false, false );

    return $template_name;

}