Get a list of terms from a shared taxonomy that are used on a particular post type only

cpt_term_list.php

<?php ?>
<label for="filter-country">Country</label>
<select id="filter-country" class="form-control" name="country">
	<option selected="" value="">All countries</option>
	<?php
	if ( false === ( $cptcountries = get_transient( 'cpt_countries' ) ) ) {
		global $wpdb;
		$cpt_countries = $wpdb->get_col( "select distinct term_taxonomy_id from $wpdb->term_relationships where object_id in ( select ID from $wpdb->posts where post_type='cpt' )" );
		$cptcountries = get_terms( array( 'taxonomy' => 'country', 'include' => $cpt_countries ) );
		if ( ! empty( $cptcountries ) && ! is_wp_error( $cptcountries ) ) {
			set_transient( 'cpt_countries', $cptcountries, 4 * HOUR_IN_SECONDS );
		}
	}
	if ( ! empty( $cptcountries ) && ! is_wp_error( $cptcountries ) ) {
		foreach ( $cptcountries as $country ) {
			echo '<option ' . selected( $cptcountry, $country->slug ) . ' value="' . esc_attr( $country->slug ) . '">' . esc_html( $country->name ) . '</option>';
		}
	}
	?>
</select>