@@ -0,0 +1,118 @@
+<?php
+
+/**
+ * Tests for walk_category_tree().
+ *
+ * @group taxonomy
+ * @group category
+ * @group walker
+ *
+ * @covers ::walk_category_tree
+ */
+class Tests_Category_WalkCategoryTree extends WP_UnitTestCase {
+
+	/**
+	 * Parent category object.
+	 *
+	 * @var WP_Term
+	 */
+	private static $parent;
+
+	/**
+	 * Child category object.
+	 *
+	 * @var WP_Term
+	 */
+	private static $child;
+
+	/**
+	 * Creates shared category fixtures.
+	 *
+	 * @param WP_UnitTest_Factory $factory Test factory.
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$parent = $factory->category->create_and_get(
+			array(
+				'name' => 'Parent Category',
+				'slug' => 'parent-category',
+			)
+		);
+
+		self::$child = $factory->category->create_and_get(
+			array(
+				'name'   => 'Child Category',
+				'slug'   => 'child-category',
+				'parent' => self::$parent->term_id,
+			)
+		);
+	}
+
+	/**
+	 * Tests that walk_category_tree() uses Walker_Category by default when no custom walker is provided.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_uses_default_walker_category() {
+		$elements = array( self::$parent, self::$child );
+		$args     = array(
+			'style'              => 'list',
+			'use_desc_for_title' => 0,
+		);
+
+		$default_walker = new Walker_Category();
+		$expected       = $default_walker->walk( $elements, 0, $args );
+
+		$this->assertStringContainsString( 'cat-item-' . self::$child->term_id, $expected );
+		$this->assertSame( $expected, walk_category_tree( $elements, 0, $args ) );
+	}
+
+	/**
+	 * Tests that walk_category_tree() falls back to Walker_Category when the 'walker' argument
+	 * is not a Walker instance.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_falls_back_to_default_walker_when_walker_arg_is_invalid() {
+		$elements = array( self::$parent, self::$child );
+		$args     = array(
+			'style'              => 'list',
+			'use_desc_for_title' => 0,
+			'walker'             => new stdClass(),
+		);
+
+		$default_walker = new Walker_Category();
+		$expected       = $default_walker->walk( $elements, 0, $args );
+
+		$this->assertStringContainsString( 'cat-item-' . self::$child->term_id, $expected );
+		$this->assertSame( $expected, walk_category_tree( $elements, 0, $args ) );
+	}
+
+	/**
+	 * Tests that walk_category_tree() uses a custom Walker instance when passed in $args['walker'].
+	 *
+	 * @ticket 65819
+	 */
+	public function test_uses_custom_walker_instance_when_provided() {
+		$custom_walker = new class() extends Walker {
+			public $tree_type = 'category';
+			public $db_fields = array(
+				'parent' => 'parent',
+				'id'     => 'term_id',
+			);
+
+			public function start_el( &$output, $data_object, $depth = 0, $args = array(), $current_object_id = 0 ) {
+				$output .= 'custom:' . $data_object->slug . '|';
+			}
+		};
+
+		$elements = array( self::$parent, self::$child );
+		$args     = array(
+			'walker' => $custom_walker,
+		);
+
+		$this->assertSame(
+			'custom:parent-category|custom:child-category|',
+			walk_category_tree( $elements, 0, $args )
+		);
+	}
+}