get_theme_mod() declaration of return type

[mensmaximus]

As of WP 4.9.8 get_theme_mod() declares @return string. If you query for navigation menu locations using get_theme_mod('nav_menu_locations') you get back an array. Modern IDEs (like PhpStorm) complain if you start to modify this array because they expect a string. The return value should be changed to mixed.

[pento]

In 44613:

Docs: Correct the @return type of get_theme_mod().

This now matches the type of the $value parameter of set_theme_mod().

Props mensmaximus.
Fixes #45319.