Move `get_meta_type()` into the `WP_Meta_Query` class as `get_cast_for_type()`. `WP_Query` can then access it like: `$this->meta_query->get_cast_for_type()`.

See #21621, [25255].



git-svn-id: https://develop.svn.wordpress.org/trunk@25269 602fd350-edb4-49c9-b593-d223f7449a82
Simplify the check for false-y `child_of` arg.

Props SergeyBiryukov.
See #25230, [25245].



git-svn-id: https://develop.svn.wordpress.org/trunk@25270 602fd350-edb4-49c9-b593-d223f7449a82