`WP_Query::parse_tax_query()` has always been documented as `protected`. Plugins should never have used it. However, it did not have a `protected` access modifier, so its visibility was `public` by default. If the access modifier had been present, accessing the method in a plugin would have produced a fatal error. The access modifier was added in [28523], along with magic methods to allow it to be accessed for BC. This one method is problematic because it expects to be passed a reference. The `WP_Query::__call()` logic does not go out of its way to fix this signature discrepancy, and so a warning is thrown: `Parameter 1 to WP_Query::parse_tax_query() expected to be a reference, value given`.

Remove the `protected` access modifier from `WP_Query::parse_tax_query()`.

Fixes #28739.

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git-svn-id: http://core.svn.wordpress.org/trunk@28776 1a063a9b-81f0-0310-95a4-ce76da25c4cd