Determining main query from `pre_get_terms`

[ssnepenthe]

Currently it is possible to hook in at pre_get_terms to modify the query args before query is actually run, but there is no good way to guarantee that the query instance is part of the terms archive.

You can check is_terms_archive(), but it is always possible that there are other term queries being run on the same page.

Core has is_main_query( $query ) and $query->is_main_query() for WP_Query...

Maybe there could be some helpers like is_term_query(), is_main_term_query() and is_term_count_query()?

[ssnepenthe]

Modifying number arg complicates things...

offset is hardcoded before pre_get_terms fires which means if the user wants to modify number they have to also modify offset.

Maybe it would be possible to hook in late on pre_get_terms or on get_terms_args to automatically handle offset after everything else is finalized?

[ssnepenthe]

Further complicated by the fact that main query and count query are instantiated early (on parse_query) after determining that the current request looks like it is for a terms archive page but before determining that the current request actually is a terms archive page.

This means you can't check Terms_Archive\is_terms_archive() from a pre_get_terms callback...