[GaryJones]

The PreGetPosts sniff only recognised an is_main_query() check when it was in an if. The same check in an elseif was ignored, which gave two false positives.

An early return in an elseif was not seen as an early return, so the code after it was flagged:

add_action( 'pre_get_posts', function( $query ) {
	if ( is_admin() ) {
		return;
	} elseif ( ! $query->is_main_query() ) {
		return;
	}
	$query->set( 'cat', '-5' ); // Was flagged.
} );

A set() inside an elseif that checks the main query was flagged as well. This is a common pattern when home and search are adjusted in separate branches:

add_action( 'pre_get_posts', function( $query ) {
	if ( $query->is_main_query() && $query->is_home() ) {
		$query->set( 'cat', '-5' );
	} elseif ( $query->is_main_query() && $query->is_search() ) {
		$query->set( 'cat', '-6' ); // Was flagged.
	}
} );

The three checks that look for the surrounding if now accept an elseif too. An elseif that does not check the main query is still flagged, and there is a test for that. else if, written as two words, already worked because PHP_CodeSniffer tokenises it as an else followed by an if. It now has a test so it stays that way.

One related case is left as it is. A set() in the else of if ( ! $query->is_main_query() ) is still flagged, because accepting it would mean the sniff has to understand the negation.

This came up while reviewing #904.