wp-includes/query.php:902Determines whether the query is the main query.
boolfunction is_main_query() { global $wp_query; if ( ! isset( $wp_query ) ) { _doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '6.1.0' ); return false; } if ( 'pre_get_posts' === current_filter() ) { _doing_it_wrong( __FUNCTION__, sprintf( /* translators: 1: pre_get_posts, 2: WP_Query->is_main_query(), 3: is_main_query(), 4: Documentation URL. */ __( 'In %1$s, use the %2$s method, not the %3$s function. See %4$s.' ), '<code>pre_get_posts</code>', '<code>WP_Query->is_main_query()</code>', '<code>is_main_query()</code>', __( 'https://developer.wordpress.org/reference/functions/is_main_query/' ) ), '3.7.0' ); } return $wp_query->is_main_query();}Introduced in 3.3.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/query.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.