When MySQL 5.7 introduced fulltext parser plugins to InnoDB, it hard-coded the plugin name "ngram" to mean something special, causing the field dict_index_t::is_ngram to be set.
Because -fsanitize=undefined was issuing warnings for the assignment in row_merge_create_index() that the value for dict_index_t::is_ngram out of range for Boolean, I would prefer to remove the field altogether.
As part of this, I made an honest attempt to import plugin/fulltext from MySQL 5.7 to MariaDB 10.2. This would result in some errors due to API mismatch:
The second one is for the field MYSQL_FTPARSER_BOOLEAN_INFO::position that was added as part of WL#6943, changing the fulltext parser plugin API and ABI in MySQL 5.7.
Also the other InnoDB fulltext parser plugin plugin/fulltext/mecab_parser/plugin_mecab.cc is filling in MYSQL_FTPARSER_BOOLEAN_INFO::position in mecab_parse().
Based on this, the InnoDB fulltext parser plugins are not directly usable with MariaDB 10.2 and we can safely remove the dict_index_t::is_ngram field and possibly revert some other WL#6943 changes.