User-defined types error (API file)

[API file] - locationData.api
STRUCT locationInfo
{
int32 latitude;
int32 longitude;
int32 altitude;
int8 fixStatus;
};

[Build Error]
locationData.api:6:0: error: no viable alternative at input u’STRUCT’

[Question]
How can I use “STRUCT” type in an API file?

This seems to be supported after legato 18.09.