In most cases, this error is displayed when the compiler figured out a package where the given definition can be found. That package will be listed on the next line. Unfortunately, Gnat uses cryptic 8 letter names for its internal packages.
If you are attempting to use non-visible operators then you probably want to add a "use ..." clause.
If you are just using that term once or twice, consider qualifying it instead (for instance, ada.interfaces.left_shift).