A decent IDE should have a check for unused variables (or similar) that will be able to work it out. PhpStorm definitely has one that shows it, as it was showing some of the const names in light grey (meaning they're unused).
So should just be a case of finding one "manually" in the repo, and then look which inspection is firing. I can have another look when the stack of patches are merged :)
I note I do have https://plugins.jetbrains.com/plugin/7622-php-inspections-ea-extended- also installed, but I don't think the inspection is necessarily (ie it's a core one)