Recently I was preparing some problems in Polygon. One of problems required custom checkers. I've implemented and set up my checker and it turned out that it gives verdict FL for all checker tests. I have tried alternating checker implementations, replacing it completely with standard wcmp.cpp code and simplifying logic into single line

quitf(_ok, "ok");

but none of that helped. Only thing that I managed to achieve is to get exit code 13131313 for roughly half of checker tests.

After messing around for a few hours I finally noticed red "Show warnings" label. (I'm not sure if this label was there when I first encountered this problem roughly a week ago.) Turns out there is that critical warning:

Turns out that I was unlucky choosing problem name set-diff-patch-notes and moving it into checker name. After renaming source file it worked just fine.

But how does this work? Are there any other bad substrings? Why there is a weird limitations for checker file names?

A friend of mine suggested that it has something to do with version control system used in Polygon. But if that's the case why any other source files (solutions, validators) have no problems with patch substring?