Report described this as a panic triggered by the sanity_check option, but that's because it was running under the fuzzer, which escalates any impossible() to panic(), rather than because nethack panicked. I couldn't find anything wrong--which doesn't mean that there isn't something wrong--with place_worm_tail_randomly() and random_dir(). They use xchar for map coordinates which should be fine as long as no negative values are generated and I couldn't discover any such. The suggested fix of changing xchar to int might indicate a compiler bug (although the odds of that are low). The bogus coordinate of -15000 in the report suggests that typedef short int schar; (which changes xchar too) is being used in the configuration but I don't recall having any problems attributable to that. This switches from xchar to int as a side-effect of replacing the offending code entirely. The new code might produce an 'ny' of -1 before goodpos() rejects it, so xchar would be inappropriate now. The old code is commented out via #if 0 _after_ changing it from xchar to int. This also adds an extra sanity_check for worm tails, unrelated to the current bug. I'm not aware of any instance where it fails. EXTRA_SANITY_CHECKS needs to be defined for it to do anything.
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