I'm confused too if I read it as not not, but if you read it as [tobool](https://github.com/D-Programming-Language/dmd/blob/3ad9c76c0d553b20f417f166ee9e35cc1d7994ee/src/lexer.h#L81) it's fairly clear.
IMHO it's easier to understand than `return rt_term() != 0`.