I'm specifically not talking about something like Typescript which is widely known to be unsound, but a type system that was believed to be "safe" before someone figured it is Turing complete.
This reminds me of that: http://litherum.blogspot.com/2019/03/addition-font.html
You can craft font files that perform arbitrary calculations… if you give the font shaping engine a (practically) unlimited stack, where it usually limits recursion to 6 levels. If you do so, you can make a font that does math (or whatever you want) as part of computing ligatures.
But in practice, you hardly ever hit those kinds of limits unless you're trying to do something silly on purpose. Sure you cannot port Doom to font substitution lookup tables, despite them being Turing complete. But that doesn't make your font shaping engine useless, or even broken.