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apparently it indicates divergence, i.e. the function does not return at all. Very interesting, I've never seen the notation before.

Swift encodes this with a noreturn attribute, and I think it's a "standard not standard" C attribute as well.

(in case you were wondering who else does this)

_Noreturn is a standard keyword in C11. C++11 has a standard attribute called [[noreturn]].