OP here. This is not actually compliant with the C standard, just a gcc extension.

It is documented at [1] and according to the documentation, the code seems perfectly legal. However, calling it twice add5(2); add5(2);

leads to an "Illegal instruction. Core dumped"

Even stranger, calling it twice but with a different argument leads to a a segfault instead of illegal instruction: add5(5); add5(5);

[1] https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html