The GCC documentation contains a good example:
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
"""
Another use for __builtin_unreachable is following a call a function that never returns but that is not declared __attribute__((noreturn)), as in this example:
void function_that_never_returns (void);
int g (int c)
{
if (c)
{
return 1;
}
else
{
function_that_never_returns ();
__builtin_unreachable ();
}
}
"""
This example is more explicit than a function which gets an "unexpected" value: such case should use Py_FatalError() if I understand correctly.
By the way, Py_FatalError() should be avoided: it's always better to report the failure to the caller :-)