The problem is that select() is a wrapper around WaitForMultipleObjectsEx() or whatever it is that select() uses, and those functions (the ones that can call APCs because entering them enters "alertable state") are extern "C" functions, which means they cannot throw exceptions, but here we have an APC throwing an exception, which is not allowed!

Now, MSFT does NOT document that APCs can't throw, but since the functions that enter alertable states (hence which can call APCs) are extern "C" functions, it follows that APCs cannot throw.

So part of the problem here is that the PAPCFUNC function type is not noexcept, therefore the compiler can't stop you from using a function that is not noexcept as an APC, thus APCs can -but must not!- throw exceptions.