Once upon a time I had to figure out an issue where I forgot a `return` statement at the end of a non-`void` function, so the C++ compiler happily omitted both RETs for some reason and let the program go straight into illegal instructions. That was fun to debug (not fun, I practically had to single-step through the entire program) because every time this happened the debugger was incredibly confused about what the fuck was going on and nothing made any sense.

Real

Another issue that happened in the same project is that for some reason whenever I compiled it with a regular C++ compiler, field writes were disappearing into the abyss. I think I actually never figured that out because it was literally the same object at the same memory address, there were no other threads and yet when a subroutine returned after writing the field, the write disappeared?? The strangest thing is that Emscripten's C++-to-WASM cross-compiler worked perfectly fine with the exact same routines. I wonder if the compiler I used simply had fuckass issues, it was an oldish (by modern standards) Apple clang from like probably macOS 10.14 or so.

That or you hit some UB elsewhere that caused the compiler to assume things that you didn't follow.

Trying UBSAN and ASAN might have been worth it. I don't think comparing debug and release builds would have helped: could be buggy optimisations or UB in your code regardless of what the outcome of that test was.

In this case I was reading memory with a debugger and could see that it literally was the same object at the same memory address, but what I did not know was whether or not different parts of the code had different views of that field for some reason, or any number of other things that could have been causing the issue. Either way, it was super weird and frustrating.

It makes no sense to me that C and C++ didn't require a diagnostic for this, rather you hit UB.

I know that at least modern GCC and Clang will warn and/or error for this (not sure which as I use -Werror), but still, this is pointless UB to have.

And no, in this case I don't buy that a C90 compiler would have been unable to check this.

Having been involved in the WG14 discussions on this topic:

The issue is that there is a contingent of users who complains about cases where the return dynamically can't be hit but that isn't obvious statically. Consider something like this:

  int do_something(enum meow koala) {
    switch (koala) {
    case enum_val_1: return 5;
    case enum_val_2: return 3;
    /* etc., covering all the enum values */
    }
  }

Should this be required to diagnose? That's the sticking point.

There may be differences between C and C++ here (and I worked with C++ more recently than C), but: if that enum doesn't specify an underlying type it would be UB to have a value that isn't in the "member list" of the enum. In that case it should be possible to determine if all cases are covered.

If an underlying type is specified, then it should error since it is legal to have those values (unless the whole range of the underlying type is covered by the cases.

Again, that is what would be sensible from a C++ perspective, I don't know if C differs here.

EDIT: Also, and now I'm talking with my Rust user hat on: it is better to not have pointless UB. Yes some is needed to practically allow for optimisation. But C and C++ had a lot of UB that doesn't really help with making your code faster, such as this.