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How can I get the same "not all control paths return a value" behaviour across Clang and MSVC?

I've recently discovered that it's not actually an error to have control reach the end of a non-void function without returning anything, it's merely undefined behaviour. I want to promote the relevant warning(s) to error.

Our product that's built on multiple platforms, using Clang on one and Microsoft Visual Studio on the other. So far, I've tried using `-Werror=return-type` for Clang and `/we4183 /we4715` for MSVC. However, there's a problem.

The following code produces no warnings at all in Clang with `-Wall -W`, but for MSVC triggers 4715, "not all control paths return a value":

```
enum class Letter
{
    A,
    B,
    C,
    END
};

int toNum(Letter letter)
{
    switch (letter)
    {
    case Letter::A:
        return 0;
    case Letter::B:
        return 1;
    case Letter::C:
        return 2;
    case Letter::END:
        return -1;
    }
}
```

It seems Clang determines that all the enum cases are handled and that control will never reach the end of the function, while MSVC does not.

Is there any way around this? It would be really, really handy to have an error triggered when someone does forget a return value, but we also can't have builds fail when our mostly Clang developers put in code that MSVC will object to.

The next-best solution would be to promote the warnings to error for Clang only, but ideally I'd like to catch any problems like this when working on Windows too...