> I am likewise unsure how a language could get enums wrong, even after reading your post.

By allowing any value of the underlying type, even if they were not values of the enum. The language most famous for this is obviously C:

    enum foo { A, B, C };

    int main() {
        enum foo x = A;
        printf("%d\n", x);
        enum foo y = 42;
        printf("%d\n", y);
    }

This will print `0` and `42` (https://godbolt.org/z/Yq8qq5bzW), because C considers enums to be integral types and will thus implicitly convert to or from them. And as you can see from the link, there is no warning under `-Wall`. Clang will warn with `-Wassign-enum` (which is included in `-Weverything`), I don't think there's any way to make gcc complain about this.

Now you might argue that this is C so obviously it fucks this up, however that C does this leads to further issues:

- For compatibility with C, C# enums have the same property. I don't know about you, but that surprised me a great deal since Java has type-safe enums (even if they're not sum types).

- Even more so, C++ added `enum class` in C++11, and obviously has inherited C's enums, but enum class still is not type safe, afaik the differences are that `enum class` is scoped (so in the snippet above you'd need `foo::A`) and it's not implicitly convertible with the underlying type. But it's still explicitly convertible (via a cast or list initialisation), meaning you can't assume a caller will remain within the enum.