[clang][request] Warn when bitfield is too small to hold all enum values

[chrchr-github]

struct S {
    enum E : unsigned char { E0, E1, E2, E3 };
    E e : 1; // warn here
};

S::E f(S::E e) {
    S s{};
    s.e = e;
    //s.e = S::E3; // [clang-diagnostic-bitfield-constant-conversion]
    return s.e;
}

int main() {
    return f(S::E3);
}

Not sure if this is appropriate for clang-tidy or diagnostics.
GCC shows a warning.
https://godbolt.org/z/WTnPYTrz7

[AaronBallman]

Clang does diagnose this under -Wbitfield-enum-conversion: https://godbolt.org/z/8TY3bqrWE but it's diagnosing the conversion rather than the declaration. I'm not certain there's benefit to diagnosing both the declaration of the bit-field and the conversion; it's the conversion that's truly problematic.

[chrchr-github]

Clang does diagnose this under -Wbitfield-enum-conversion: https://godbolt.org/z/8TY3bqrWE but it's diagnosing the conversion rather than the declaration. I'm not certain there's benefit to diagnosing both the declaration of the bit-field and the conversion; it's the conversion that's truly problematic.

I'd be happy with that, but why must the warning be enabled explicitly (or by -Weverything) in this case?

Here -Wall is sufficient: https://godbolt.org/z/esMqxWGh3

[AaronBallman]

I'm not certain why it needs to be explicitly enabled; this feels like it should be safe enough for -Wall (or -Wextra). We'd have to dig into the history to see why this is off by default (it's possible that's just an oversight).