clang-diagnostics: -Wswitch-default and -Wcovered-switch-default contradictory warnings

[FalcoGer]

When having both -Wswitch-default and -Wcovered-switch-default this generates contradictory warnings. When you don't have a default case for a switch statement that handles every enum value you get a warning for not having it; and if you do you get a warning for having it. However you require both those flags if you wish to get related warnings elsewhere when dealing with switch statements not related to enums or enums but not all the values.

Example

#include <cstdint>

enum class ETest : std::uint8_t
{
    TEST1,
    TEST2,
};

auto main() -> int
{
    ETest test = ETest::TEST2;
    [[maybe_unused]]
    int   a {};

    switch (test)
    {
        case ETest::TEST1: a = 1; break;
        case ETest::TEST2: a = 2; break;
        // default:           a = 99; break;
    }

    return 0;
}

What I would expect is that -Wswitch-default is suppressed when dealing with enums and every case is covered, but active otherwise.

The way it is now, you will miss out on at least one of those three possible cases that require a warning:

• A switch statement that is not handling an enum doesn't have a default case (which is bad)
• A switch statement that is dealing with an enum is fully defined but has a default case (which is bad)
• A switch statement that is dealing with an enum is not fully defined and doesn't have a default case (which is bad)

This was not the case in earlier versions of clang, which handled this correctly.

My version is

Ubuntu clang version 19.0.0 (++20240222031214+307409a8872f-1~exp1~20240222151237.1514)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[shafik]

Looks like this diagnostic was added to be compatible w/ gcc: #73077

and it looks like we are matching their behavior here: https://godbolt.org/z/oe1z1a8fh

So this looks like behaves as expected.

CC @dongjianqiang2 @hstk30-hw @aaronpuchert @AaronBallman for additional feedback

[aaronpuchert]

Correct, the intention was to have the warning behave like GCC's implementation.

What I would expect is that -Wswitch-default is suppressed when dealing with enums and every case is covered, but active otherwise.

That is the behavior of clang-tidy's bugprone-switch-missing-default-case. I guess a discussion can be had about having this in the core compiler, but we'd need some good arguments.

This was not the case in earlier versions of clang, which handled this correctly.

Earlier versions of Clang didn't implement -Wswitch-default at all. You can see that #73077 implemented the warning which was previously only a no-op. If you want to restore the behavior of earlier versions, simply disable the warning.

[aaronpuchert]

Duplicates #81354.