Clang-cl should map `/Wall` to `-Wall`, not `-Weverything`

[fredemmott]

This is not the same issue as #102583 - that issue is saying that -Wall works the same way as /-Wall, which is likely intentional

This issue is that the documented behavior of clang-cl /Wall is extremely surprising - both -Wall and /Wall should be mapped to clang's -Wall, not -Weverything

It was surprising as:

• I had a clean build with MSVC cl.exe /Wall /WX
• I had a clean build with clang++.exe -Wall -Werror
• I had a failing build with clang-cl.exe /Wall /WX

The set of 'everything' is particularly problematic as even if you're passing /std:c++20, you still get -Wc++98-compat - of course my code that targets C++20 is not compatible with C++98.

While it could be argued that /W4 is closer to clang's -Wall, the above makes the current mapping basically unusable for modern code. A potential middle ground would be to map /Wall to -Wall -Wextra ?

[efriedma-quic]

This was done in f9b08a3 . It might be worth reconsidering. CC @rnk .

[davidstone]

I think clang-cl's behavior is correct here. /Wall is documented as turning on all available warnings, which is the same conceptual behavior as -Weverything. Even for the warnings that match in gcc and clang, each of them warns one certain code that the other does not.

[fredemmott]

The set of 'everything' is particularly problematic as even if you're passing /std:c++20, you still get -Wc++98-compat - of course my code that targets C++20 is not compatible with C++98.

I'm not expecting identical behavior, but this is the part I strongly disagree with. Perhaps explicitly excluding the '-compat' warnings would fit both?

I don't think it's reasonable for /std:c++20 /Wall /WX to raise errors saying that I'm not writing C++98.

[rnk]

My email shows that @zmodem thanked me for landing this change in 2017, so I assume he has some opinion on this.

I propose we alias clang-cl /Wall to -Wall -Wextra. IIRC Wextra has all the extra warnings that a reasonable, normal person might wish to enable. I believe it excludes all of these compat warnings.

[CaseyCarter]

/Wall literally means "turn on all the warnings". If you don't want every single warning - including those that are useless or contradict each other - you should use an argument other than /Wall. Changing the meaning to disagree with what it means to MSVC creates friction for users who want the capability to compile code portably with both compilers.

[rnk]

I think the argument for compatibility here is a bit thin, since Clang and MSVC will never produce anything close to the same set of warnings. I think it makes more sense to focus on the main use case for these flags, which I believe is warning discovery. Users use these flags to look for new useful warnings that they may wish to enable in their build. I think the right question is, which is better for that use case, -Wall+-Wextra, or -Weverything?

Or perhaps, do we have too many low-value warnings in -Weverything that we should get rid of? C++98 compat feels pretty low-value in 2024.

[CaseyCarter]

Users use these flags to look for new useful warnings that they may wish to enable in their build.

That's certainly the intended use, but I've seen overzealous users take the "turn on everything and disable the warnings we don't want" approach in production too many times. Said users invariably complain when the compiler "breaks their build" by adding new warnings. I'm concerned that this practice will become more common if clang-cl changes /Wall to an actually useful subset of warnings.

[zmodem]

clang's -Wall works like GCC's (enables more warnings), and clang-cl's /Wall works like MSVC's (enables all warnings). I don't think we should change that.

I don't think it's reasonable for /std:c++20 /Wall /WX to raise errors saying that I'm not writing C++98.

That's a fair point. Perhaps -Wc++98-compat should not fire in C++20 mode. Similarly, maybe -Wunsafe-buffer-usage doesn't make sense for C code.

On the other hand, users of /Wall most likely need a list of disabled warnings anyway, so why not just pass -Wno-c++98-compat?

[rnk]

/std:c++20 /Wall /WX is not a reasonable MSVC build configuration, just as -Weverything -Werror is not a reasonable build configuration. It's just asking for pain, and we should do nothing to support it. It is already documented to contain contradictory warnings, specifically recommending -Wno-c++98-compat.

This does point to a gap in our warning reference, since I guess Weverything is special:
https://clang.llvm.org/docs/DiagnosticsReference.html