Allow `maybe_unused` in pre-C++17 / pre-C23 modes

[ahatanak]

[[maybe_unused]] is helpful for suppressing warnings about declarations that may or may not be referenced. However, clang currently accepts it only in c++17 (or c23 for C) and later. Users who target older standards often rely on inconvenient workarounds, such as wrapping code in #pragma clang diagnostic push/pop to silence warnings.

It would be useful if clang accepted a spelling like __attribute__((maybe_unused)) for earlier language modes.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Akira Hatanaka (ahatanak)

`[[maybe_unused]]` is helpful for suppressing warnings about declarations that may or may not be referenced. However, clang currently accepts it only in c++17 (or c23 for C) and later. Users who target older standards often rely on inconvenient workarounds, such as wrapping code in `#pragma clang diagnostic push/pop` to silence warnings.

It would be useful if clang accepted a spelling like __attribute__((maybe_unused)) for earlier language modes.

[philnik777]

What do you mean by "clang currently accepts it only in c++17 (or c23 for C) and later"? Clang seems to be happy as-is: https://godbolt.org/z/zE3ff1r9j

[jeremyhu]

What do you mean by "clang currently accepts it only in c++17 (or c23 for C) and later"? Clang seems to be happy as-is: https://godbolt.org/z/zE3ff1r9j

I think your link just exposes a bug in clang. That should fail becuase the requested language mode is C++03 which doesn't support [[maybe_unused]].

However, even considering [[maybe_unused]] is supported in older C++ modes, there is no way to test for that support at compile time. We need to write code that supports being built with C++11 language mode running older clang++ and g++ versions which predate [[maybe_unused]].

Plus, the above doesn't work without warning for C language modes earlier than C23:

<source>:3:3: warning: [[]] attributes are a C23 extension [-Wc23-extensions]
    3 |   [[maybe_unused]] int i = 0;
      |   ^

Adding an attribute which does the same thing and can be checked for would allow clients to write properly portable code that will compile correctly with older compilers across language models and that will avoid emitting these warnings when using current compiler versions.

[shafik]

CC @erichkeane

[erichkeane]

We definitely accept [[maybe_unused]] in all language modes as far as I can tell. Yes, attribute spelling that way is an extension, so we diagnose it. If you don't want that, just disable that warning? Or, more correctly: don't enable it in the first place. You have to go beyond -Wextra to enable that one.

We also support __attribute__((unused)) which does the same thing (but doesn't have a language-extensions warning).

I don't see an issue here.

[cor3ntin]

__has_cpp_attribute can be used to check whether an attribute exists, this works for both standard and non standard attributes.

__has_cpp_attribute is a standard feature supported by all compilers.

you can check

#if defined(__has_cpp_attribute) if you need to support fairly old toolchains

[frederick-vs-ja]

I think your link just exposes a bug in clang. That should fail becuase the requested language mode is C++03 which doesn't support [[maybe_unused]].

This is not a bug. It's possible to use the -pedantic or -pedantic-errors option to make Clang issue conforming diagnostic messages.

However, even considering [[maybe_unused]] is supported in older C++ modes, there is no way to test for that support at compile time.

There is. In addition to above, you can use __has_c_attribute in C.

[AaronBallman]

However, even considering [[maybe_unused]] is supported in older C++ modes, there is no way to test for that support at compile time.

There is. In addition to above, you can use __has_c_attribute in C.

And __has_attribute for __attribute__ style spellings, these are documented starting at: https://clang.llvm.org/docs/LanguageExtensions.html#has-cpp-attribute

This is working as-designed, so closing the issue.