tree 3be2e19f15486a42a7039e51e684b4abcbb82b78
parent 4e444514007d049659f8c8e18327302ae6f80d9b
author Aaron Ballman <aaron@aaronballman.com> 1638895687 -0500
committer Copybara-Service <copybara-worker@google.com> 1638896197 -0800

Add diagnostic groups for attribute extensions

Some users have a need to control attribute extension diagnostics
independent of other extension diagnostics. Consider something like use
of [[nodiscard]] within C++11:
```
[[nodiscard]]
int f();
```
If compiled with -Wc++17-extensions enabled, this will produce warning:
use of the 'nodiscard' attribute is a C++17 extension. This diagnostic
is correct -- using [[nodiscard]] in C++11 mode is a C++17 extension.
And the behavior of __has_cpp_attribute(nodiscard) is also correct --
we support [[nodiscard]] in C++11 mode as a conforming extension. But
this makes use of -Werror or -pedantic-errors` builds more onerous.

This patch adds diagnostic groups for attribute extensions so that
users can selectively disable attribute extension diagnostics. I
believe this is preferable to requiring users to specify additional
flags because it means -Wc++17-extensions continues to be the way we
enable all C++17-related extension diagnostics. It would be quite easy
for someone to use that flag thinking they're protected from some
portability issues without realizing it skipped attribute extensions if
we went the other way.

This addresses PR33518.

GitOrigin-RevId: a18632adc884397e0cd7f27bdb8ea3a822b63000