[Bug c++/105377] New: Likely a misleading clang warning -Wc++20-attribute-extensions

[Bug c++/105377] New: Likely a misleading clang warning -Wc++20-attribute-extensions

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

            Bug ID: 105377
           Summary: Likely a misleading clang warning
                    -Wc++20-attribute-extensions
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marxin at gcc dot gnu.org
                CC: mpolacek at gcc dot gnu.org, redi at gcc dot gnu.org
  Target Milestone: ---

I noticed the following warning:

clang++  -I/home/marxin/Programming/gcc/libcpp -I.
-I/home/marxin/Programming/gcc/libcpp/../include
-I/home/marxin/Programming/gcc/libcpp/include  -g -O2     -W -Wall
-Wno-narrowing -Wwrite-strings -Wmissing-format-attribute -pedantic
-Wno-long-long  -fno-exceptions -fno-rtti -I/home/marxin/Programming/gcc/libcpp
-I. -I/home/marxin/Programming/gcc/libcpp/../include
-I/home/marxin/Programming/gcc/libcpp/include  -fPIC -fcf-protection -c -o
lex.o -MT lex.o -MMD -MP -MF .deps/lex.Tpo
/home/marxin/Programming/gcc/libcpp/lex.cc -std=c++17
/home/marxin/Programming/gcc/libcpp/lex.cc:1289:7: warning: use of the 'likely'
attribute is a C++20 extension [-Wc++20-attribute-extensions]
      ATTR_LIKELY case kind::NONE:
      ^~~~~~~~~~~
/home/marxin/Programming/gcc/libcpp/system.h:427:25: note: expanded from macro
'ATTR_LIKELY'
#  define ATTR_LIKELY [[likely]]
                      ~~^~~~~~~~

[Bug c++/105377] Likely a misleading clang warning -Wc++20-attribute-extensions

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Yes, it is an extension, but if the compiler doesn't recognize it, it shouldn't
do anything with it.  So it is just fine to use it...
Especially when gcc only uses it when the compiler tells it it supports it...
#ifdef __has_cpp_attribute
# if __has_cpp_attribute(likely)
#  define ATTR_LIKELY [[likely]]
# elif __has_cpp_attribute(__likely__)
#  define ATTR_LIKELY [[__likely__]]
# else
#  define ATTR_LIKELY
# endif
#else
# define ATTR_LIKELY
#endif

[Bug c++/105377] Likely a misleading clang warning -Wc++20-attribute-extensions

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Yes, I'd prefer to keep it the way it is.

[Bug c++/105377] Likely a misleading clang warning -Wc++20-attribute-extensions

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
it's probably due to the combination of -pedantic and -std=c++17; what happens
if you use -std=gnu++17 instead for the standard?

[Bug c++/105377] Likely a misleading clang warning -Wc++20-attribute-extensions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But we don['t want to use gnu++17 because we want the compiler to be built
using portable ISO C++17. An unrecognized attribute is a portable ISO C++17
construct, it just doesn't do anything (except maybe give a warning).

We could add:

#ifdef __clang__
#pragma clang diagnostic ignored "-Wc++20-attribute-extensions"
#endif

To disable this warning, since we know we're already guarding the attribute
with __has_cpp_attribute.

[Bug c++/105377] Likely a misleading clang warning -Wc++20-attribute-extensions

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #1)
> Yes, it is an extension, but if the compiler doesn't recognize it, it
> shouldn't do anything with it.  So it is just fine to use it...

I agree with that and I'm going to silente the warning in
contrib/filter-clang-warnings.py.