[Bug c++/89160] New: -Wattributes too eager on C++11 attributes

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89160

            Bug ID: 89160
           Summary: -Wattributes too eager on C++11 attributes
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

A random testcase to illustrate:

[[hahaha]] int i;

$ g++-snapshot a.c -c -std=gnu++2a
a.c:1:16: warning: 'hahaha' attribute directive ignored [-Wattributes]
    1 | [[hahaha]] int i;
      |                ^

The standard, in [dcl.attr.grammar], says: "Any attribute-token that is not
recognized by the implementation is ignored." I believe the intent is that it
should be **silently** ignored, unless I really ask for it clearly with
something like -Wunknown-c++-attribute (definitely not part of -Wall). There is
a strong requirement that standard attributes have to be safe to ignore. I
believe I should be able to use newer (C++20) attributes without having older
(C++17) compilers complain so noisily. Sadly, clang is just as bad, and ISTR
reading that visual studio was even worse, rejecting programs with C++17
attributes in C++14 mode. As a result, we keep having to write a lot of
preprocessor nonsense :-(