[Bug c++/101316] New: [[maybe_unused]] to the right of array size accepted

[gcc at alanwu dot email via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101316

            Bug ID: 101316
           Summary: [[maybe_unused]] to the right of array size accepted
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at alanwu dot email
  Target Milestone: ---

GCC 11.1.0 and trunk accepts [[maybe_unused]] in the following in a way
the standard doesn't seem to allow:

    void func() {
        int one, two[2] [[maybe_unused]];
    }

    $ g++ -c -std=c++20 -Wall test.cc
    test.cc: In function ‘void func()’:
    test.cc:2:13: warning: unused variable ‘one’ [-Wunused-variable]
        2 |         int one, two[2] [[maybe_unused]];
          |             ^~~

There is no unused variable warning about `two` because it
accepts the attribute.

The declaration for `two` matches the form outlined in 
[dcl.array]p3 which says the attribute appertains to the array type.
[dcl.attr.unused]p2 doesn't allow [[maybe_unused]] to apply to type entities.
[dcl.attr.grammar]p5 says that when an attribute appertains to an entity
that it's not allowed to apply to, the program is ill-formed.

Should g++ reject the program in this case?

https://godbolt.org/z/EWr4q63cT