[Bug c++/97038] New: [[no_unique_address]] support anonymous unions

[no_unique_address at mail dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97038

            Bug ID: 97038
           Summary: [[no_unique_address]] support anonymous unions
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: no_unique_address at mail dot de
  Target Milestone: ---

Compiling the following source file with g++ -std=c++20 -Wall -pedantic
--
struct A {};
struct B {};
struct C
{
    [[no_unique_address]] union
    {
        [[no_unique_address]] A a;
        [[no_unique_address]] B b;
    } /*u*/;
    char c;
};

static_assert(sizeof(C) == sizeof(char));
--
leads to the following output:

<source>:6:5: warning: attribute ignored in declaration of 'union C::<unnamed>'
[-Wattributes]

    6 |     {

      |     ^

<source>:6:5: note: attribute for 'union C::<unnamed>' must follow the 'union'
keyword

<source>:13:25: error: static assertion failed

   13 | static_assert(sizeof(C) == sizeof(char));

      |               ~~~~~~~~~~^~~~~~~~~~~~~~~

Compiler returned: 1

See also: https://godbolt.org/z/GcW9cr

This works fine if the union is not anonymous.
This also works on clang 10.

It would be nice if it could be supported on gcc.