[Bug c++/96078] New: [10/11 Regression] flatten attribute on constructor and destructor causes spurious warning

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96078

            Bug ID: 96078
           Summary: [10/11 Regression] flatten attribute on constructor
                    and destructor causes spurious warning
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

struct A {
    [[gnu::flatten]] A() {}
    [[gnu::flatten]] ~A() {}
};

A a;

Produces no warnings with GCC 9 but on GCC 10 and trunk warns:

<source>:3:22: warning: 'flatten' attribute is ignored on aliases
[-Wattributes]

    3 |     [[gnu::flatten]] ~A() {}

      |                      ^

<source>:2:22: warning: 'flatten' attribute is ignored on aliases
[-Wattributes]

    2 |     [[gnu::flatten]] A() {}

      |                      ^

The warning appears to be introduced by the fix to PR92372.

Marking `A` as `final` eliminates the warnings, as does giving it an virtual
base class. Presumably the issue is that in the problematic case one of the
complete/base object c/dtors was treated as an alias for the other?