[Bug c++/108114] New: Designated initialization of parent class results in error in C++20 mode

["gawain.bolton at free dot fr" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108114

            Bug ID: 108114
           Summary: Designated initialization of parent class results in
                    error in C++20 mode
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gawain.bolton at free dot fr
  Target Milestone: ---

Created attachment 54095
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54095&action=edit
Code accepted with -std=c++17 and rejected with -std=c++20

The attached code generates the following error when compiled with -std=c++20:

<source>: In function 'int main()':
<source>:11:19: error: either all initializer clauses should be designated or
none of them should be
   11 |     B b{{.a = 0}, .b = 1};
      |                   ^

From my understanding of the C++20 standard (cf.
https://eel.is/c++draft/dcl.init.general#nt:brace-or-equal-initializer)
this code attached is valid since a designated-initializer-clause may be a
braced-init-list.

This code has no error or warning about designated initialization with
-std=c++17

Clang generates a warning about mixed designated/non-designated initialization,
which I believe is not merited, but this is not an error.