[Bug c++/96868] New: C++20 designated initializer erroneous warnings

["matt at godbolt dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96868

            Bug ID: 96868
           Summary: C++20 designated initializer erroneous warnings
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at godbolt dot org
  Target Milestone: ---

The following code, with -Wall -Wextra, GCC 10.x or trunk, -std=c++20:

```
struct MyObj {
    MyObj();
};

struct Test {
    int a{};
    MyObj obj;
};

Test t() {
    Test t{.a = 1};

    return t;
}

```

Creates a warning:

```
<source>: In function 'Test t()':

<source>:11:18: warning: missing initializer for member 'Test::obj'
[-Wmissing-field-initializers]
   11 |     Test t{.a = 1};
      |                  ^
```

The "obj" is not missing initialization: the generated code correctly calls
MyObj::MyObj().

clang issues no diagnostic on the same code (even with -Weverything).

May be a duplicate of bug #82283 or bug #84685, but those have far more
complex-looking initialiser lists.

CE link: https://godbolt.org/z/b75P6r