[Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors

["iamsupermouse at mail dot ru" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771

            Bug ID: 111771
           Summary: Incorrect "is used uninitialized" warning, as if
                    zero-initialization didn't propagate through
                    user-provided default constructors
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iamsupermouse at mail dot ru
  Target Milestone: ---

Here's the code. GCC with `-Wall -O3` warns that `x` is used uninitialized,
even though it's zeroed.

    struct A
    {
        A() {}
        int x;
    };

    struct B : A {};

    int main()
    {
        B b = B();
        return b.x;
    }

Since `B` doesn't have a user-provided default constructor, value-initializing
it like this performs zero-initialization, which propagates recursively
(http://eel.is/c++draft/dcl.dcl#dcl.init.general-6.2) over all members, zeroing
everything. Yet GCC incorrectly warns about `x` being uninitialized.