[Bug c++/107206] New: Bogus -Wuninitialized in std::optional

["ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107206

            Bug ID: 107206
           Summary: Bogus -Wuninitialized in std::optional
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Since 12, at -std=c++17 -O -Wuninitialized:

#include <optional>
struct X {
    X() = default;
    X(X const& r) : i(r.i) {}
    int i;
};
struct Y {
    Y() : x() {}
    X x;
    std::optional<int> o;
};
struct Z {
    Y y;
    explicit Z(Y y) : y(y) {}
};
void f(Y const&);
void test() {
    Y const y;
    Z z(y);
    z.y.o = 1;
    auto const w = z;
    f(w.y);
}

In copy constructor 'Y::Y(const Y&)',
    inlined from 'void test()' at <source>:19:10:
<source>:7:8: warning: '*(int*)((char*)&y + offsetof(const Y,
Y::o.std::optional<int>::<unnamed>.std::_Optional_base<int, true,
true>::<unnamed>))' is used uninitialized [-Wuninitialized]
    7 | struct Y {
      |        ^
<source>: In function 'void test()':
<source>:18:13: note: 'y' declared here
   18 |     Y const y;
      |             ^

Most similar to #80635 and #86465, I think? Although this is -Wuninitialized,
not -Wmaybe-uninitialized.