[Bug tree-optimization/80635] [8/9/10 regression] std::optional and bogus -Wmaybe-uninitialized warning

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80635

--- Comment #41 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Jason Merrill from comment #38)
> This is better:

Complete revised testcase:

#ifdef USE_STD

#include <optional>
using std::optional;

#else

using size_t = decltype(sizeof(1));
inline void *operator new (size_t, void *p) { return p; }
template<typename T>
struct optional
{
  optional () : m_dummy (), live (false) {}
  void emplace () { new (&m_item) T (); live = true; }
  ~optional () { if (live) m_item.~T (); }

  union
  {
    struct {} m_dummy;
    T m_item;
  };
  bool live;
};

#endif

extern int get ();
extern void set (int);

struct A
{
  A () : m (get ()) {}
  ~A () { set (m); }

  int m;
};

struct B
{
  B ();
  ~B ();
};

void func ()
{
  optional<A> maybe_a;
  optional<B> maybe_b;

  maybe_a.emplace ();
  maybe_b.emplace ();
}