[Bug c++/111284] New: Some passing-by-value parameters are miscompiled since GCC 9

["de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111284

            Bug ID: 111284
           Summary: Some passing-by-value parameters are miscompiled since
                    GCC 9
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid, rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

GCC incorrectly rejects the following program since GCC 9
(https://godbolt.org/z/cGK1a1dqK):
```
class self_locator {
public:
    self_locator() = default;
    constexpr self_locator(const self_locator&) noexcept : this_{this} {}
    constexpr self_locator& operator=(const self_locator&) noexcept { return
*this; }

    constexpr bool valid() const noexcept { return this_ == this; }

private:
    self_locator *this_ = this;
};

constexpr bool demonstrator(self_locator x) noexcept
{
    return x.valid();
}

static_assert(demonstrator(self_locator{}), "");
static_assert([](self_locator x){ return x.valid(); }(self_locator{}), "");
```

The `valid` member function should always return true. But if `self_locator` is
passed by value, GCC can sometimes render the parameter in an inconsistent
state (perhaps due to incorrect bitwise copy).