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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111284] New: Some passing-by-value parameters are
 miscompiled since GCC 9
Date: Mon, 04 Sep 2023 11:16:43 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.0
X-Bugzilla-Keywords: accepts-invalid, rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: de34 at live dot cn
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111284

            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() =3D default;
    constexpr self_locator(const self_locator&) noexcept : this_{this} {}
    constexpr self_locator& operator=3D(const self_locator&) noexcept { ret=
urn
*this; }

    constexpr bool valid() const noexcept { return this_ =3D=3D this; }

private:
    self_locator *this_ =3D 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).=