From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 72A103858C2B; Tue, 10 Nov 2020 21:09:41 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 72A103858C2B
From: "ldalessandro at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/97790] New: constexpr evaluation reports false positive
 memory leak
Date: Tue, 10 Nov 2020 21:09:41 +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: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: ldalessandro at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-97790-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 10 Nov 2020 21:09:41 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97790

            Bug ID: 97790
           Summary: constexpr evaluation reports false positive memory
                    leak
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldalessandro at gmail dot com
  Target Milestone: ---

The constexpr evaluator seems to lose track of dynamic allocations in some
contexts, resulting in false-positive leaks and rejected valid code.=20

This particular example uses an immediately invoked function expression
referencing an NTTP, which seems like a corner case but is actually extreme=
ly
common in two-pass constexpr algorithms (i.e., compute constexpr size of
result, then compute result to return).

https://godbolt.org/z/Tnn6xh

```
struct vector {
    int *data;
    int n;
    constexpr vector() : data(new int[1]{}), n(1) {}
    constexpr ~vector() { delete [] data; }
};

struct Foo {
    constexpr auto foo() const {
        return vector();
    }
};

template <auto& f>
constexpr static auto bar() {
    [] { return f.foo().n; }();
    return true;
}

constexpr Foo foo;
constexpr auto dd =3D bar<foo>();
```=