[Bug c++/65896] New: Erroneous uninitialized variable access error in constexpr function with temporary variables

[Bug c++/65896] New: Erroneous uninitialized variable access error in constexpr function with temporary variables

[david at doublewise dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65896

            Bug ID: 65896
           Summary: Erroneous uninitialized variable access error in
                    constexpr function with temporary variables
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david at doublewise dot net
  Target Milestone: ---

file.cpp
==================================================
struct base{};

struct derived : base {
        constexpr derived():
                base{},
                m_value(0) {
        }
        constexpr derived(derived & other):
                base{},
                m_value(other) {
        }
        constexpr explicit operator int() const {
                return m_value;
        }
        int m_value;
};

constexpr int by_ref(derived && value) {
        return (derived(value), 0);
}

constexpr int value = by_ref(derived{});
==================================================

c++ -std=c++11 -o /dev/null file.cpp -c


gcc 4.8.3 and 4.9.3 and clang 3.5.0 successfully compile this program. gcc
5.1.1 fails with:

file.cpp:22:29:   in constexpr expansion of ‘by_ref(derived())’
file.cpp:22:39:   in constexpr expansion of ‘derived((* & value))’
file.cpp:10:16:   in constexpr expansion of ‘(& other)->derived::operator
int()’
file.cpp:22:39: error: accessing uninitialized member ‘derived::m_value’
 constexpr int value = by_ref(derived{});
                                       ^


Changing to C++1y / C++14 does not change this.
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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/65892] gcc fails to implement N685 aliasing of union members
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide892

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |---

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Thanks for the pointer!  I had looked for a related bug report but couldn't
find it.

There's an important difference between the test cases in pr14319 and the one
here that's easy to overlook.  The rule only applies to structs defined in
unions, not those defined at file scope and only used to declare union members,
and to translation units in which the union definition is visible.  I would
recommend closing pr14319 as NOTABUG.  I have reopened this bug.

[Bug c++/65896] Erroneous uninitialized variable access error in constexpr function with temporary variables

[david at doublewise dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65896

David Stone <david at doublewise dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |david at doublewise dot net

--- Comment #1 from David Stone <david at doublewise dot net> ---
I have simplified the test case further:


struct base {};

struct derived :  base {
        constexpr derived():
                base{},
                m_value(0) {
        }
        int m_value;
};

constexpr int by_ref(derived && value) {
        return value.m_value;
}

constexpr int value = by_ref(derived{});

[Bug c++/65896] [5/6 Regression] Erroneous uninitialized variable access error in constexpr function with temporary variables

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65896

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-04-27
      Known to work|                            |4.8.3, 4.9.2
            Summary|Erroneous uninitialized     |[5/6 Regression] Erroneous
                   |variable access error in    |uninitialized variable
                   |constexpr function with     |access error in constexpr
                   |temporary variables         |function with temporary
                   |                            |variables
     Ever confirmed|0                           |1
      Known to fail|                            |5.1.0, 6.0

[Bug c++/65896] [5/6 Regression] Erroneous uninitialized variable access error in constexpr function with temporary variables

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65896

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Tue Apr 28 21:27:17 2015
New Revision: 222549

URL: https://gcc.gnu.org/viewcvs?rev=222549&root=gcc&view=rev
Log:
        PR c++/65896
        * constexpr.c (cxx_eval_store_expression): Don't try to actually
        store an empty class.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-empty9.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/constexpr.c

[Bug c++/65896] [5/6 Regression] Erroneous uninitialized variable access error in constexpr function with temporary variables

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65896

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|---                         |FIXED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
   Target Milestone|---                         |5.2

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for 5.2, 6

[Bug c++/65896] [5/6 Regression] Erroneous uninitialized variable access error in constexpr function with temporary variables

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65896

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Wed Apr 29 00:57:50 2015
New Revision: 222557

URL: https://gcc.gnu.org/viewcvs?rev=222557&root=gcc&view=rev
Log:
        PR c++/65896
        * constexpr.c (cxx_eval_store_expression): Don't try to actually
        store an empty class.

Added:
    branches/gcc-5-branch/gcc/testsuite/g++.dg/cpp0x/constexpr-empty9.C
Modified:
    branches/gcc-5-branch/gcc/cp/ChangeLog
    branches/gcc-5-branch/gcc/cp/constexpr.c