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

["david at doublewise dot net" <gcc-bugzilla@gcc.gnu.org>]

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
Date: Sun, 26 Apr 2015 17:39:00 -0000
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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.