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From: "nightstrike at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107531] New: List of references calls destructors, add
 warning?
Date: Sat, 05 Nov 2022 05:16:24 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.0
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X-Bugzilla-Who: nightstrike at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107531

            Bug ID: 107531
           Summary: List of references calls destructors, add warning?
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nightstrike at gmail dot com
  Target Milestone: ---

The following code results in a destructor being calling twice:

#include <initializer_list>
struct S {
    S() { __builtin_printf("ctor\n"); }
    ~S() { __builtin_printf("dtor\n"); }
};

template<typename ... Args>
void f(Args const & ... args) {
    for (auto const & s: { args... })
        ;
}

int main() {
    S s;
    f(s);
}

$ ./a.out
ctor
dtor
dtor


Changing line 9 to use pointers fixes it:

for (auto const * s: { &args... })

$ ./a.out
ctor
dtor


I think that if my code is exploiting some kind of bad behavior (and it lik=
ely
is), a warning at -Wall would be a nice addition (currently, there is none)=
.=20
If what I'm doing is allowed, I don't think calling a destructor twice is t=
he
best idea :) :).  I'd be betting money on the first thing, though.  Maybe
adding something from -fsanitize=3Dundefined would be an option?=